Worked example: Lewis diagram of formaldehyde (CH₂O)

WEBVTT
Kind: captions
Language: en

00:00:00.950 --> 00:00:01.950
- [Instructor] What we're
gonna do in this video

00:00:01.950 --> 00:00:03.320
is get a little bit more practice

00:00:03.320 --> 00:00:05.990
constructing Lewis diagrams,

00:00:05.990 --> 00:00:08.170
and in particular, we're going to try

00:00:08.170 --> 00:00:11.650
to construct the Lewis
diagram for formaldehyde.

00:00:11.650 --> 00:00:15.750
Formaldehyde has one
carbon, two hydrogens,

00:00:15.750 --> 00:00:18.710
and an oxygen, CH2O.

00:00:18.710 --> 00:00:20.470
So pause this video and have a go at it.

00:00:20.470 --> 00:00:23.630
Try to construct a valid Lewis structure,

00:00:23.630 --> 00:00:26.133
or a Lewis diagram for formaldehyde.

00:00:27.050 --> 00:00:29.950
All right, now let's do this together.

00:00:29.950 --> 00:00:32.870
Now the first step, and we
saw this in a previous video,

00:00:32.870 --> 00:00:35.190
we want to think about all
of the valence electrons

00:00:35.190 --> 00:00:36.930
for this molecule.

00:00:36.930 --> 00:00:39.190
So we want to account,

00:00:39.190 --> 00:00:42.560
account for the valence electrons.

00:00:42.560 --> 00:00:43.830
Now the reason why we wanna do that

00:00:43.830 --> 00:00:46.640
is so that while we're trying
to create this structure,

00:00:46.640 --> 00:00:49.320
we are making use of all
of the valence electrons.

00:00:49.320 --> 00:00:52.160
And to figure out how many
total valence electrons we have,

00:00:52.160 --> 00:00:54.230
we can look at a periodic
table of elements.

00:00:54.230 --> 00:00:56.720
We can see that carbon,

00:00:56.720 --> 00:00:58.750
it's in that second row,
in that second period,

00:00:58.750 --> 00:01:01.140
so its second shell is its outer shell.

00:01:01.140 --> 00:01:02.170
And in that shell,

00:01:02.170 --> 00:01:05.660
it has one, two, three,
four valence electrons.

00:01:05.660 --> 00:01:08.660
So, carbon has four valence electrons.

00:01:08.660 --> 00:01:10.400
A neutral free hydrogen atom

00:01:10.400 --> 00:01:12.270
is going to have one valence electron,

00:01:12.270 --> 00:01:14.090
but we have two of them here,

00:01:14.090 --> 00:01:15.980
so it's gonna be two times one.

00:01:15.980 --> 00:01:20.290
And then, oxygen, it also
is in the second period,

00:01:20.290 --> 00:01:21.720
and in its second shell

00:01:21.720 --> 00:01:26.320
it has one, two, three, four,
five, six valence electrons.

00:01:26.320 --> 00:01:28.860
And so the total valence
electrons in this molecule

00:01:28.860 --> 00:01:32.660
are gonna be four plus two,
which is six, plus six,

00:01:32.660 --> 00:01:37.330
which is equal to 12 valence electrons.

00:01:37.330 --> 00:01:41.580
Now the next step is to
try to draw a structure.

00:01:41.580 --> 00:01:44.640
Try to draw,

00:01:44.640 --> 00:01:49.140
draw single bonds, I'll say, single bonds.

00:01:49.140 --> 00:01:50.320
And a key question is,

00:01:50.320 --> 00:01:52.690
what do we think is going
to be our central atom?

00:01:52.690 --> 00:01:56.420
And the rule of thumb is the
least electronegative atom,

00:01:56.420 --> 00:01:58.530
that is not hydrogen,

00:01:58.530 --> 00:02:01.300
is a good candidate for our central atom.

00:02:01.300 --> 00:02:02.630
So we can rule out hydrogen.

00:02:02.630 --> 00:02:05.500
So between carbon and
oxygen, we know that oxygen

00:02:05.500 --> 00:02:07.980
is one of the most electronegative atoms,

00:02:07.980 --> 00:02:09.490
well one of the most
electronegative elements

00:02:09.490 --> 00:02:10.750
on the periodic table of elements.

00:02:10.750 --> 00:02:12.170
It's very close to fluorine.

00:02:12.170 --> 00:02:15.810
And so carbon is a good
candidate for the central atom.

00:02:15.810 --> 00:02:20.600
So let's put the carbon right over here,

00:02:20.600 --> 00:02:24.000
and then let's put these
other atoms around it.

00:02:24.000 --> 00:02:25.830
We could call them terminal atoms.

00:02:25.830 --> 00:02:29.340
So, let's put our oxygen right over there,

00:02:29.340 --> 00:02:31.340
and then we have two hydrogens.

00:02:31.340 --> 00:02:33.840
Hydrogen there, a hydrogen there.

00:02:33.840 --> 00:02:35.980
And let me draw the bonds.

00:02:35.980 --> 00:02:37.130
So that's a single bond.

00:02:37.130 --> 00:02:39.380
That accounts for two valence electrons.

00:02:39.380 --> 00:02:41.490
That accounts for two valence electrons.

00:02:41.490 --> 00:02:43.070
That accounts for two valence electrons.

00:02:43.070 --> 00:02:46.260
So I've just used two,
four, six valence electrons.

00:02:46.260 --> 00:02:49.410
So if I subtract six valence electrons,

00:02:49.410 --> 00:02:53.580
I am now left with six valence electrons,

00:02:53.580 --> 00:02:55.910
six valence electrons.

00:02:55.910 --> 00:02:57.410
So the next step is

00:02:57.410 --> 00:02:59.660
allocate the remaining valence electrons,

00:02:59.660 --> 00:03:01.970
trying to get to the octet rule

00:03:01.970 --> 00:03:04.690
for atoms that are not
hydrogen, and then for hydrogen,

00:03:04.690 --> 00:03:06.880
trying to get it to have
two valence electrons.

00:03:06.880 --> 00:03:11.480
So allocate, allocate the remaining,

00:03:11.480 --> 00:03:15.890
remaining valence electrons.

00:03:15.890 --> 00:03:17.650
All right, so let's
start with this oxygen.

00:03:17.650 --> 00:03:19.940
This oxygen already
has these two electrons

00:03:19.940 --> 00:03:21.760
that it's sharing hanging around.

00:03:21.760 --> 00:03:25.150
So in order to get to the
octet rule, it needs six more.

00:03:25.150 --> 00:03:27.910
So let's give it six electrons.

00:03:27.910 --> 00:03:32.910
So, one, two, three, four, five, six.

00:03:33.560 --> 00:03:38.100
Well I've just used up the
remaining six valence electrons.

00:03:38.100 --> 00:03:40.480
So I don't really have
any more to play with,

00:03:40.480 --> 00:03:43.020
but let's see how the
other atoms are feeling.

00:03:43.020 --> 00:03:46.880
So hydrogen here, it's able
to share these two electrons

00:03:46.880 --> 00:03:49.080
that are in this covalent
bond, so it's feeling good.

00:03:49.080 --> 00:03:52.430
It can kind of pretend that
it has a full outer shell,

00:03:52.430 --> 00:03:54.310
'cause its outer shell is just that one,

00:03:54.310 --> 00:03:56.970
that first shell, that's
filled with two electrons.

00:03:56.970 --> 00:03:58.840
Same thing for this other hydrogen.

00:03:58.840 --> 00:04:00.560
So at least the terminal atoms,

00:04:00.560 --> 00:04:02.480
the oxygen and the two hydrogens,

00:04:02.480 --> 00:04:05.070
are feeling like they
have a full outer shell.

00:04:05.070 --> 00:04:06.580
But then in the fourth step,

00:04:06.580 --> 00:04:09.540
we're going to look at our central atom.

00:04:09.540 --> 00:04:14.470
So, let's focus on the
central atom, central atom,

00:04:14.470 --> 00:04:16.660
and do we need more bonds,

00:04:16.660 --> 00:04:19.430
or do we need to do
something interesting here?

00:04:19.430 --> 00:04:22.030
And what we see is that carbon,

00:04:22.030 --> 00:04:25.610
it's able to have two, four, six electrons

00:04:25.610 --> 00:04:28.670
hanging around it, but it
would love to have eight.

00:04:28.670 --> 00:04:31.450
Carbon would love to
have a full outer shell,

00:04:31.450 --> 00:04:33.280
so how could we do that?

00:04:33.280 --> 00:04:36.000
Well, we could add more bonds.

00:04:36.000 --> 00:04:37.930
Where could the bonds come from?

00:04:37.930 --> 00:04:40.230
Well it would come from
some lone pair of electrons.

00:04:40.230 --> 00:04:41.850
Well the only lone pairs of electrons

00:04:41.850 --> 00:04:43.570
are hanging around this oxygen.

00:04:43.570 --> 00:04:45.730
So what if we were to take,

00:04:45.730 --> 00:04:48.300
say, this lone pair of electrons,

00:04:48.300 --> 00:04:51.980
and then construct another
covalent bond with that?

00:04:51.980 --> 00:04:55.950
Then, our Lewis diagram
will look like this.

00:04:55.950 --> 00:04:57.610
I will actually redraw it.

00:04:57.610 --> 00:04:59.370
So you have your carbon,

00:04:59.370 --> 00:05:04.370
you have your three
original covalent bonds,

00:05:05.220 --> 00:05:08.460
you had a hydrogen, a hydrogen,

00:05:08.460 --> 00:05:13.460
and then you had your
oxygen, right over here,

00:05:13.490 --> 00:05:18.490
and now we've formed a new
covalent bond, just like this,

00:05:18.680 --> 00:05:21.360
and then you have these
two other lone pairs

00:05:21.360 --> 00:05:22.510
around the oxygen.

00:05:22.510 --> 00:05:24.210
So let me draw that.

00:05:24.210 --> 00:05:28.430
So, two, then another
two around the oxygen.

00:05:28.430 --> 00:05:31.660
And this is looking pretty
good, because the oxygen,

00:05:31.660 --> 00:05:33.880
it still has eight
electrons hanging around,

00:05:33.880 --> 00:05:35.050
four in lone pairs,

00:05:35.050 --> 00:05:37.680
and then four, they're in this double bond

00:05:37.680 --> 00:05:39.070
that it is sharing.

00:05:39.070 --> 00:05:41.520
The hydrogens still have two
electrons hanging around.

00:05:41.520 --> 00:05:43.670
They're able to share the electrons

00:05:43.670 --> 00:05:45.270
in each of these covalent bonds.

00:05:45.270 --> 00:05:47.890
And now the carbon is participating in,

00:05:47.890 --> 00:05:49.710
you could think of it
as four covalent bonds,

00:05:49.710 --> 00:05:51.620
two single bonds and one double bond,

00:05:51.620 --> 00:05:54.410
so each of those have two
electrons associated with it,

00:05:54.410 --> 00:05:56.790
so it has eight electrons hanging around.

00:05:56.790 --> 00:05:59.680
So this is looking really good

00:05:59.680 --> 00:06:03.330
as a legitimate Lewis structure,

00:06:03.330 --> 00:06:06.603
or Lewis diagram for formaldehyde.


Give a gift that will change a life.

WEBVTT
Kind: captions
Language: en

00:00:00.239 --> 00:00:03.572
(soft uplifting music)


Causation from 1980-2020

WEBVTT
Kind: captions
Language: en

00:00:00.360 --> 00:00:01.510
- [Instructor] From our first lesson

00:00:01.510 --> 00:00:03.970
focusing on the migration
of indigenous people

00:00:03.970 --> 00:00:07.520
to the landmass that today
comprises the United States,

00:00:07.520 --> 00:00:10.460
we've made it all the way to the present,

00:00:10.460 --> 00:00:14.250
a journey in time of
more than 15,000 years.

00:00:14.250 --> 00:00:17.870
We've looked most closely
at the last 500 years

00:00:17.870 --> 00:00:21.480
starting with the arrival of
Europeans in the Americas.

00:00:21.480 --> 00:00:23.880
Now, in this last unit we're focused

00:00:23.880 --> 00:00:27.820
just on the last 40
years of American history

00:00:27.820 --> 00:00:30.590
from 1980 until the present.

00:00:30.590 --> 00:00:32.010
How can we examine something

00:00:32.010 --> 00:00:34.570
that's so close to us in time?

00:00:34.570 --> 00:00:36.550
We're still in this era.

00:00:36.550 --> 00:00:37.930
It doesn't have a name,

00:00:37.930 --> 00:00:41.620
not like the Gilded Age
or the Revolutionary Era,

00:00:41.620 --> 00:00:42.860
at least not yet.

00:00:42.860 --> 00:00:46.150
Maybe this is the post-Cold War era

00:00:46.150 --> 00:00:49.210
or the post-9/11 era?

00:00:49.210 --> 00:00:51.450
Maybe it will be defined by the changes

00:00:51.450 --> 00:00:52.580
brought by technology

00:00:52.580 --> 00:00:55.020
and we'll call it the Information Age.

00:00:55.020 --> 00:00:57.940
Trying to think historically
about the present

00:00:57.940 --> 00:00:59.820
gives us a better understanding

00:00:59.820 --> 00:01:02.360
of what it was like to live in the past,

00:01:02.360 --> 00:01:04.330
to not know what was coming next

00:01:04.330 --> 00:01:07.980
or how to interpret
all the complex threads

00:01:07.980 --> 00:01:11.380
of politics, society, and culture

00:01:11.380 --> 00:01:13.050
weaving together around you.

00:01:13.050 --> 00:01:15.010
Life is messy and confusing.

00:01:15.010 --> 00:01:17.780
We don't know if we're
living in a tragic era

00:01:17.780 --> 00:01:19.540
or a triumphant one,

00:01:19.540 --> 00:01:22.330
neither did the people who lived in 1940

00:01:22.330 --> 00:01:25.040
or 1860 or 1770.

00:01:25.040 --> 00:01:27.500
So years from now when historians

00:01:27.500 --> 00:01:30.990
write about the period from 1980 to 2020

00:01:30.990 --> 00:01:32.230
in the United States

00:01:32.230 --> 00:01:33.500
what will they say?

00:01:33.500 --> 00:01:35.890
Well, let's pretend for a few minutes

00:01:35.890 --> 00:01:39.030
that we are those
historians from the future.

00:01:39.030 --> 00:01:41.410
Maybe we're living on a space station

00:01:41.410 --> 00:01:43.120
a few hundred years from now

00:01:43.120 --> 00:01:46.060
enjoying a Raktajino by the Replicator

00:01:46.060 --> 00:01:47.700
and discussing the United States

00:01:47.700 --> 00:01:50.290
at the turn of the 21st century.

00:01:50.290 --> 00:01:53.840
Would we be discussing a
golden age or a dark period?

00:01:53.840 --> 00:01:55.640
Let's apply some of our historical

00:01:55.640 --> 00:01:57.290
thinking skills to this era

00:01:57.290 --> 00:01:59.120
and see if we can determine

00:01:59.120 --> 00:02:02.210
what effects changes in this period had

00:02:02.210 --> 00:02:04.540
on American national identity.

00:02:04.540 --> 00:02:08.040
First, let's talk about
America's role in the world.

00:02:08.040 --> 00:02:10.770
In 1980, the United
States was still locked

00:02:10.770 --> 00:02:12.950
in a Cold War with the Soviet Union,

00:02:12.950 --> 00:02:15.660
and when Ronald Reagan
took over as president

00:02:15.660 --> 00:02:18.930
he moved the country away
from the policy of Detente,

00:02:18.930 --> 00:02:21.420
or relaxation of tension,

00:02:21.420 --> 00:02:24.080
kind of live and let live with the soviets

00:02:24.080 --> 00:02:26.170
that was pursued in the 1970s

00:02:26.170 --> 00:02:30.460
towards a more active,
anticommunist stance.

00:02:30.460 --> 00:02:33.910
The end of the decade saw the
collapse of the Soviet Union.

00:02:33.910 --> 00:02:38.030
So after more then 40
years of foreign policy

00:02:38.030 --> 00:02:40.710
that was aimed at containing communism

00:02:40.710 --> 00:02:42.730
and the influence of the Soviet Union,

00:02:42.730 --> 00:02:45.180
capitalism and democracy were now

00:02:45.180 --> 00:02:47.800
the dominant economic
and political systems,

00:02:47.800 --> 00:02:51.790
and the United States was
the world's lone superpower.

00:02:51.790 --> 00:02:53.100
After the Cold War ended

00:02:53.100 --> 00:02:55.010
it wasn't immediately clear

00:02:55.010 --> 00:02:58.350
what the new US role
in the world should be.

00:02:58.350 --> 00:03:01.510
Should it return to an isolationist stance

00:03:01.510 --> 00:03:03.540
like it had before World War II.

00:03:03.540 --> 00:03:06.780
Should it serve as the
world's police officer

00:03:06.780 --> 00:03:09.720
keeping the peace and
countering the actions

00:03:09.720 --> 00:03:12.530
of hostile powers that might try to arise.

00:03:12.530 --> 00:03:15.240
During the 1990s, the United States had

00:03:15.240 --> 00:03:18.240
a few limited engagements abroad

00:03:18.240 --> 00:03:21.210
and the Gulf War, the United
States defended Kuwait

00:03:21.210 --> 00:03:23.180
from an Iraqi invasion,

00:03:23.180 --> 00:03:25.430
and it sent troops to Kosovo

00:03:25.430 --> 00:03:28.410
as part of a NATO peacekeeping force.

00:03:28.410 --> 00:03:32.460
But the terrorist attacks
of September 11th, 2001

00:03:32.460 --> 00:03:35.120
on the World Trade Center in New York City

00:03:35.120 --> 00:03:37.620
and the Pentagon in Washington, D.C.

00:03:37.620 --> 00:03:41.610
began a new era of
American foreign policy.

00:03:41.610 --> 00:03:44.460
US President George W. Bush articulated

00:03:44.460 --> 00:03:46.630
what's been called the Bush Doctrine

00:03:46.630 --> 00:03:48.350
which asserted that the United States

00:03:48.350 --> 00:03:51.660
has the right to secure
itself against countries

00:03:51.660 --> 00:03:54.810
that harbor or give aid
to terrorist groups.

00:03:54.810 --> 00:03:57.200
So under this doctrine, the United States

00:03:57.200 --> 00:04:02.200
went to war with Afghanistan
in 2001 and Iraq in 2003

00:04:02.780 --> 00:04:05.270
after those countries refused to surrender

00:04:05.270 --> 00:04:08.100
Osama bin Laden and the
terrorists responsible

00:04:08.100 --> 00:04:09.510
for September 11th.

00:04:09.510 --> 00:04:12.980
The war in Afghanistan
became the longest war

00:04:12.980 --> 00:04:16.380
in US history continuing through 2019.

00:04:16.380 --> 00:04:20.860
At the end of this era from 1980 to 2020,

00:04:20.860 --> 00:04:23.590
Americans were continuing to debate

00:04:23.590 --> 00:04:25.810
the proper US role in the world.

00:04:25.810 --> 00:04:28.770
Should the United States
withdraw from wars abroad

00:04:28.770 --> 00:04:31.010
to save the lives of American troops

00:04:31.010 --> 00:04:34.010
and the massive budget
expenditures of war?

00:04:34.010 --> 00:04:35.670
Or would leaving the Middle East

00:04:35.670 --> 00:04:37.650
further destabilize the region

00:04:37.650 --> 00:04:40.170
and lead to even bigger
problems in the future?

00:04:40.170 --> 00:04:41.980
Some questions that we might think about

00:04:41.980 --> 00:04:44.720
in the broader scope of US history,

00:04:44.720 --> 00:04:47.340
how does the Bush doctrine compare to

00:04:47.340 --> 00:04:50.470
earlier presidential
doctrines on foreign policy

00:04:50.470 --> 00:04:53.520
like the Nixon Doctrine,
the Truman Doctrine,

00:04:53.520 --> 00:04:55.550
or even the Monroe Doctrine?

00:04:55.550 --> 00:04:57.780
And how does the war in Afghanistan

00:04:57.780 --> 00:05:00.990
compare to the United
States' second longest war,

00:05:00.990 --> 00:05:02.470
the war in Vietnam.

00:05:02.470 --> 00:05:03.990
The end of the Cold War

00:05:03.990 --> 00:05:06.410
and the rise of the war on terror

00:05:06.410 --> 00:05:09.700
weren't the only major
changes in this time period.

00:05:09.700 --> 00:05:12.790
There were also
far-reaching social changes.

00:05:12.790 --> 00:05:14.580
How work was done,

00:05:14.580 --> 00:05:16.370
how much people were paid for it,

00:05:16.370 --> 00:05:17.810
and who did that work.

00:05:17.810 --> 00:05:19.480
In the late 20th century,

00:05:19.480 --> 00:05:22.950
the emergence of the personal
computer and the internet

00:05:22.950 --> 00:05:24.640
made it possible to communicate

00:05:24.640 --> 00:05:28.280
and to do business all over
the world in an instant.

00:05:28.280 --> 00:05:32.320
At the same time, barriers to
trade fell between countries.

00:05:32.320 --> 00:05:35.030
In 1994, the United States signed

00:05:35.030 --> 00:05:38.510
the North American Free
Trade Agreement or NAFTA

00:05:38.510 --> 00:05:41.500
to reduce or eliminate
tariffs on trade goods

00:05:41.500 --> 00:05:44.550
between the United
States, Mexico and Canada.

00:05:44.550 --> 00:05:49.460
The growing international
interdependence of business

00:05:49.460 --> 00:05:51.510
and the mixing of cultures that business

00:05:51.510 --> 00:05:55.060
carries along with it
is called globalization.

00:05:55.060 --> 00:05:58.240
But the availability of
cheap goods and cheap labor

00:05:58.240 --> 00:06:00.370
that globalization made possible

00:06:00.370 --> 00:06:04.550
also had some economic
consequences for American workers.

00:06:04.550 --> 00:06:07.410
Manufacturing generally moved overseas

00:06:07.410 --> 00:06:10.510
to take advantage of lower
wages and regulations

00:06:10.510 --> 00:06:13.640
and union membership fell to a record low.

00:06:13.640 --> 00:06:15.880
Meanwhile, the share of Americans

00:06:15.880 --> 00:06:18.820
working in service-oriented jobs rose.

00:06:18.820 --> 00:06:21.250
The largest employer in the United States

00:06:21.250 --> 00:06:23.820
in 2019 was Walmart.

00:06:23.820 --> 00:06:26.010
Wages have stagnated for low

00:06:26.010 --> 00:06:27.990
and middle class American workers

00:06:27.990 --> 00:06:31.000
while wages have soared
for the richest Americans.

00:06:31.000 --> 00:06:33.250
A study by the federal reserve found that

00:06:33.250 --> 00:06:36.580
although the total net
worth of US households

00:06:36.580 --> 00:06:40.500
more than quadrupled
between 1989 and 2018,

00:06:40.500 --> 00:06:42.450
most of those gains were for the wealthy,

00:06:42.450 --> 00:06:45.090
with the top 10% of households controlling

00:06:45.090 --> 00:06:47.640
64% of overall wealth.

00:06:47.640 --> 00:06:51.130
The top 1% alone controlled 32%.

00:06:51.130 --> 00:06:53.270
Another social change of the late 20th

00:06:53.270 --> 00:06:56.680
and early 21st century was
the growth of immigration.

00:06:56.680 --> 00:06:59.370
The 1965 immigration act ended

00:06:59.370 --> 00:07:01.410
national quotas in immigration

00:07:01.410 --> 00:07:04.070
which had barred immigration
for most countries

00:07:04.070 --> 00:07:05.700
other than Western Europe.

00:07:05.700 --> 00:07:08.440
As a result, immigration
from Latin America,

00:07:08.440 --> 00:07:10.770
Asia and Africa grew

00:07:10.770 --> 00:07:13.900
changing the racial demographics
of the United States.

00:07:13.900 --> 00:07:16.620
Some of the biggest questions
of American politics

00:07:16.620 --> 00:07:18.230
at the end of this era

00:07:18.230 --> 00:07:21.930
concerned the effects of
globalization and immigration.

00:07:21.930 --> 00:07:24.730
Putting this in a broader
historical context

00:07:24.730 --> 00:07:26.970
we might ask how did the developments

00:07:26.970 --> 00:07:29.510
in this time period compare with

00:07:29.510 --> 00:07:33.420
say the Second Industrial
Revolution in the Gilded Age

00:07:33.420 --> 00:07:36.220
when new manufacturing
technology opened up

00:07:36.220 --> 00:07:37.920
many new factory jobs,

00:07:37.920 --> 00:07:40.920
and lured many immigrants
to American cities,

00:07:40.920 --> 00:07:43.920
but also bred great inequality of wealth.

00:07:43.920 --> 00:07:47.500
Politics in this period
also differed sharply

00:07:47.500 --> 00:07:49.770
from the era that came before it.

00:07:49.770 --> 00:07:52.710
From 1932 to 1980,

00:07:52.710 --> 00:07:56.700
liberal ideas had
dominated American politics

00:07:56.700 --> 00:07:58.710
that government should be active

00:07:58.710 --> 00:08:00.580
in securing the welfare of people,

00:08:00.580 --> 00:08:03.540
and that taxation should
pay for those initiatives.

00:08:03.540 --> 00:08:06.190
But there was a growing
conservative movement

00:08:06.190 --> 00:08:07.970
starting in the 1960s

00:08:07.970 --> 00:08:10.550
and Ronald Reagan's election in 1980

00:08:10.550 --> 00:08:13.750
began a period of limiting
government regulation

00:08:13.750 --> 00:08:15.220
and lowering taxes.

00:08:15.220 --> 00:08:19.050
And when democrat Bill
Clinton took office in 1993,

00:08:19.050 --> 00:08:21.530
he largely adopted that same stance.

00:08:21.530 --> 00:08:24.630
He declared that the era
of big government is over

00:08:24.630 --> 00:08:27.160
and reduced welfare
benefits significantly.

00:08:27.160 --> 00:08:31.230
The 1990s saw the emergence
of the culture wars.

00:08:31.230 --> 00:08:34.580
Battles over the growing multiculturalism,

00:08:34.580 --> 00:08:37.340
secularism, and cultural acceptance

00:08:37.340 --> 00:08:40.990
of non-traditional marriage
and family relationships.

00:08:40.990 --> 00:08:43.360
Many conservative Christians feared that

00:08:43.360 --> 00:08:46.200
the traditional American nuclear family

00:08:46.200 --> 00:08:49.240
with Christian,
heterosexual married parents

00:08:49.240 --> 00:08:50.400
was disappearing.

00:08:50.400 --> 00:08:53.000
Meanwhile, liberal progressive celebrated

00:08:53.000 --> 00:08:56.190
the growing acceptance of LGBTQ citizens

00:08:56.190 --> 00:08:58.050
in diversity and public life.

00:08:58.050 --> 00:09:01.710
This was also a time of
bitter partisan divisions

00:09:01.710 --> 00:09:03.810
with citizens not only more strongly

00:09:03.810 --> 00:09:05.400
identifying with one party

00:09:05.400 --> 00:09:08.410
but increasingly vilifying
members of the other party

00:09:08.410 --> 00:09:11.090
is immoral or unpatriotic.

00:09:11.090 --> 00:09:14.130
To put this in a broader
historical context

00:09:14.130 --> 00:09:16.670
we might ask, do these partisan divisions

00:09:16.670 --> 00:09:19.380
suggest that American national identity

00:09:19.380 --> 00:09:21.400
was fractured beyond repair

00:09:21.400 --> 00:09:24.450
or that a party realignment was underway?

00:09:24.450 --> 00:09:27.740
How does this period
compare with the 1850s

00:09:27.740 --> 00:09:29.970
when the beliefs of the
north and south diverged

00:09:29.970 --> 00:09:32.870
sharply over the institution of slavery?

00:09:32.870 --> 00:09:34.780
These are very difficult question

00:09:34.780 --> 00:09:37.170
and I'm glad we're here
in the 23rd century

00:09:37.170 --> 00:09:38.900
enjoying the benefits of hindsight

00:09:38.900 --> 00:09:42.753
to assess that strange
period from 1980 to 2020.


Worked example: Using formal charges to evaluate nonequivalent resonance structures

WEBVTT
Kind: captions
Language: en

00:00:01.322 --> 00:00:02.229
- [Instructor] We're told

00:00:02.229 --> 00:00:04.242
that three possible resonance structures

00:00:04.242 --> 00:00:08.060
for the thiocyanate ion are shown below.

00:00:08.060 --> 00:00:09.213
All right, there we have them.

00:00:10.092 --> 00:00:11.673
Based on formal charges,

00:00:12.579 --> 00:00:15.531
which of the three
structures contributes most

00:00:15.531 --> 00:00:19.183
to the resonance hybrid of thiocyanate?

00:00:20.402 --> 00:00:22.470
And they have given us
some extra information.

00:00:22.470 --> 00:00:24.925
They've given us the various elements

00:00:24.925 --> 00:00:26.686
in these resonance structures,

00:00:26.686 --> 00:00:30.055
and they've told us their
Pauling scale electronegativity,

00:00:30.055 --> 00:00:32.286
so maybe that is going to be useful

00:00:32.286 --> 00:00:34.000
for thinking about basing on the,

00:00:34.000 --> 00:00:35.457
based on the formal charges,

00:00:35.457 --> 00:00:37.020
which of the three
structures contributes most

00:00:37.020 --> 00:00:40.948
to the resonance hybrid of thiocyanate?

00:00:40.948 --> 00:00:42.723
So pause this video and see
if you can figure that out.

00:00:44.025 --> 00:00:45.960
All right, now let's work
through this together.

00:00:45.960 --> 00:00:50.180
So there's really two things
we want to optimize for

00:00:50.180 --> 00:00:51.476
when we're thinking about which

00:00:51.476 --> 00:00:53.110
of these resonance
structures contributes most

00:00:53.110 --> 00:00:54.523
to the resonance hybrid?

00:00:55.650 --> 00:00:58.770
One, we want to figure out
the resonance structures

00:00:59.647 --> 00:01:03.340
where individual atoms have formal charges

00:01:03.340 --> 00:01:05.932
as close to zero as possible.

00:01:05.932 --> 00:01:07.316
So let me write that down.

00:01:07.316 --> 00:01:08.149
Individual,

00:01:09.218 --> 00:01:10.680
individual atoms

00:01:12.150 --> 00:01:16.480
have formal charge

00:01:19.014 --> 00:01:20.613
as close to zero as possible.

00:01:21.464 --> 00:01:25.082
As close to zero as possible.

00:01:25.082 --> 00:01:27.044
We're not talking about the
charge of the entire ion.

00:01:27.044 --> 00:01:29.723
We're talking about individual
atoms' formal charges,

00:01:30.659 --> 00:01:31.959
close to zero as possible.

00:01:33.198 --> 00:01:35.198
And then the electronegativity is useful

00:01:36.098 --> 00:01:38.241
because we also want to see

00:01:38.241 --> 00:01:40.669
if there's any negative formal charge

00:01:40.669 --> 00:01:42.467
on an individual atom

00:01:42.467 --> 00:01:45.204
that ideally, that would be
on the most electronegative

00:01:45.204 --> 00:01:46.755
of the atoms.

00:01:46.755 --> 00:01:49.300
So any formal charge,

00:01:49.300 --> 00:01:50.133
so once again, we're not
talking about the charge

00:01:50.133 --> 00:01:51.343
of the entire ion.

00:01:52.258 --> 00:01:53.283
Any formal charge,

00:01:55.230 --> 00:01:56.886
any negative,

00:01:56.886 --> 00:01:58.450
any negative formal charge

00:01:58.450 --> 00:02:00.183
on individual atom,

00:02:01.207 --> 00:02:03.493
individual atom,

00:02:05.762 --> 00:02:07.570
ideally, ideally

00:02:09.038 --> 00:02:11.113
on most electronegative ones,

00:02:12.204 --> 00:02:13.604
or most electronegative one.

00:02:14.456 --> 00:02:15.789
Electronegative.

00:02:17.080 --> 00:02:19.959
All right, now with these two principles,

00:02:19.959 --> 00:02:22.980
let's figure out which of
these resonance structures

00:02:23.923 --> 00:02:26.263
get closest to these ideals.

00:02:26.263 --> 00:02:27.183
So to do that,

00:02:27.183 --> 00:02:29.288
let's just calculate the formal charges

00:02:29.288 --> 00:02:31.519
in each of these resonance structures.

00:02:31.519 --> 00:02:35.009
So the way that we do that is
for each of these elements,

00:02:35.009 --> 00:02:37.742
if you had just a free atom
of it that was neutral,

00:02:37.742 --> 00:02:39.852
how many valence electrons would it have?

00:02:39.852 --> 00:02:42.318
And actually, let me make
another column right over here,

00:02:42.318 --> 00:02:44.459
which is just the valence electrons.

00:02:44.459 --> 00:02:46.623
You can look it up on a
periodic table of elements

00:02:46.623 --> 00:02:47.595
or you might already know

00:02:47.595 --> 00:02:51.393
that carbon has four valence
electrons, six total,

00:02:51.393 --> 00:02:52.813
but four in that second shell.

00:02:53.786 --> 00:02:56.751
Nitrogen has five valence electrons,

00:02:56.751 --> 00:02:58.180
a neutral nitrogen,

00:02:58.180 --> 00:02:59.637
seven overall electrons,

00:02:59.637 --> 00:03:01.905
but it has five in its outer shell,

00:03:01.905 --> 00:03:03.893
and sulfur has six valence electrons.

00:03:05.034 --> 00:03:07.375
And the way that we
calculate formal charge

00:03:07.375 --> 00:03:09.120
of the individual atoms

00:03:09.120 --> 00:03:11.481
in each of these resonance structures

00:03:11.481 --> 00:03:12.314
is we say, all right,

00:03:12.314 --> 00:03:15.329
how many valence electrons
would say, sulfur,

00:03:15.329 --> 00:03:18.033
a neutral, free sulfur
atom typically have?

00:03:18.033 --> 00:03:20.207
And we know that that is six.

00:03:20.207 --> 00:03:21.184
And then we say, well,

00:03:21.184 --> 00:03:23.210
how many outer electrons are hanging out

00:03:23.210 --> 00:03:26.073
around the sulfur in
this resonance structure?

00:03:27.191 --> 00:03:29.152
And the outer electrons that we see here,

00:03:29.152 --> 00:03:30.952
it's really from this Lewis diagram,

00:03:31.799 --> 00:03:33.783
we can see one, two,

00:03:34.809 --> 00:03:36.663
three, four, five.

00:03:37.721 --> 00:03:41.534
So five electrons versus
six valence electrons

00:03:41.534 --> 00:03:43.723
in a typically neutral sulfur free atom,

00:03:44.705 --> 00:03:46.142
and so it's one less electron.

00:03:46.142 --> 00:03:49.454
So you would expect a plus
one formal charge here.

00:03:49.454 --> 00:03:50.550
Another way you could think
about it is typically,

00:03:50.550 --> 00:03:52.833
six valence electrons and,

00:03:53.704 --> 00:03:55.880
but we are only seeing five hanging out

00:03:56.801 --> 00:03:58.800
in this Lewis structure,

00:03:58.800 --> 00:04:01.286
so that's where we get our plus one from.

00:04:01.286 --> 00:04:04.688
Now we can do the same
exercise for the carbon here.

00:04:04.688 --> 00:04:07.874
Carbon typically has
four valence electrons

00:04:07.874 --> 00:04:08.825
when it's neutral,

00:04:08.825 --> 00:04:10.251
and this Lewis structure,

00:04:10.251 --> 00:04:11.526
in this resonance structure,

00:04:11.526 --> 00:04:15.110
we can see that four outer
electrons are hanging out,

00:04:15.110 --> 00:04:16.605
the same as you would expect

00:04:16.605 --> 00:04:18.658
for a neutral carbon atom.

00:04:18.658 --> 00:04:21.508
And so four minus four, you
have zero formal charge here.

00:04:22.682 --> 00:04:24.144
And then for the nitrogen,

00:04:24.144 --> 00:04:26.667
we have one, two, three,

00:04:26.667 --> 00:04:28.703
four, five, six, seven.

00:04:29.666 --> 00:04:31.820
We can say outer electrons hanging out.

00:04:31.820 --> 00:04:34.983
Neutral nitrogen would have
five valence electrons,

00:04:36.061 --> 00:04:37.300
so five valence electrons,

00:04:37.300 --> 00:04:38.828
we have two more than that.

00:04:38.828 --> 00:04:41.135
Five minus seven is negative two.

00:04:41.135 --> 00:04:44.483
So since we have two more
outer electrons hanging out

00:04:44.483 --> 00:04:46.986
than we would typically
have for a neutral nitrogen,

00:04:46.986 --> 00:04:49.363
we have a negative two formal charge.

00:04:50.371 --> 00:04:52.743
Now let's go to this
resonance structure here.

00:04:52.743 --> 00:04:53.803
So same idea.

00:04:54.760 --> 00:04:56.960
Here, we have one, two, three,

00:04:56.960 --> 00:05:01.623
four, five, six outer electrons
hanging out, the sulfur.

00:05:02.628 --> 00:05:06.202
Now that's the same as a neutral
sulfur valence electrons.

00:05:06.202 --> 00:05:07.943
So here, we have no formal charge.

00:05:08.944 --> 00:05:09.813
You could think about it,

00:05:09.813 --> 00:05:11.363
six minus six is equal to zero.

00:05:12.220 --> 00:05:16.776
Carbon, we have four outer
electrons hanging around

00:05:16.776 --> 00:05:17.976
from this Lewis diagram,

00:05:19.378 --> 00:05:21.160
and that's typical of
the valence electrons

00:05:21.160 --> 00:05:22.253
of a neutral carbon,

00:05:23.141 --> 00:05:25.433
so once again, four minus four,

00:05:25.433 --> 00:05:27.774
we have no formal charge there,

00:05:27.774 --> 00:05:29.827
and then we move onto the nitrogen.

00:05:29.827 --> 00:05:32.470
We have one, two, three,

00:05:32.470 --> 00:05:35.973
four, five, six outer
electrons hanging out.

00:05:37.098 --> 00:05:38.848
Nitrogen would typically have five.

00:05:39.724 --> 00:05:43.103
Five minus six, we have one
extra electron hanging out,

00:05:44.043 --> 00:05:46.655
which gives us a negative
one formal charge,

00:05:46.655 --> 00:05:49.857
the nitrogen right over there
in this resonance structure,

00:05:49.857 --> 00:05:52.000
and then last, but not least,

00:05:52.000 --> 00:05:53.692
in this resonance structure,

00:05:53.692 --> 00:05:56.530
we have one, two, three, four, five,

00:05:56.530 --> 00:05:59.941
six, seven electrons hanging around,

00:05:59.941 --> 00:06:02.241
outer electrons hanging
out around the sulfur.

00:06:03.249 --> 00:06:05.716
Neutral sulfur would have
six valence electrons.

00:06:05.716 --> 00:06:07.133
Six minus this seven,

00:06:08.036 --> 00:06:09.763
we have one extra electron.

00:06:09.763 --> 00:06:12.363
That's what gives us this
negative one formal charge

00:06:13.441 --> 00:06:15.975
for the sulfur in that
resonance structure.

00:06:15.975 --> 00:06:19.173
The carbon is still
having four hanging out,

00:06:20.350 --> 00:06:21.698
which is typical of carbon

00:06:21.698 --> 00:06:23.283
and neutral carbon's valence electrons,

00:06:23.283 --> 00:06:25.138
so no formal charge there,

00:06:25.138 --> 00:06:28.420
and then the nitrogen has one, two, three,

00:06:28.420 --> 00:06:31.563
four, five outer electrons hanging out,

00:06:32.572 --> 00:06:33.472
which is equivalent

00:06:33.472 --> 00:06:36.363
to a neutral nitrogen's valence electrons,

00:06:37.306 --> 00:06:38.999
and so five minus five,

00:06:38.999 --> 00:06:40.854
you have no formal charge.

00:06:40.854 --> 00:06:41.752
So there you have it.

00:06:41.752 --> 00:06:43.644
We've looked at the formal
charges on all of these,

00:06:43.644 --> 00:06:45.123
and now let's look at these ideals.

00:06:46.036 --> 00:06:48.681
So individual atoms have
formal charges close

00:06:48.681 --> 00:06:49.681
to zero as possible.

00:06:50.827 --> 00:06:52.786
In this first resonance structure,

00:06:52.786 --> 00:06:54.240
we have two individual atoms

00:06:55.127 --> 00:06:57.010
whose formal charges are
not zero, and in fact,

00:06:57.010 --> 00:06:59.103
nitrogen is quite far from zero,

00:07:00.096 --> 00:07:02.359
while in these other two
resonance structures,

00:07:02.359 --> 00:07:05.623
we only have one atom whose
formal charge is not zero.

00:07:06.595 --> 00:07:09.129
So I'm liking, just based
on this first principle,

00:07:09.129 --> 00:07:11.976
I'm liking these second
two resonance structures

00:07:11.976 --> 00:07:13.690
as contributing more
to the resonance hybrid

00:07:13.690 --> 00:07:14.983
than this first one.

00:07:15.915 --> 00:07:17.403
So I will rule that one out,

00:07:18.503 --> 00:07:20.345
and then if we had to
pick between these two,

00:07:20.345 --> 00:07:21.941
we could go to the second principle.

00:07:21.941 --> 00:07:24.755
Any negative formal charge
on an individual atom,

00:07:24.755 --> 00:07:26.083
ideally on the most electronegative.

00:07:27.367 --> 00:07:29.126
So in this resonance structure here,

00:07:29.126 --> 00:07:30.983
I guess the second resonance structure,

00:07:30.983 --> 00:07:33.376
the negative formal charge is on nitrogen.

00:07:33.376 --> 00:07:34.347
While on this third one,

00:07:34.347 --> 00:07:36.709
the negative formal charge is on sulfur.

00:07:36.709 --> 00:07:38.826
And we can see from this table

00:07:38.826 --> 00:07:41.653
that nitrogen is more
electronegative than sulfur.

00:07:42.645 --> 00:07:45.005
So it's in the second resonance structure,

00:07:45.005 --> 00:07:48.030
you have the negative
formal charge on an atom

00:07:48.030 --> 00:07:50.784
that is more electronegative than nitrogen

00:07:50.784 --> 00:07:53.448
than in this third resonance structure,

00:07:53.448 --> 00:07:54.548
and so this is the one

00:07:55.553 --> 00:07:57.160
that I believe contributes most

00:07:57.160 --> 00:08:00.611
to the resonance hybrid of thiocyanate

00:08:00.611 --> 00:08:01.711
for these two reasons.


Representing alloys using particulate models

WEBVTT
Kind: captions
Language: en

00:00:00.230 --> 00:00:02.620
- [Instructor] In many videos
we have already talked about

00:00:02.620 --> 00:00:05.410
metals and metallic bonds.

00:00:05.410 --> 00:00:08.210
And in this video we're going
to dig a little bit deeper

00:00:08.210 --> 00:00:11.720
and in particular, we're
going to talk about alloys,

00:00:11.720 --> 00:00:13.850
which are mixtures of elements,

00:00:13.850 --> 00:00:17.600
but still have metallic properties.

00:00:17.600 --> 00:00:20.490
So first of all, what
are metallic properties?

00:00:20.490 --> 00:00:23.170
Well those tend to be
things like they're shiny,

00:00:23.170 --> 00:00:24.130
they reflect light.

00:00:24.130 --> 00:00:27.040
This is actually a pure
iron sample right over here.

00:00:27.040 --> 00:00:28.980
You can see that it reflects light.

00:00:28.980 --> 00:00:30.440
It tends to be malleable,

00:00:30.440 --> 00:00:32.810
which means you can bend
it without breaking it.

00:00:32.810 --> 00:00:35.610
And it tends to conduct electricity.

00:00:35.610 --> 00:00:37.560
And alloys are when you can mix

00:00:37.560 --> 00:00:39.040
multiple elements together

00:00:39.040 --> 00:00:41.980
and still have most of these properties.

00:00:41.980 --> 00:00:45.460
And just as a review of where
these properties come from,

00:00:45.460 --> 00:00:47.910
we can imagine metallic bonds.

00:00:47.910 --> 00:00:50.010
And there's a whole video on this,

00:00:50.010 --> 00:00:51.730
but in metallic bonds, let's say we were

00:00:51.730 --> 00:00:54.630
to take a bunch of iron and
you can see right over here,

00:00:54.630 --> 00:00:58.250
iron, Fe, it is a transition metal.

00:00:58.250 --> 00:01:00.310
And what happens with metals is,

00:01:00.310 --> 00:01:02.330
is when they form bonds with each other,

00:01:02.330 --> 00:01:03.760
they're valence electrons,

00:01:03.760 --> 00:01:06.380
because each of the atoms
aren't that electronegative,

00:01:06.380 --> 00:01:07.910
they don't want to hog the electrons.

00:01:07.910 --> 00:01:09.720
They don't want them, just for themselves.

00:01:09.720 --> 00:01:11.900
They're willing to share
their valence electrons

00:01:11.900 --> 00:01:14.950
into a bit of a communal
pool of electrons.

00:01:14.950 --> 00:01:17.950
And so even though you
have a bunch of neutral,

00:01:17.950 --> 00:01:21.080
let's say iron atoms, you
could actually view them

00:01:21.080 --> 00:01:26.080
as positively charged ions
in a sea of electrons.

00:01:29.070 --> 00:01:32.850
And so you have a bunch of electrons here.

00:01:32.850 --> 00:01:35.010
And where did these electrons come from?

00:01:35.010 --> 00:01:36.830
Well these are the valence electrons

00:01:36.830 --> 00:01:40.880
from the neutral atoms that
get contributed to this sea.

00:01:40.880 --> 00:01:42.630
And this is why most metals are good

00:01:42.630 --> 00:01:44.210
at conducting electricity.

00:01:44.210 --> 00:01:46.463
This is why they are malleable.

00:01:47.613 --> 00:01:48.446
And depending on the metal,

00:01:48.446 --> 00:01:51.180
if you're talking about a Group one metal,

00:01:51.180 --> 00:01:52.570
you could imagine that the charge

00:01:52.570 --> 00:01:55.990
of these ions right over
here would be a plus one.

00:01:55.990 --> 00:01:57.810
But if we're talking
about a Group two metal

00:01:57.810 --> 00:02:01.380
or a transition metal, they
have more valence electrons

00:02:01.380 --> 00:02:03.700
that they might be able to
contribute to this pool.

00:02:03.700 --> 00:02:05.480
And so if you're thinking
about these ions,

00:02:05.480 --> 00:02:07.460
they can even have a positive two charge

00:02:07.460 --> 00:02:09.780
or a positive three charge.

00:02:09.780 --> 00:02:13.160
But as promised in this
video we're gonna talk about

00:02:13.160 --> 00:02:15.190
the notion of alloys.

00:02:15.190 --> 00:02:17.720
And we're going to do
these particulate diagrams

00:02:17.720 --> 00:02:19.240
that we have seen in other videos.

00:02:19.240 --> 00:02:20.940
And in the particulate diagrams,

00:02:20.940 --> 00:02:24.310
we're not going to show
this sea of electrons,

00:02:24.310 --> 00:02:25.840
but they're going to help us visualize

00:02:25.840 --> 00:02:27.840
the structure of the alloys.

00:02:27.840 --> 00:02:29.690
So let's imagine what
iron could look like.

00:02:29.690 --> 00:02:32.030
And we're just going to look
at a two-dimensional slice

00:02:32.030 --> 00:02:35.270
of a solid of iron,
where all the iron atoms

00:02:35.270 --> 00:02:36.680
have formed metallic bonds.

00:02:36.680 --> 00:02:37.850
And as I said, we're not going

00:02:37.850 --> 00:02:39.620
to draw this sea of electrons,

00:02:39.620 --> 00:02:42.230
but they might form a
pretty regular structure,

00:02:42.230 --> 00:02:43.960
something like this.

00:02:43.960 --> 00:02:48.130
And so each of these circles
represent an iron atom.

00:02:48.130 --> 00:02:51.150
But as promised, this
video is about alloys.

00:02:51.150 --> 00:02:53.840
So let's imagine what
steel might look like.

00:02:53.840 --> 00:02:57.090
This is a steel blade and
steel is a bunch of iron,

00:02:57.090 --> 00:03:01.330
so once again, we can
visualize each of these

00:03:01.330 --> 00:03:05.140
as an iron atom, but
mixed in with that iron

00:03:05.140 --> 00:03:07.090
is a little bit of carbon.

00:03:07.090 --> 00:03:09.190
And when you look at the
periodic table of elements

00:03:09.190 --> 00:03:12.380
you can see that carbon
is a good bit higher

00:03:12.380 --> 00:03:13.510
on the periodic table of elements

00:03:13.510 --> 00:03:16.080
and to the right of iron.

00:03:16.080 --> 00:03:19.300
Neutral iron has 26
protons and 26 electrons,

00:03:19.300 --> 00:03:22.720
and neutral carbon only has
six protons and six electrons.

00:03:22.720 --> 00:03:25.490
The valence electrons in carbon
are in their second shell.

00:03:25.490 --> 00:03:29.050
The valence electrons of
iron are in the fourth shell.

00:03:29.050 --> 00:03:32.000
So carbon is a good bit smaller.

00:03:32.000 --> 00:03:34.570
And so, when you mix that carbon in,

00:03:34.570 --> 00:03:35.910
because it is smaller,

00:03:35.910 --> 00:03:39.130
it's able to fit in the
gaps between the irons.

00:03:39.130 --> 00:03:41.380
So you might have, I'll
draw this right here.

00:03:41.380 --> 00:03:43.240
You might have a little
bit of carbon there.

00:03:43.240 --> 00:03:45.703
You might have a little
bit of carbon there.

00:03:46.689 --> 00:03:48.230
You might have a little
bit of carbon there.

00:03:48.230 --> 00:03:50.400
And so when you form an alloy,

00:03:50.400 --> 00:03:53.690
where one atom has a larger radius

00:03:53.690 --> 00:03:56.100
or a significantly larger
radius than the other,

00:03:56.100 --> 00:03:58.297
you tend to form things like this,

00:03:58.297 --> 00:04:00.297
which are known as interstitial alloys,

00:04:04.160 --> 00:04:09.050
and basic carbon steel
is a good example of it.

00:04:09.050 --> 00:04:11.730
Now you have other situations
where you have alloys

00:04:11.730 --> 00:04:14.610
between atoms of similar size.

00:04:14.610 --> 00:04:17.550
And this right over here, this is a brass,

00:04:17.550 --> 00:04:19.460
I don't know if this is
a clock or an astrolabe,

00:04:19.460 --> 00:04:20.870
or something like this,

00:04:20.870 --> 00:04:25.870
but brass is made up of
a mix of copper and zinc.

00:04:27.040 --> 00:04:28.660
And so when you have an alloy like this,

00:04:28.660 --> 00:04:32.010
that's between atoms of similar radius,

00:04:32.010 --> 00:04:34.480
this is called a substitutional alloy.

00:04:34.480 --> 00:04:36.560
You can imagine that some of the copper

00:04:36.560 --> 00:04:39.390
has been substituted with zinc.

00:04:39.390 --> 00:04:42.860
So this is substitutional alloy.

00:04:42.860 --> 00:04:44.880
Now the last thing you
might be wondering about is

00:04:44.880 --> 00:04:46.870
can you have a combination of both?

00:04:46.870 --> 00:04:48.910
And you indeed can.

00:04:48.910 --> 00:04:50.270
This is over here are panels

00:04:50.270 --> 00:04:52.290
on the International Space Station,

00:04:52.290 --> 00:04:54.160
and it's made out of stainless steel.

00:04:54.160 --> 00:04:56.840
You're likely to have stainless
steel in your kitchen.

00:04:56.840 --> 00:04:59.510
And stainless steel, you could view it as

00:04:59.510 --> 00:05:03.590
it's basic steel but instead
of just iron and carbon,

00:05:03.590 --> 00:05:06.730
It also has a little bit
of chromium mixed in.

00:05:06.730 --> 00:05:09.690
And so we can visualize this.

00:05:09.690 --> 00:05:11.760
If this is stainless steel,

00:05:11.760 --> 00:05:15.360
maybe the blue ones, we say are iron,

00:05:15.360 --> 00:05:16.990
but it has a little bit of chromium.

00:05:16.990 --> 00:05:18.550
I'll do that with red.

00:05:18.550 --> 00:05:21.570
Chromium has a similar radius to iron.

00:05:21.570 --> 00:05:24.170
It's not exactly the
same, but it is close.

00:05:24.170 --> 00:05:26.210
So maybe a little chromium there,

00:05:26.210 --> 00:05:29.340
a little bit of chromium right over there,

00:05:29.340 --> 00:05:33.367
a little bit of chromium right over there.

00:05:33.367 --> 00:05:34.200
And if it was just iron chromium

00:05:34.200 --> 00:05:35.530
we would call it substitutional,

00:05:35.530 --> 00:05:39.210
but it also has carbon and
carbon has a smaller radius.

00:05:39.210 --> 00:05:41.110
So maybe a little bit of carbon fitting

00:05:41.110 --> 00:05:43.940
in the gaps between
the larger atoms there.

00:05:43.940 --> 00:05:45.490
A little bit of carbon there.

00:05:45.490 --> 00:05:47.960
A little bit of carbon right over here.

00:05:47.960 --> 00:05:51.170
And so this is an example of an alloy,

00:05:51.170 --> 00:05:55.340
that is both interstitial
and substitutional.

00:05:55.340 --> 00:05:56.610
Now one final question, you're like okay

00:05:56.610 --> 00:05:57.760
this is all interesting,

00:05:57.760 --> 00:06:02.380
but why have we decided to put
things like carbon in iron?

00:06:02.380 --> 00:06:04.390
Well it turns out that even by putting

00:06:04.390 --> 00:06:06.220
a little bit of carbon in or mixing in

00:06:06.220 --> 00:06:09.620
with other metals, you're able to change

00:06:09.620 --> 00:06:13.100
the properties and for
example, steel as an alloy,

00:06:13.100 --> 00:06:17.290
is much stronger than iron, by itself.

00:06:17.290 --> 00:06:20.210
And stainless steel once
you mix that chromium in,

00:06:20.210 --> 00:06:24.270
it's much more resistant to
corrosion, than basic steel.

00:06:24.270 --> 00:06:25.510
So I'll leave you there.

00:06:25.510 --> 00:06:29.293
You just learned a little bit
more about metals and alloys.


Representing ionic solids using particulate models

WEBVTT
Kind: captions
Language: en

00:00:00.290 --> 00:00:01.770
- [Instructor] In this video,
we're gonna think about

00:00:01.770 --> 00:00:04.102
how ions will arrange themselves

00:00:04.102 --> 00:00:07.120
when they form solid crystals.

00:00:07.120 --> 00:00:10.310
When they form these lattice structures.

00:00:10.310 --> 00:00:12.380
So just in very broad brush terms,

00:00:12.380 --> 00:00:17.030
let's say that we have a
bunch of this white cation,

00:00:17.030 --> 00:00:22.030
and we have a bunch of this
green, or this blue-green anion.

00:00:22.890 --> 00:00:25.050
So let's say they're
in a one-to-one ratio.

00:00:25.050 --> 00:00:26.846
How will that look?

00:00:26.846 --> 00:00:28.320
How will the solid look,
if you were to take

00:00:28.320 --> 00:00:30.080
a two-dimensional slice of it?

00:00:30.080 --> 00:00:31.890
To imagine that, we can draw

00:00:31.890 --> 00:00:34.010
what we could call particulate models.

00:00:34.010 --> 00:00:36.930
We're just imagining a
two-dimensional slice of the solid,

00:00:36.930 --> 00:00:40.250
and we're just drawing
these ions as particles.

00:00:40.250 --> 00:00:42.060
Would it look something like this,

00:00:42.060 --> 00:00:47.060
where maybe the positive
ion is all on one side,

00:00:49.240 --> 00:00:54.110
and then the negative
ion is on the other side,

00:00:54.110 --> 00:00:57.190
is on the bottom if we
were to take a slice?

00:00:57.190 --> 00:01:01.410
Would something like this make sense?

00:01:01.410 --> 00:01:03.000
Or maybe it's random.

00:01:03.000 --> 00:01:05.460
Maybe you have a positive there,

00:01:05.460 --> 00:01:09.810
and then you have some
negatives right over there.

00:01:09.810 --> 00:01:14.130
And then, maybe you have
a positive and a positive,

00:01:14.130 --> 00:01:16.760
and then a positive right over there.

00:01:16.760 --> 00:01:21.054
And then maybe you have some
negatives right over there.

00:01:21.054 --> 00:01:23.810
Would this be a reasonable configuration,

00:01:23.810 --> 00:01:26.493
as they form these ionic bonds?

00:01:27.400 --> 00:01:29.340
Well, when we think about Coulomb forces,

00:01:29.340 --> 00:01:31.840
we know that like charges repel each other

00:01:31.840 --> 00:01:35.180
and unlike charges, or opposite
charges, attract each other.

00:01:35.180 --> 00:01:37.820
And so, when these ionic solids form,

00:01:37.820 --> 00:01:39.810
they're unlikely to form in this way,

00:01:39.810 --> 00:01:41.830
or even in this way,
because they're gonna form

00:01:41.830 --> 00:01:44.950
in a way that maximizes
the attractive forces

00:01:44.950 --> 00:01:48.580
and minimizes the repulsive,
the repelling forces.

00:01:48.580 --> 00:01:50.820
And so what would be an
arrangement that does that?

00:01:50.820 --> 00:01:53.210
Pause this video and think about it.

00:01:53.210 --> 00:01:54.440
Well, all the positive charges

00:01:54.440 --> 00:01:55.920
are gonna try to get as close as possible

00:01:55.920 --> 00:01:57.940
to the negative charges
and as far as possible

00:01:57.940 --> 00:01:59.500
from other positive charges.

00:01:59.500 --> 00:02:01.113
And the same thing is going to
be true of negative charges.

00:02:01.113 --> 00:02:02.880
They're gonna try to get as far away

00:02:02.880 --> 00:02:04.770
from other negative charges as possible,

00:02:04.770 --> 00:02:07.780
and as close to other
positive charges as possible.

00:02:07.780 --> 00:02:10.350
So the arrangement that
you are likely to see

00:02:10.350 --> 00:02:13.090
is going to look something more
like a checkerboard pattern.

00:02:13.090 --> 00:02:14.990
So it may be a positive there,

00:02:14.990 --> 00:02:18.610
a positive there, a positive there,

00:02:18.610 --> 00:02:21.020
a positive there, and a positive there.

00:02:21.020 --> 00:02:22.000
These are all the same ion,

00:02:22.000 --> 00:02:24.340
I'm not drawing it perfectly,
they'd be the same size.

00:02:24.340 --> 00:02:27.010
And when you do these
two-dimensional representations,

00:02:27.010 --> 00:02:28.000
these particulate models,

00:02:28.000 --> 00:02:29.480
it is important to get the size right,

00:02:29.480 --> 00:02:31.280
'cause we're gonna think
about that in a second.

00:02:31.280 --> 00:02:34.720
And then the negative
charges would be in between.

00:02:34.720 --> 00:02:35.553
So notice.

00:02:35.553 --> 00:02:37.660
In this configuration,

00:02:37.660 --> 00:02:39.620
every negative is surrounded by positives,

00:02:39.620 --> 00:02:41.480
and every positive is
surrounded by negatives.

00:02:41.480 --> 00:02:43.671
So it's maximizing the attractive forces

00:02:43.671 --> 00:02:46.630
and it's minimizing the repulsive forces.

00:02:46.630 --> 00:02:48.550
And if you were to think
about it in three dimensions,

00:02:48.550 --> 00:02:49.790
you would have a lattice structure

00:02:49.790 --> 00:02:51.200
that looks something like that.

00:02:51.200 --> 00:02:54.580
And we have seen this in other videos.

00:02:54.580 --> 00:02:56.970
Now another interesting
thing to think about

00:02:56.970 --> 00:03:01.970
is the size of the ions
that form that ionic solid.

00:03:02.150 --> 00:03:07.150
Let's say we wanted to
deal with rubidium bromide.

00:03:07.580 --> 00:03:10.080
Rubidium bromide.

00:03:10.080 --> 00:03:12.180
What would this look
like if I were to draw it

00:03:12.180 --> 00:03:14.670
in a two-dimensional
particulate model like this,

00:03:14.670 --> 00:03:17.850
and I wanted to make the
size roughly comparable

00:03:17.850 --> 00:03:21.390
to what we would see between
the rubidium and the bromide?

00:03:21.390 --> 00:03:22.670
Pause this video and think about that,

00:03:22.670 --> 00:03:24.470
and I'll give you a little bit of a hint.

00:03:24.470 --> 00:03:25.740
It might be useful to look at

00:03:25.740 --> 00:03:27.543
this periodic table of elements.

00:03:29.020 --> 00:03:32.550
All right, if we were to
separate this out into its ions,

00:03:32.550 --> 00:03:36.757
it is a rubidium cation,
and a bromide anion.

00:03:41.486 --> 00:03:44.800
Now a rubidium cation,
it has lost an electron.

00:03:44.800 --> 00:03:46.980
So even though it still has 37 protons,

00:03:46.980 --> 00:03:51.240
its electron configuration now
looks like that of krypton.

00:03:51.240 --> 00:03:56.210
Now, the bromide anion, even
though it only has 35 protons,

00:03:56.210 --> 00:03:58.960
it's going to gain an electron
to become a bromide anion,

00:03:58.960 --> 00:04:02.060
and it also has an electron
configuration of krypton.

00:04:02.060 --> 00:04:06.030
So both of these have the
same number of electrons,

00:04:06.030 --> 00:04:09.970
but rubidium has two more
protons than bromide does.

00:04:09.970 --> 00:04:13.040
And so the rubidium is going to attract

00:04:13.040 --> 00:04:14.570
that outer shell of electrons,

00:04:14.570 --> 00:04:16.220
that fourth shell of electrons,

00:04:16.220 --> 00:04:20.210
more than the bromide nucleus is going to.

00:04:20.210 --> 00:04:23.130
And so, the rubidium in this example

00:04:23.130 --> 00:04:27.740
is going to be smaller than the bromide.

00:04:27.740 --> 00:04:31.990
And so if I were to draw
one of these diagrams,

00:04:31.990 --> 00:04:33.880
it would look something like this.

00:04:33.880 --> 00:04:36.990
Let me draw the bromide first.

00:04:36.990 --> 00:04:41.880
So I have a bromide anion, I
have another bromide anion,

00:04:41.880 --> 00:04:45.702
another bromide anion,
maybe I have a bromide anion

00:04:45.702 --> 00:04:49.480
right over here, bromide anion over there,

00:04:49.480 --> 00:04:51.180
maybe a few more.

00:04:51.180 --> 00:04:53.890
Make 'em a little bit, if I
was doing this with a computer,

00:04:53.890 --> 00:04:56.490
I would make them all the same size.

00:04:56.490 --> 00:04:59.000
So these are our bromide anions.

00:04:59.000 --> 00:05:02.250
And then your rubidium cations
would be a bit smaller.

00:05:02.250 --> 00:05:06.080
And so, our particulate
model right over here

00:05:06.080 --> 00:05:07.300
might look something like this.

00:05:07.300 --> 00:05:09.900
We wanna make it clear that the cation

00:05:09.900 --> 00:05:13.740
is a bit smaller than the anion.

00:05:13.740 --> 00:05:16.580
It would arrange, it would likely arrange

00:05:16.580 --> 00:05:18.400
in a pattern that looks like this.

00:05:18.400 --> 00:05:21.050
And notice, I am trying to make the sizes

00:05:24.040 --> 00:05:26.060
roughly accurate, to show that the cation

00:05:26.060 --> 00:05:28.580
is indeed smaller than the anion.

00:05:28.580 --> 00:05:30.290
Although it wouldn't be
dramatically smaller.

00:05:30.290 --> 00:05:31.830
Remember, they have the
same number of electrons.

00:05:31.830 --> 00:05:33.190
And they don't have that dramatically

00:05:33.190 --> 00:05:34.840
different number of protons.

00:05:34.840 --> 00:05:37.200
And this is just a very rough drawing.

00:05:37.200 --> 00:05:38.890
If they were dramatically different,

00:05:38.890 --> 00:05:41.693
you might show that in
the sizes on this diagram.


Lattice energy

WEBVTT
Kind: captions
Language: en

00:00:00.660 --> 00:00:02.220
- [Instructor] You may already be familiar

00:00:02.220 --> 00:00:04.740
with Coulomb's law,

00:00:04.740 --> 00:00:07.310
which is really the most important

00:00:07.310 --> 00:00:10.400
or underlying law behind
all of what we know

00:00:10.400 --> 00:00:12.690
about electrostatics

00:00:12.690 --> 00:00:16.730
and how things with charge
attract or repulse each other,

00:00:16.730 --> 00:00:19.160
but a simplified version of Coulomb's law

00:00:19.160 --> 00:00:23.200
is just that the force
between charged particles,

00:00:23.200 --> 00:00:24.940
the magnitude of the force

00:00:24.940 --> 00:00:29.940
is going to be proportional
to the product of the charges,

00:00:30.280 --> 00:00:31.900
so q one would be the charge

00:00:31.900 --> 00:00:33.320
of one of the charged particles.

00:00:33.320 --> 00:00:34.900
Maybe this is an ion.

00:00:34.900 --> 00:00:38.250
Q two would the charge
of the other particle.

00:00:38.250 --> 00:00:39.710
Maybe that's an ion,

00:00:39.710 --> 00:00:42.200
divided by r squared.

00:00:42.200 --> 00:00:43.570
And if we're talking about ions,

00:00:43.570 --> 00:00:47.840
r is going to be the distance
between their nuclei,

00:00:47.840 --> 00:00:49.460
and if the charges are different,

00:00:49.460 --> 00:00:50.940
it's going to be force of attraction.

00:00:50.940 --> 00:00:52.380
If the charges are the same,

00:00:52.380 --> 00:00:54.680
it's going to be a force of repulsion.

00:00:54.680 --> 00:00:59.680
And we can use Coulomb's law
to think about ionic compounds.

00:00:59.710 --> 00:01:03.430
So let's go with maybe the
most common ionic compound

00:01:03.430 --> 00:01:06.450
in our daily life, and that is table salt.

00:01:06.450 --> 00:01:09.010
Table salt is sodium chloride,

00:01:09.010 --> 00:01:12.230
so sodium chloride.

00:01:12.230 --> 00:01:14.810
We have talked about this in other videos.

00:01:14.810 --> 00:01:19.810
It is made up of
positively-charged sodium cations,

00:01:20.350 --> 00:01:23.040
so you have an Na plus,

00:01:23.040 --> 00:01:25.230
so sodium is a group one element.

00:01:25.230 --> 00:01:28.100
It's very easy to nab
an electron off of it

00:01:28.100 --> 00:01:29.830
and then it has a positive charge,

00:01:29.830 --> 00:01:33.220
and it's made up of a chloride anion,

00:01:33.220 --> 00:01:35.770
so Cl minus.

00:01:35.770 --> 00:01:39.130
Chloride is a group seven element.

00:01:39.130 --> 00:01:41.730
It really wants to get that extra electron

00:01:41.730 --> 00:01:45.100
to have eight valence electrons
in its outermost shell,

00:01:45.100 --> 00:01:48.570
and so it's very likely
to grab an electron maybe

00:01:48.570 --> 00:01:49.530
from a sodium,

00:01:49.530 --> 00:01:52.610
and so these two characters
are going to be attracted

00:01:52.610 --> 00:01:53.443
to each other.

00:01:53.443 --> 00:01:55.400
Notice, they have opposite charges.

00:01:55.400 --> 00:01:59.830
And when you have a bunch of
sodium and chloride together,

00:01:59.830 --> 00:02:03.510
you'll have a structure that
looks something like this.

00:02:03.510 --> 00:02:08.340
And in chemistry, we call this a lattice.

00:02:08.340 --> 00:02:09.910
Now in everyday language,

00:02:09.910 --> 00:02:12.580
you might associate things like lattices

00:02:12.580 --> 00:02:16.750
with kind of a crossing pattern like that,

00:02:16.750 --> 00:02:18.950
and in chemistry, when we're
talking about a lattice,

00:02:18.950 --> 00:02:21.470
we're talking about a
three-dimensional structure

00:02:21.470 --> 00:02:24.650
of atoms or three-dimensional
structure of ions

00:02:24.650 --> 00:02:26.840
that have a repeating pattern to them,

00:02:26.840 --> 00:02:27.840
and you can see that here,

00:02:27.840 --> 00:02:30.320
and in future videos,
we'll go into more detail

00:02:30.320 --> 00:02:32.010
onto lattice structures,

00:02:32.010 --> 00:02:34.870
but you can see in this picture,

00:02:34.870 --> 00:02:38.091
the purples are the sodium cations

00:02:38.091 --> 00:02:43.091
and the greens are the chloride anions.

00:02:43.142 --> 00:02:47.270
And the reason why the
sodium cations are so small,

00:02:47.270 --> 00:02:49.840
you can see that if you
look at the periodic table

00:02:49.840 --> 00:02:51.100
of elements here.

00:02:51.100 --> 00:02:52.920
We have said that as you go to the right,

00:02:52.920 --> 00:02:55.150
your radius decreases,

00:02:55.150 --> 00:02:57.900
but what's happening is when sodium loses

00:02:57.900 --> 00:02:59.830
that outermost electron,

00:02:59.830 --> 00:03:04.470
then its electrons have a noble
gas configuration of neon.

00:03:04.470 --> 00:03:07.850
So it really loses that
third shell, it gets smaller,

00:03:07.850 --> 00:03:09.090
and not only does it
lose that third shell,

00:03:09.090 --> 00:03:10.540
but it has 11 protons,

00:03:10.540 --> 00:03:12.880
so it's going to have a very strong pull

00:03:12.880 --> 00:03:15.880
on those electrons in that second shell.

00:03:15.880 --> 00:03:18.950
And similarly, chloride is
going to gain an electron

00:03:18.950 --> 00:03:22.270
so it's going to have a noble
gas configuration of argon.

00:03:22.270 --> 00:03:24.853
So it is going to be bigger.

00:03:25.700 --> 00:03:27.410
Now when we talked about covalent bonds,

00:03:27.410 --> 00:03:29.200
we talked about the bond energy,

00:03:29.200 --> 00:03:31.130
the energy needed to pull apart the atoms

00:03:31.130 --> 00:03:33.700
that were forming the covalent bonds.

00:03:33.700 --> 00:03:37.730
There's a similar notion
for ionic bonds like this

00:03:37.730 --> 00:03:39.560
and that is lattice energy,

00:03:39.560 --> 00:03:42.110
and that is energy necessary

00:03:42.110 --> 00:03:44.320
to pull the ions apart

00:03:44.320 --> 00:03:47.470
so that they are infinitely
far apart from each other,

00:03:47.470 --> 00:03:51.530
and lattice energy is usually measured

00:03:51.530 --> 00:03:54.090
in kilojoules per mole,

00:03:54.090 --> 00:03:57.540
which is also what we
measure bond energy in

00:03:57.540 --> 00:03:59.730
because they're really the same notion,

00:03:59.730 --> 00:04:00.750
except lattice energy,

00:04:00.750 --> 00:04:02.810
you're breaking up a lattice of ions,

00:04:02.810 --> 00:04:03.800
while in bond energy,

00:04:03.800 --> 00:04:06.770
you're normally talking
about covalent bonds.

00:04:06.770 --> 00:04:08.290
Now I want you to think about something.

00:04:08.290 --> 00:04:11.000
What's going to have a
higher lattice energy?

00:04:11.000 --> 00:04:13.740
Would it be sodium chloride,

00:04:13.740 --> 00:04:16.060
or let's pick something else.

00:04:16.060 --> 00:04:20.690
Let's say we had rubidium.

00:04:20.690 --> 00:04:23.600
Rubidium chloride,

00:04:23.600 --> 00:04:26.720
which is going to have
a higher lattice energy?

00:04:26.720 --> 00:04:28.820
What's going to take more energy

00:04:28.820 --> 00:04:31.180
to pull the ions apart?

00:04:31.180 --> 00:04:32.460
And I'll give you a hint

00:04:32.460 --> 00:04:35.083
with this periodic table of elements.

00:04:36.500 --> 00:04:38.640
All right, well, rubidium chloride,

00:04:38.640 --> 00:04:41.770
that's made up, instead
of a sodium cation,

00:04:41.770 --> 00:04:44.520
that's made up of a rubidium cation,

00:04:44.520 --> 00:04:47.010
so you have Rb plus, and of course,

00:04:47.010 --> 00:04:51.120
you have the chloride anion, Cl minus,

00:04:51.120 --> 00:04:52.780
and so what's the difference here?

00:04:52.780 --> 00:04:56.830
The anion is both, is
chloride in both cases,

00:04:56.830 --> 00:04:59.430
but when you look at
rubidium versus sodium,

00:04:59.430 --> 00:05:01.250
rubidium, when it loses an electron,

00:05:01.250 --> 00:05:03.280
it's going to have a noble gas structure,

00:05:03.280 --> 00:05:05.520
electron structure of krypton,

00:05:05.520 --> 00:05:07.860
while sodium, once it loses an electron,

00:05:07.860 --> 00:05:09.130
it's, its electron,

00:05:09.130 --> 00:05:12.810
its electron configuration
is going to look like neon.

00:05:12.810 --> 00:05:16.030
So the sodium cation is smaller,

00:05:16.030 --> 00:05:17.743
and what does that tell us?

00:05:18.640 --> 00:05:21.370
Well, if this one right over here,

00:05:21.370 --> 00:05:23.030
let me circle it like this.

00:05:23.030 --> 00:05:25.533
If this is smaller,

00:05:26.750 --> 00:05:28.830
and we have similar charges on top,

00:05:28.830 --> 00:05:31.930
you have a plus one and
a negative one on top,

00:05:31.930 --> 00:05:34.480
that's the charges between the two ions,

00:05:34.480 --> 00:05:37.730
but now you have a smaller
radius between the nuclei

00:05:37.730 --> 00:05:40.550
because sodium is smaller than rubidium.

00:05:40.550 --> 00:05:42.210
While the radius goes down,

00:05:42.210 --> 00:05:44.430
the force goes up,

00:05:44.430 --> 00:05:47.460
so you're going to have
stronger Coulomb forces

00:05:47.460 --> 00:05:49.510
in a lattice of sodium chloride

00:05:49.510 --> 00:05:51.970
than in a lattice of rubidium chloride.

00:05:51.970 --> 00:05:54.070
Because the force of
attraction is stronger,

00:05:54.070 --> 00:05:56.560
it's going to take more
energy to pull it apart.

00:05:56.560 --> 00:05:57.520
So because of that,

00:05:57.520 --> 00:05:59.690
you're going to have a higher,

00:05:59.690 --> 00:06:02.330
higher lattice energy.

00:06:02.330 --> 00:06:07.330
Lattice energy for sodium
chloride than rubidium chloride.

00:06:07.700 --> 00:06:09.410
Let's think about another ionic compound.

00:06:09.410 --> 00:06:14.410
Let's say we were to think
about magnesium fluoride, F two,

00:06:16.040 --> 00:06:20.340
and this is made up of a magnesium cation

00:06:20.340 --> 00:06:22.950
that has a positive two charge,

00:06:22.950 --> 00:06:27.950
so two plus, in a lattice with
a bunch of fluoride anions,

00:06:28.290 --> 00:06:31.660
so with a bunch of fluoride anions.

00:06:31.660 --> 00:06:33.280
So how would the lattice energy

00:06:33.280 --> 00:06:35.860
of magnesium fluoride compare

00:06:35.860 --> 00:06:38.080
to what we just saw up here?

00:06:38.080 --> 00:06:40.560
So magnesium has a larger charge

00:06:40.560 --> 00:06:42.860
than these cations up here,

00:06:42.860 --> 00:06:46.220
so if you viewed the charge
of magnesium as q one,

00:06:46.220 --> 00:06:49.090
you're going to have
something larger up there

00:06:49.090 --> 00:06:54.090
and that fluoride is a
smaller anion than chloride.

00:06:54.500 --> 00:06:57.850
We can see that if we
look at the periodic table

00:06:57.850 --> 00:06:59.150
of elements again.

00:06:59.150 --> 00:07:01.860
Florine is smaller than chlorine,

00:07:01.860 --> 00:07:04.150
and so even if you added an
electron to both of them,

00:07:04.150 --> 00:07:06.070
fluoride is still going to be smaller,

00:07:06.070 --> 00:07:09.950
and magnesium, when you take
two electrons off of it,

00:07:09.950 --> 00:07:11.770
it's going to have the
noble gas configure,

00:07:11.770 --> 00:07:14.020
electron configuration of neon,

00:07:14.020 --> 00:07:16.480
but it's going to pull even more on those,

00:07:16.480 --> 00:07:18.380
that, those second shell electrons

00:07:18.380 --> 00:07:22.520
because it has 12 protons
versus sodium only has 11.

00:07:22.520 --> 00:07:27.510
So what we see here is not only
does magnesium have a larger

00:07:27.510 --> 00:07:31.090
positive charge than
the sodium cation does,

00:07:31.090 --> 00:07:33.160
but it's going to be smaller.

00:07:33.160 --> 00:07:37.530
And the fluoride has a comparable
charge to the chloride,

00:07:37.530 --> 00:07:39.650
but it too is going to be smaller.

00:07:39.650 --> 00:07:42.160
So we have a larger charge on top,

00:07:42.160 --> 00:07:43.840
at least for the magnesium,

00:07:43.840 --> 00:07:47.110
and you have smaller radii for the bottom,

00:07:47.110 --> 00:07:48.510
so in magnesium fluoride,

00:07:48.510 --> 00:07:51.490
the Coulomb forces between
the ions and the lattice

00:07:51.490 --> 00:07:53.120
are even stronger,

00:07:53.120 --> 00:07:54.530
and so the lattice energy,

00:07:54.530 --> 00:07:56.480
the energy necessary to pull it apart,

00:07:56.480 --> 00:07:58.720
is going to be higher,

00:07:58.720 --> 00:08:00.780
so out of the three we just looked at,

00:08:00.780 --> 00:08:03.880
the highest lattice energy is
going to be magnesium fluoride

00:08:03.880 --> 00:08:06.490
followed by sodium chloride

00:08:06.490 --> 00:08:08.683
followed by rubidium chloride.


All in for Education with Khan Academy. Give today!

WEBVTT
Kind: captions
Language: en

00:00:00.780 --> 00:00:01.613
- Hi, everyone.

00:00:01.613 --> 00:00:04.330
Sal Khan here from the
not-for-profit Khan Academy

00:00:04.330 --> 00:00:06.980
and I just wanted to
announce our first-ever

00:00:06.980 --> 00:00:09.830
fundraising campaign on YouTube.

00:00:09.830 --> 00:00:12.780
As you know, we are a
not-for-profit organization

00:00:12.780 --> 00:00:14.740
and we are able to do the work we do

00:00:14.740 --> 00:00:17.890
because of donations
from folks like yourself,

00:00:17.890 --> 00:00:21.830
so we are trying to
raise $10000 on YouTube.

00:00:21.830 --> 00:00:25.940
Every dollar is equal to three
hours of learning some place

00:00:25.940 --> 00:00:26.860
on the planet.

00:00:26.860 --> 00:00:29.620
This is learning for you,
learning for your family,

00:00:29.620 --> 00:00:32.450
or learning for a young child
in some part of the world

00:00:32.450 --> 00:00:35.440
that might not have resources otherwise.

00:00:35.440 --> 00:00:38.450
So please donate by
clicking on the blue button

00:00:38.450 --> 00:00:40.380
which is either there or there,

00:00:40.380 --> 00:00:41.213
but you'll find it.

00:00:41.213 --> 00:00:43.500
There should be a blue
button around me someplace,

00:00:43.500 --> 00:00:46.010
and please support us on this mission

00:00:46.010 --> 00:00:48.250
of a free, world-class education

00:00:48.250 --> 00:00:49.950
for anyone, anywhere.

00:00:49.950 --> 00:00:50.783
Thank you.


Worked example: Interpreting potential energy curves of diatomic molecules

WEBVTT
Kind: captions
Language: en

00:00:00.320 --> 00:00:01.330
- [Instructor] In a previous video,

00:00:01.330 --> 00:00:04.450
we began to think about
potential energy as a function

00:00:04.450 --> 00:00:09.210
of internuclear distance
for diatomic molecules.

00:00:09.210 --> 00:00:11.190
What do I mean by diatomic molecules?

00:00:11.190 --> 00:00:13.957
Well, we looked at
molecular hydrogen, or H2,

00:00:14.953 --> 00:00:18.080
which is just two hydrogens
covalently bonded to each other.

00:00:18.080 --> 00:00:20.580
And at standard temperature and pressure,

00:00:20.580 --> 00:00:22.720
there, they would naturally,

00:00:22.720 --> 00:00:25.770
the distance between the two nuclei

00:00:26.948 --> 00:00:28.360
would be based on where there is

00:00:28.360 --> 00:00:30.200
the lowest potential energy.

00:00:30.200 --> 00:00:32.070
And if you were to squeeze them together,

00:00:32.070 --> 00:00:34.030
you would have to put
energy into the system

00:00:34.030 --> 00:00:35.650
and have a higher potential energy.

00:00:35.650 --> 00:00:36.980
Or if you were to pull them apart,

00:00:36.980 --> 00:00:38.660
you would have to put
energy into the system

00:00:38.660 --> 00:00:41.260
and have a higher potential energy.

00:00:41.260 --> 00:00:42.610
What I want to do in this video

00:00:42.610 --> 00:00:44.750
is do a little bit of a worked example.

00:00:44.750 --> 00:00:47.360
Over here, I have three potential energies

00:00:47.360 --> 00:00:50.200
as a function of
internuclear distance graphs.

00:00:50.200 --> 00:00:52.150
And what I'm going to tell you is

00:00:52.150 --> 00:00:55.900
one of these is molecular hydrogen,

00:00:55.900 --> 00:01:00.090
one of these is molecular
nitrogen or diatomic nitrogen, N2,

00:01:00.090 --> 00:01:02.980
and one of these is diatomic oxygen.

00:01:02.980 --> 00:01:05.500
And what I want you to think
about, pause this video,

00:01:05.500 --> 00:01:08.830
is which graph is the potential energy

00:01:08.830 --> 00:01:10.700
as a function of internuclear distance

00:01:10.700 --> 00:01:13.640
for each of these diatomic molecules.

00:01:13.640 --> 00:01:15.980
And I'll give you a hint.

00:01:15.980 --> 00:01:18.370
Look at the low point in potential energy.

00:01:18.370 --> 00:01:20.750
The low point in potential energy is

00:01:20.750 --> 00:01:22.610
what you would typically observe

00:01:22.610 --> 00:01:25.300
that diatomic molecule's
internuclear distance

00:01:25.300 --> 00:01:27.680
to be at standard
temperature and pressure.

00:01:27.680 --> 00:01:29.520
And this distance right over here is going

00:01:29.520 --> 00:01:33.200
to be a function of two things.

00:01:33.200 --> 00:01:35.050
It's going to be a function of

00:01:35.050 --> 00:01:37.620
how small the atoms actually are,

00:01:37.620 --> 00:01:39.650
how small their radii are.

00:01:39.650 --> 00:01:41.790
So smaller atoms are, in general,

00:01:41.790 --> 00:01:44.650
going to have a shorter
stable internuclear distance.

00:01:44.650 --> 00:01:47.540
But the other thing to think
about is the bond order

00:01:47.540 --> 00:01:49.280
between these atoms,

00:01:49.280 --> 00:01:51.130
and I'll give you a little bit of a hint.

00:01:51.130 --> 00:01:55.480
Diatomic hydrogen, you just
have a single covalent bond.

00:01:55.480 --> 00:01:59.380
For diatomic nitrogen,
it is a triple bond.

00:01:59.380 --> 00:02:03.600
And for diatomic oxygen,
it is a double bond.

00:02:03.600 --> 00:02:05.730
So the higher order the bond,

00:02:05.730 --> 00:02:09.150
that will also bring the
two atoms closer together,

00:02:09.150 --> 00:02:13.460
and it also makes it have
a higher bond energy,

00:02:13.460 --> 00:02:16.090
the energy required to separate the atoms.

00:02:16.090 --> 00:02:18.190
Remember, we talked about
it in the previous video.

00:02:18.190 --> 00:02:22.910
This right over here is the bond energy.

00:02:22.910 --> 00:02:25.410
And so with that said, pause the video,

00:02:25.410 --> 00:02:26.790
and try to figure it out.

00:02:26.790 --> 00:02:29.100
Which of these is the graphs of H2,

00:02:29.100 --> 00:02:30.983
which is N2, and which is O2?

00:02:32.110 --> 00:02:35.500
So let's first just think about
it in terms of bond energy.

00:02:35.500 --> 00:02:36.333
If you look at it,

00:02:36.333 --> 00:02:39.170
the single bond, double
bond, triple bond here,

00:02:39.170 --> 00:02:41.650
you would expect the
highest order bond here

00:02:41.650 --> 00:02:43.670
to have the highest bond energy,

00:02:43.670 --> 00:02:45.040
and the highest bond energy

00:02:45.040 --> 00:02:48.360
is this salmon-colored
one right over here.

00:02:48.360 --> 00:02:50.150
So just based on that,

00:02:50.150 --> 00:02:54.020
I would say that this is
a good candidate for N2.

00:02:54.020 --> 00:02:56.630
So this one right over here,

00:02:56.630 --> 00:03:00.493
this looks like diatomic nitrogen to me.

00:03:01.340 --> 00:03:04.740
Then the next highest bond energy,

00:03:04.740 --> 00:03:06.180
if you look at it carefully,

00:03:06.180 --> 00:03:09.540
it looks like this purple
one right over here.

00:03:09.540 --> 00:03:13.030
And so just based on bond order,

00:03:13.030 --> 00:03:16.260
I would say this is a
good candidate for O2.

00:03:16.260 --> 00:03:19.950
And then the lowest bond energy

00:03:19.950 --> 00:03:23.030
is this one right over here.

00:03:23.030 --> 00:03:25.810
And so just based on the bond order here,

00:03:25.810 --> 00:03:27.670
it's just a single covalent bond,

00:03:27.670 --> 00:03:31.810
this looks like a good
candidate for diatomic hydrogen.

00:03:31.810 --> 00:03:36.460
But let's also think about
the radii of these atoms.

00:03:36.460 --> 00:03:40.280
If we get a periodic
table of elements here,

00:03:40.280 --> 00:03:43.190
we can see that hydrogen
only has one electron

00:03:43.190 --> 00:03:44.750
in that first shell,

00:03:44.750 --> 00:03:47.320
and so it's going to be the smallest.

00:03:47.320 --> 00:03:51.000
So that makes sense over
here, that your distance,

00:03:51.000 --> 00:03:52.830
where you have the
lowest potential energy,

00:03:52.830 --> 00:03:56.420
is shortest for the diatomic molecule

00:03:56.420 --> 00:03:59.040
that's made up of the smallest atoms.

00:03:59.040 --> 00:04:00.920
But then when you look at the other two,

00:04:00.920 --> 00:04:02.610
something interesting happens.

00:04:02.610 --> 00:04:06.160
Remember, your radius
for an atom increases

00:04:06.160 --> 00:04:08.310
as you go down a column.

00:04:08.310 --> 00:04:11.640
But as you go to the right on
a row, your radius decreases.

00:04:11.640 --> 00:04:13.120
'Cause you're adding
more and more electrons

00:04:13.120 --> 00:04:16.690
to the same shell, but the
Coulomb forces are increasing

00:04:16.690 --> 00:04:20.630
between that outermost
shell and your nucleus.

00:04:20.630 --> 00:04:22.800
And so if you just look at that trend,

00:04:22.800 --> 00:04:25.070
as you go from nitrogen to oxygen,

00:04:25.070 --> 00:04:27.200
you would actually
expect your atomic radius

00:04:27.200 --> 00:04:28.970
to get a little bit smaller.

00:04:28.970 --> 00:04:30.210
They're right next to each other.

00:04:30.210 --> 00:04:32.390
They might be close, but
you say, okay, oxygen,

00:04:32.390 --> 00:04:35.040
you have one extra electron
in that same second shell,

00:04:35.040 --> 00:04:37.390
maybe it's going to be
a little bit smaller.

00:04:37.390 --> 00:04:39.440
So if you were to base
things just on that,

00:04:39.440 --> 00:04:42.200
you'd say, all right, well,
the internuclear distance

00:04:42.200 --> 00:04:44.290
for this salmon-colored one
is a little bit shorter,

00:04:44.290 --> 00:04:48.380
maybe that one is oxygen, and
maybe this one is nitrogen.

00:04:48.380 --> 00:04:50.870
But they would be close,
and I would say, in general,

00:04:50.870 --> 00:04:52.870
the bond order would trump things.

00:04:52.870 --> 00:04:56.810
And the bond order, because
you see this high bond energy,

00:04:56.810 --> 00:04:58.690
that's the biggest
giveaway that this is going

00:04:58.690 --> 00:05:03.690
to be the higher bond order
diatomic molecule or N2.

00:05:04.180 --> 00:05:06.140
They're close in atomic radius,

00:05:06.140 --> 00:05:09.970
but this is what makes
all of the difference.

00:05:09.970 --> 00:05:12.260
And we'll take those two nitrogen atoms

00:05:12.260 --> 00:05:15.420
and squeeze them together
just a little bit more,

00:05:15.420 --> 00:05:18.200
even though they might
be a little bit bigger.

00:05:18.200 --> 00:05:20.553
And so I feel pretty
good with this labeling.


You Can Learn Anything (30 sec)

WEBVTT
Kind: captions
Language: en

00:00:00.670 --> 00:00:02.890
- [Narrator] Nobody's born smart.

00:00:02.890 --> 00:00:05.890
There was a time when
Einstein couldn't count to 10

00:00:05.890 --> 00:00:08.030
and Shakespeare had to learn his ABC's

00:00:08.030 --> 00:00:10.110
just like the rest of us.

00:00:10.110 --> 00:00:13.430
You stumble, slip, crawl, fall, and fail.

00:00:13.430 --> 00:00:16.130
But failing is just
another word for growing

00:00:16.130 --> 00:00:17.630
and you keep going.

00:00:17.630 --> 00:00:19.920
One foot in front of the other, one idea

00:00:19.920 --> 00:00:22.810
on top of the next until you get it.

00:00:22.810 --> 00:00:24.600
This is learning.

00:00:24.600 --> 00:00:26.200
You only have to know one thing,

00:00:27.130 --> 00:00:28.793
you can learn anything.


Bond length and bond energy

WEBVTT
Kind: captions
Language: en

00:00:00.160 --> 00:00:02.340
- [Instructor] If you
were to find a pure sample

00:00:02.340 --> 00:00:04.320
of hydrogen,

00:00:04.320 --> 00:00:07.950
odds are that the individual
hydrogen atoms in that sample

00:00:07.950 --> 00:00:10.750
aren't just going to be
separate atoms floating around,

00:00:10.750 --> 00:00:13.270
that many of them, and
if not most of them,

00:00:13.270 --> 00:00:15.610
would have bonded with each other,

00:00:15.610 --> 00:00:18.640
forming what's known as diatomic hydrogen,

00:00:18.640 --> 00:00:20.710
which we would write as H2.

00:00:20.710 --> 00:00:23.920
Another way to write it
is you have each hydrogen

00:00:23.920 --> 00:00:27.290
in diatomic hydrogen would have
bonded to another hydrogen,

00:00:27.290 --> 00:00:29.730
to form a diatomic molecule like this.

00:00:29.730 --> 00:00:31.900
This molecule's only made up of hydrogen,

00:00:31.900 --> 00:00:33.720
but it's two atoms of hydrogen.

00:00:33.720 --> 00:00:35.820
And this makes sense, why it's stable,

00:00:35.820 --> 00:00:40.390
because each individual hydrogen
has one valence electron

00:00:40.390 --> 00:00:41.440
if it is neutral.

00:00:41.440 --> 00:00:43.110
So that's one hydrogen there.

00:00:43.110 --> 00:00:44.200
That's another one there.

00:00:44.200 --> 00:00:47.640
And if they could share
their valence electrons,

00:00:47.640 --> 00:00:51.720
they can both feel like they
have a complete outer shell.

00:00:51.720 --> 00:00:54.280
And so this dash right over here,

00:00:54.280 --> 00:00:56.770
you can view as a pair
of electrons being shared

00:00:56.770 --> 00:00:58.810
in a covalent bond.

00:00:58.810 --> 00:01:00.660
Now, what we're going to do in this video

00:01:00.660 --> 00:01:04.730
is think about the
distance between the atoms.

00:01:04.730 --> 00:01:08.570
So just as an example, imagine
two hydrogens like this.

00:01:08.570 --> 00:01:09.910
So that's one hydrogen atom,

00:01:09.910 --> 00:01:11.860
and that is another hydrogen atom.

00:01:11.860 --> 00:01:14.690
It turns out, at standard
temperature, pressure,

00:01:14.690 --> 00:01:17.990
the distance between
the centers of the atoms

00:01:17.990 --> 00:01:21.020
that we observe, that
distance right over there,

00:01:21.020 --> 00:01:24.640
is approximately 74 picometers.

00:01:24.640 --> 00:01:27.630
And just as a refresher of
how small a picometer is,

00:01:27.630 --> 00:01:30.840
a picometer is one trillionth of a meter.

00:01:30.840 --> 00:01:33.920
So this is 74 trillionths of a meter,

00:01:33.920 --> 00:01:36.390
so we're talking about
a very small distance.

00:01:36.390 --> 00:01:39.260
But one interesting question
is why is it this distance?

00:01:39.260 --> 00:01:41.940
What would happen if we tried
to squeeze them together?

00:01:41.940 --> 00:01:44.370
What would happen if we
tried to pull them apart?

00:01:44.370 --> 00:01:45.800
And to think about that,

00:01:45.800 --> 00:01:47.320
I'm gonna make a little bit of a graph

00:01:47.320 --> 00:01:51.020
that deals with potential
energy and distance.

00:01:51.020 --> 00:01:53.170
So in the vertical axis,

00:01:53.170 --> 00:01:56.100
this is going to be potential energy,

00:01:56.100 --> 00:01:58.230
potential energy.

00:01:58.230 --> 00:01:59.560
And I won't give the units just yet.

00:01:59.560 --> 00:02:02.280
I'll just think in very
broad-brush conceptual terms,

00:02:02.280 --> 00:02:04.900
then we could think about
the units in a little bit.

00:02:04.900 --> 00:02:07.433
And then this over here is the distance,

00:02:08.330 --> 00:02:10.810
distance between the centers of the atoms.

00:02:10.810 --> 00:02:13.290
You could view it as the
distance between the nuclei.

00:02:13.290 --> 00:02:16.290
And let's give this in picometers.

00:02:16.290 --> 00:02:18.110
Now, potential energy,
when you think about it,

00:02:18.110 --> 00:02:20.160
it's all relative to something else.

00:02:20.160 --> 00:02:22.140
And so let's just arbitrarily say

00:02:22.140 --> 00:02:25.620
that at a distance of 74 picometers,

00:02:25.620 --> 00:02:27.540
our potential energy is right over here.

00:02:27.540 --> 00:02:30.760
I'm not even going to label this axis yet.

00:02:30.760 --> 00:02:33.810
Now, what's going to happen
to the potential energy

00:02:33.810 --> 00:02:37.373
if we wanted to pull
these two atoms apart?

00:02:38.230 --> 00:02:40.270
Well, this is what we
typically find them at.

00:02:40.270 --> 00:02:41.730
This is probably a low point,

00:02:41.730 --> 00:02:44.290
or this is going to be a low
point in potential energy.

00:02:44.290 --> 00:02:46.500
So if you make the distances go apart,

00:02:46.500 --> 00:02:47.840
you're going to have
to put energy into it,

00:02:47.840 --> 00:02:51.800
and that makes the
potential energy go higher.

00:02:51.800 --> 00:02:54.930
And to think about why that makes sense,

00:02:54.930 --> 00:02:57.180
imagine a spring right over here.

00:02:57.180 --> 00:02:59.490
If you want to pull it apart,

00:02:59.490 --> 00:03:01.960
if you pull on either sides of a spring,

00:03:01.960 --> 00:03:03.740
you are putting energy in,

00:03:03.740 --> 00:03:05.610
which increases the potential energy.

00:03:05.610 --> 00:03:09.510
Because if you let go, they're
just going to come back to,

00:03:09.510 --> 00:03:11.240
they're going to accelerate
back to each other.

00:03:11.240 --> 00:03:12.570
So as you pull it apart,

00:03:12.570 --> 00:03:14.430
you're adding potential energy to it.

00:03:14.430 --> 00:03:16.690
So as you have further
and further distances

00:03:16.690 --> 00:03:19.730
between the nuclei, the
potential energy goes up.

00:03:19.730 --> 00:03:21.750
And if you go really far,

00:03:21.750 --> 00:03:24.030
it's going to asymptote
towards some value,

00:03:24.030 --> 00:03:26.180
and that value's
essentially going to be the

00:03:26.180 --> 00:03:30.170
potential energy if these two
atoms were not bonded at all,

00:03:30.170 --> 00:03:32.800
if they, to some degree, weren't
associated with each other,

00:03:32.800 --> 00:03:35.060
if they weren't interacting
with each other.

00:03:35.060 --> 00:03:37.150
And so that's actually the point

00:03:37.150 --> 00:03:39.260
at which most chemists or physicists

00:03:39.260 --> 00:03:43.440
or scientists would label
zero potential energy,

00:03:43.440 --> 00:03:45.410
the energy at which they are infinitely

00:03:45.410 --> 00:03:46.550
far away from each other.

00:03:46.550 --> 00:03:48.800
And that's what this
is asymptoting towards,

00:03:48.800 --> 00:03:51.490
and so let me just draw
that line right over here.

00:03:51.490 --> 00:03:54.840
So let's call this zero right over here.

00:03:54.840 --> 00:03:56.310
And actually, let me now give units.

00:03:56.310 --> 00:04:00.113
Let's say all of this is
in kilojoules per mole.

00:04:01.610 --> 00:04:04.060
Now, once again, if
you're pulling them apart,

00:04:04.060 --> 00:04:06.050
as you pull further and
further and further apart,

00:04:06.050 --> 00:04:08.260
you're getting closer and closer to these,

00:04:08.260 --> 00:04:10.400
these two atoms not interacting.

00:04:10.400 --> 00:04:11.280
Why is that?

00:04:11.280 --> 00:04:14.150
Because as you get further
and further and further apart,

00:04:14.150 --> 00:04:16.510
the Coulomb forces between them are going

00:04:16.510 --> 00:04:19.410
to get weaker and weaker
and weaker and weaker.

00:04:19.410 --> 00:04:20.470
And so that's why they like

00:04:20.470 --> 00:04:23.430
to think about that as
zero potential energy.

00:04:23.430 --> 00:04:25.010
Now, what if we think about
it the other way around?

00:04:25.010 --> 00:04:28.290
What if we want to squeeze
these two together?

00:04:28.290 --> 00:04:31.140
Well, once again, if you
think about a spring,

00:04:31.140 --> 00:04:33.020
if you imagine a spring like this,

00:04:33.020 --> 00:04:35.210
just as you would have to add energy

00:04:35.210 --> 00:04:36.940
or increase the potential
energy of the spring

00:04:36.940 --> 00:04:39.150
if you want to pull the spring apart,

00:04:39.150 --> 00:04:42.320
you would also have to do it
to squeeze the spring more.

00:04:42.320 --> 00:04:43.760
And so to get these two atoms

00:04:43.760 --> 00:04:46.400
to be closer and closer
and closer together,

00:04:46.400 --> 00:04:49.650
you have to add energy into the system

00:04:49.650 --> 00:04:51.870
and increase the potential energy.

00:04:51.870 --> 00:04:55.300
And why, why are you having
to put more energy into it?

00:04:55.300 --> 00:04:58.520
Because the more that you squeeze
these two things together,

00:04:58.520 --> 00:05:00.670
you're going to have the positive charges

00:05:00.670 --> 00:05:02.930
of the nuclei repelling each other,

00:05:02.930 --> 00:05:04.660
so you're gonna have to
try to overcome that.

00:05:04.660 --> 00:05:06.750
That puts potential
energy into the system.

00:05:06.750 --> 00:05:08.480
And these electrons are starting

00:05:08.480 --> 00:05:10.310
to really overlap with each other,

00:05:10.310 --> 00:05:12.770
and they will also want
to repel each other.

00:05:12.770 --> 00:05:16.110
And so what we've drawn here,
just as just conceptually,

00:05:16.110 --> 00:05:17.560
is this idea of if you wanted them

00:05:17.560 --> 00:05:19.920
to really overlap with each other,

00:05:19.920 --> 00:05:22.490
you're going to have a
pretty high potential energy.

00:05:22.490 --> 00:05:24.020
And if you're going to have them

00:05:24.020 --> 00:05:25.590
very separate from each other,

00:05:25.590 --> 00:05:28.040
you're not going to have as
high of a potential energy,

00:05:28.040 --> 00:05:29.910
but this is still going to be higher

00:05:29.910 --> 00:05:32.240
than if you're at this stable point.

00:05:32.240 --> 00:05:35.350
This stable point is stable
because that is a minimum point.

00:05:35.350 --> 00:05:38.460
It is a low point in this
potential energy graph.

00:05:38.460 --> 00:05:40.660
You could view this as just right.

00:05:40.660 --> 00:05:44.110
And it turns out that
for diatomic hydrogen,

00:05:44.110 --> 00:05:47.740
this difference between zero
and where you will find it

00:05:47.740 --> 00:05:49.790
at standard temperature and pressure,

00:05:49.790 --> 00:05:54.790
this distance right over here
is 432 kilojoules per mole.

00:05:55.120 --> 00:05:59.050
So this is at the point negative
432 kilojoules per mole.

00:05:59.050 --> 00:06:00.380
And so one interesting thing

00:06:00.380 --> 00:06:02.550
to think about a diagram like this is

00:06:02.550 --> 00:06:05.790
how much energy would it take
to separate these two atoms,

00:06:05.790 --> 00:06:07.700
to completely break this bond?

00:06:07.700 --> 00:06:10.610
Well, it'd be the energy of
completely pulling them apart.

00:06:10.610 --> 00:06:13.240
And so it would be this energy.

00:06:13.240 --> 00:06:16.080
It would be this energy right over here,

00:06:16.080 --> 00:06:19.100
or 432 kilojoules.

00:06:19.100 --> 00:06:22.140
And that's what people
will call the bond energy,

00:06:22.140 --> 00:06:26.110
the energy required to separate the atoms.

00:06:26.110 --> 00:06:27.910
And we'll see in future videos,

00:06:27.910 --> 00:06:30.400
the smaller the individual atoms

00:06:30.400 --> 00:06:32.700
and the higher the order of the bonds,

00:06:32.700 --> 00:06:35.520
so from a single bond to a
double bond to a triple bond,

00:06:35.520 --> 00:06:37.220
the higher order of the bonds,

00:06:37.220 --> 00:06:41.113
the higher of a bond energy
you're going to be dealing with.