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PROFESSOR: OK, a couple
of announcements.
Tomorrow will be the Periodic
Table test. I provide the
numbers, you provide
the letters.
Friday, the contest
ends, 5:00 pm.
Get the submissions to me,
either over the internet or if
you have something majestic,
you have to cart it in with
the riggers.
Get it to my office.
Last day, we had a lesson
on the Aufbau principle.
And the Aufbau principle
gave us the--
There's still too
much talking.
Still too much talking.
I will tolerate zero,
zero talking.
When you walk through that door,
I'm assuming it's an act
of free will.
And when you walk through
that door, we
enter into a contract.
The contract goes something like
this: You have certain
expectations of me.
You expect me to come
to class prepared.
You expect me to treat you
with respect, not to use
vulgar language, not to say
things that are insulting,
insensitive.
And I have certain expectations
of you.
I expect you to come
to class prepared.
I expect that you've
done the reading.
And I expect that you're
going to sit quietly.
Absolutely quietly.
Because it's my duty to preserve
a fertile learning
environment.
If you don't want to learn,
I don't care.
I don't care.
But if you impair the ability
of your neighbor to learn, I
will take action.
It's very simple.
Very simple.
Where was I?
I think I was talking about
the Aufbau principle.
And the Aufbau principle tells
us what the filling sequence
of electrons is in a
multielectron atom, which
ultimately we rationalized in
terms of the four quantum
numbers and the filling in
ascending order of energy.
And then we tried to understand
what's behind the
Aufbau principle.
And lastly, we took a journey
through central Europe in the
1920s and 1930s.
And we met de Broglie,
who gave us the
concept of matter waves.
If matter has wave-like
properties, the wavelength
would be given by the ratio of
the Planck constant to the
Newtonian momentum.
Heisenberg taught us that we
can't deal with precision down
to atomic dimensions, and
there's a certain degree of
uncertainty or indeterminacy.
And it's given by this
relationship here.
And then, lastly, we saw
Schroedinger, who wrote the
wave equation saying that if we
have wave-like properties
then let's model the atom as
a wave, and he gave us this
equation here, which is
essentially the equation of
the simple harmonic
oscillator.
The simple harmonic oscillator,
which allows you
to tell us what's going on
with the plucked string.
And it's essentially just this,
the double derivative x
double dot plus kx goes
to some function.
And the solution of that is a
bunch of sines and cosines.
And that's this equation, but
dressed up for night-time.
It's a little bit more
sophisticated and gives us the
three dimensions, eigenfunctions
that
ultimately, we can take and
generate the pretty pictures
of the orbitals.
So, today what I want to do is
go a little more deeply into
properties.
And if you learn nothing else
from me in 3.091, this is what
I want you to retain.
It's that electronic structure
dictates properties.
Electronic structure dictates
properties.
You know this on
your death bed.
It's that simple.
Electronic structure dictates
properties.
Then we're going to take 14
weeks and describe electronic
structure, right?
But that's it.
That's it.
So I want to go back to this.
We got into this energy, Aufbau
principle because we
saw on the Periodic Table
there's some problems here.
If we just go in terms of
quantum numbers, we see
there's two elements in
n equals 1 shell.
There's 8 elements in
n equals 2 shell.
And, if we just kept going,
we would expect to find 18
elements in n equals 3 shell,
but instead we found only 8.
And that's where the
journey began.
Well, there's a way to sort of
help remember what the filling
sequence is.
And that rule is encapsulated by
the n plus l relationship.
So for equivalent, n plus l
values, for equivalent n plus
l values, you fill
in ascending n.
Fill in ascending n.
So let's take a look.
So, here's Aufbau.
n plus l rule.
Here's an example where
we have 3d.
d means l equals 2.
p means l equals 1. s
means l equals 0.
So 5 plus 0 is 5.
4 plus 1 is 5.
3 plus 2 is 5.
So what do I do?
Well, it's saying go in order
of ascending n plus l.
And so, you can snake your way
through this n plus l, and get
the filling sequence for the
whole Periodic Table.
So let's take a look.
We'll start with 1s.
Well, that's easy.
That's hydrogen and helium.
Then we go next to 2s.
There's lithium and beryllium.
And then next goes 2p.
I mean, nobody's going to try
to stick 3s ahead of 2p.
That's pretty straightforward.
2p, and that gets us all
the way over to neon.
And then we wrap around
over here to 3s.
That gets us from sodium
to magnesium.
And then we jump to
3p, gets us from
aluminum over to argon.
And then, we don't go to 3d,
because 3p is 3 plus 1 is 4.
4s is 4 plus 0 is 4.
So it says we fill
in ascending n.
So we go to 4s next.
And that gets us potassium
and calcium.
Then comes 3d.
So that gets us scandium over
to zinc. Then comes 4p,
gallium to krypton.
5s.
I'm having fun here, I'm
going to keep going.
5s gets us over to strontium.
Next comes 4d.
Gets us all the way
over to cadmium.
5p gets us to xenon.
Now it's turn 6s
cesium, barium.
4f.
We've got to jump down here.
Finally we're going to
get to the f shell.
See?
The 4f is in here.
Now, this is an item
of convenience.
Strictly speaking, lanthanum
is 57, cerium is 58.
So all of these elements
belong in here.
But if we were to put this to
scale, the Periodic Table
would be way over here.
It wouldn't fit on a page.
So people, over time, have
gotten used to just
putting it down here.
But these elements are
in the middle here.
So you go 4d, 5p, 6s, then 4f.
Finally we get over to the edge
here, to erbium, and then
we jump over here to hafnium,
5d, over to
mercury, 6p, et cetera.
So the n plus l rule gives you
the filling sequence in
ascending order.
That's good.
So we've got a nice compact
way of grabbing this e
goes to n plus l.
Now let's look at
some properties.
We said it's a table of
the elements, but
it's a periodic table.
So let's see what this
periodicity looks like.
Now, here's a variation of
first ionization energy.
So here it is in kilojoules per
mole, and here's my pet
unit, the electron
volt per atom.
Because here's hydrogen, of 1.6
megajoules per mole, but I
like the 13.6 electron volts
because I can remember 13.6.
I can't remember 1312
megajoules per mole,
kilojoules per mole,
whatever it is.
13.6 electronic volts.
So here's hydrogen,
there's helium.
And then we drop down to
lithium, and then we move
across up to neon, and so on.
So you can see 1s, we
filled the 1s shell.
Now, we're going to go
to shell n equals 2.
Here's 2s.
And then we go to 2p.
And you can even see, look,
boron, carbon, oxygen, the
ascending value of the first
ionization energy.
And then there's a little
jog here at oxygen.
Because here we've got
the three unpaired
electrons for nitrogen.
And then there's the oxygen
starts to pair, and we
continue to neon, and so on.
3s, 3p, 4s.
So you can see the relationship
between that
property, and the place,
the element, in
the Periodic Table.
So how do we measure
these values?
Well, I thought it might be a
good opportunity to look at,
revisit the whole question
of gas dynamics, and also
understand the measurement.
Measurement of ionization
energies.
And the technique that's used
is called photoelectron
spectroscopy.
Photoelectron spectroscopy.
All right.
And it's got a three-letter
initialization, PES.
I want to show you a cartoon
of how this works.
Actually, I took one --
this is taken from the
text we used to use.
I like the text that we're using
now better, but there
are a few things in the other
text that were good.
So, we've scanned these few
pages and posted them at the
website if you want to go
through and read this stuff.
So let's jump over to that.
So here's an example of
a terrible drawing.
I look at this and I haven't got
the faintest idea how this
thing works.
It's not the artist's fault.
Somebody should have proofread
this thing.
What they should be doing is
showing something like this.
So, let's see what's going on.
So what we're going to do is,
we're going to bombard a
specimen right here.
Which they never show.
I suppose this atom beam is
supposed to be colliding with
the photons.
That would be quite
an apparatus to
build, let me tell you.
Instead, what you have is, you
have the apparatus sitting so
that you've got the material in
the center of the chamber.
And you irradiate
with photons.
And the photons have very, very
high energy, and they
dislodge electrons.
When they dislodge electrons
that's the ionization event.
And then what we do is, we
measure the velocity of those
ejected electrons.
And if we know the velocity,
we know their energy.
We know the energy of the
incident energy and by
difference, we calculate the
binding energy of the electron
and the atom.
So let's take a look.
I'm going to draw a cartoon here
that gives you a sense of
what's going on.
So, here's the n equals 1
shell, or the k shell.
And then I'm going to show,
say, a second shell here.
One, two, three.
So this is the l shell,
or n equals 2.
And you might say, gee,
he's got that wrong.
It's kind of simple,
and so on.
It's as complex as it needs
to be for the explanation.
Don't let accuracy
trump clarity.
The real accurate picture here
is too detailed to convey what
I'm trying to convey.
So, this is the specimen,
right?
Let's call this the specimen.
And I'm going to irradiate the
specimen with some radiation
of high energy.
And so, I generally indicate the
photon by a squiggly line
with an arrow.
That's to indicate it's got
wave-like properties.
And I'll usually write h nu next
to it, to indicate that
it's a photon of energy h nu.
So the photon comes in, and if
the energy is high enough, it
will dislodge an electron.
So this electron is dislodged.
It's now ballistic.
It's rendered ballistic.
It's free.
It's ionized.
It's gone.
It's gone.
And what we're going to do is,
we're going to take an energy
balance here.
And at some point you may
come back and learn
some quantum mechanics.
This photon is annihilated
at this collision.
So the photon doesn't act the
way an incident electron does
where it loses some
of its energy.
All of the energy is lost.
So this photon's gone.
All of the energy is given
to this electron.
So let's do an energy
balance here.
So I can say that the energy
of the incident photon, the
energy of the incident photon,
is now going to be given to
the electron.
It's now got some kind
of kinetic energy.
E kinetic plus the energy
it took to pull
it out of the atom.
Plus E, let's call it binding.
E binding
And we know the incident energy
of the photon because
we're running the experiment.
So we set the value of
the incident energy
of the photon beam.
And then we send this
to a detector.
This goes to a detector.
And the detector measures the
velocity and ultimately gives
us the energy of the
scattered electron.
This is called the dislodged
electron, or photoelectron.
It's the electron that was
kicked out by the photon.
So it's known as the
photoelectron.
Photoelectron.
Pardon me, let's
try that again.
Photoelectron energy is measured
at the detector.
And then by difference, we
get the binding energy.
We get the binding energy.
So this is h nu.
This one here is going to be a
1/2 m b squared, and then by
difference we get the
binding energy.
And what kind of photon
energies do we need?
We need fairly high energies.
And so typically, we might use
wavelengths down around 1
angstrom, which then
makes it an X-ray.
And so, over here in Building
13, we have such
instrumentation, and the
material scientists
would call this XPS.
X-ray photoelectron
spectroscopy.
And if you don't want to blast
all of the electrons out of
the specimen and just get the
most weakly bound, then you
want a lower incident photon
energy, which means a longer
wavelength.
You might come in at around
100 angstroms, which is
ultraviolet.
You know, people, the general
public, is so afraid, they
don't know science.
If they hear X-ray they
get all panicked.
You know, it's radiation.
Their children are going
to die and all
this kind of stuff.
So, what people will do is,
they'll say instead of soft
X-ray they'll say hard
ultraviolet.
And then the public thinks, oh,
as long I wear sunscreen
I'm going to be OK.
So if you use hard ultraviolet,
it's called UPS.
That's nothing to do
with brown trucks.
It's ultraviolet photoelectron
spectroscopy.
And collectively, this
is known as PES--
photoelectron spectroscopy.
All right.
So now, let's go back
to this schematic.
Now you can see how bad this
schematic was that these
photons are coming in as so.
And striking the specimen
here, ejecting the
photoelectrons which then are
focused and ultimately
measured here at the detector.
So there's the energy balance.
The energy of the incident
photon is distributed across
the kinetic energy of the
photoelectron plus the binding
energy, and this is how we
measure all of these
quantities.
Since you're using very, very
high intensity radiation,
there's nothing saying
that you're going to
eject only one electron.
You can eject a plurality
of electrons.
And the different
electrons have
different binding energies.
So you're going to get a set
of binding energies, which
means you'll get
a set of lines.
You'll get a spectrum
of energies.
So this is what the spectrum
looks like, and this is, by
the way, just a general way of
looking at any spectrum so
that you train you eye to
know what to look for.
If I look at any spectrum, I'm
going to have a plot of some
kind of intensity, some kind of
intensity unit versus some
kind of energy unit, versus
some kind of energy unit.
And sometimes, energy increases
from left to right.
But sometimes they might put
energy increasing from right
to left as you see in
these spectrum.
But energy is along here.
And intensity is related
to population.
All other things being equal, if
I have a specimen that has
two times the concentration of
active species, I'm going to
get twice the strength
of the line.
So the intensity measures
population.
The energy is related to the
characteristic binding energy
of a particular electrons, so
the energy is related to the
identity of the species.
This is how we can identify
certain species.
I've already shown you if you
walk in and you see those four
Balmer series lines on a
spectrum, you know that's
characteristic of
atomic hydrogen.
Nothing else.
So those are energies.
And then, where do we go?
So we have a line here, but we
know that real data have a
little bit of a spread
to them.
So you don't see discrete lines;
you'll see a spread
centered at about the value.
So let's look at these.
These are highly stylized, but
here you can see-- here's
boron for example.
So these are the 1s electrons of
boron, and they're held at
a binding energy of about 19.3
megajoules per mole.
And then this is 2s,
and here's 2p.
Now there's a difference between
2s and 2p, but they're
of the same order.
They're roughly about a
megajoule per mole.
There's subtle differences.
And this goes back to
Sommerfeld, who said that even
though there are s and p
orbitals, there's a circular
orbit and there's an
elliptical orbit.
But they're roughly in
the same shell and
that's what you see.
But there's a huge difference in
going from shell n equals 1
to n equals 2.
This is not to scale.
You see they got a little
break there.
And now look, what's the
electronic structure of boron?
1s2 2s2 2s1.
There are two 2s electrons,
but only one 2p electron.
And this is trying to show that
the height here is twice
the height of the 2s peak.
So you see that we've got two
electrons here, one electron
here, and it's easiest to pull
out the 2p electrons first.
And you go on, there's
beryllium and so on.
So we see that we can build
data in this fashion here.
We keep going.
Here's carbon, which
is 2, 2, 2.
And then oxygen has 2s2--
pardon me.
1s2, 2s2, 2p4.
So this is two electrons,
this is four electrons.
Showing that the 2p height
is twice the 2s height.
And then finally, neon
is 2s2, 2p6.
So this is roughly three times
the height of the 2p.
But all of the 2p numbers are
roughly of the same order of
magnitude and decidedly
smaller than
what's going on at 1s.
And what else do you see?
Well, the carbon has 6
protons, so the inner
electrons of carbon are held
not as tightly as the inner
electrons of neon, which
has 10 protons.
All of this comes out.
The data are all good.
So now what do we want
to talk about?
What's all this about?
I said electronic structure
leads to properties.
The first thing we want to
talk about is reactivity,
chemical reactivity.
Well it's pretty clear from
looking at these XPS data that
those inner shell electrons
aren't going anywhere.
They're too tightly held.
So first thing is the only
electrons that we can even
think about involving in
chemical reactivity must be
the electrons in the
outermost shell.
These are called valence
electrons.
The outermost shell is called
the valence shell.
So let's get that down because
that's probably important.
So chemical reactivity
determined by only electrons
in outermost shell.
And we're going to term that
the valence shell.
And those electrons are called
the valence electrons.
And, these are the only ones
that we expect to see involved
in chemical reactivity.
And we want to have a measure.
You can see that there are
subtle differences between the
energies that hold the valence
electrons of carbon as opposed
the valence electrons of neon.
And so what we can do is have
a measure of the ability for
valence electrons to react.
That is to say, to participate
in chemical activity.
And that measure is
called the average
valence electron energy.
And there's no special symbol
for that, so we
just call it AVEE.
And it's just the values
of the valence
electron energies averaged.
So if I want to take the average
valence electron
energy for say, oxygen.
So I can take it right
off of there.
I can see I'm going to need
two times the ionization
energy of 2s plus four times the
ionization energy of 2p.
And those data are up there.
All divided by 6.
2 plus 4, the total number
of valence electrons.
And if I go through the math up
there I get 1.91 megajoules
per mole, which I prefer to
expresses as 19.8 electron
volts per atom.
And when we go through and
calculate the values of
average valence electron
energies, we find trends.
And here's what the
trends look like.
So you have, first of all,
following along with the
ionization energies, very
similar values.
Pardon me.
So here's the plot of average
valence electron energy in bar
heights arranged as the elements
are found on the
Periodic Table.
So you have hydrogen here,
helium on this scale
would be way up.
And then here's lithium,
beryllium, boron.
And so you see a monotonic
increase as you move
from left to right.
And you see a monotonic decrease
if you stay in the
same column from low
mass to high mass.
So what's going on there?
Why do we have that change?
We have similar electronics
structures.
We have similar electronics
structures in a given column
and lithium is s1, sodium
is s1, potassium is
s1, rubidium is s1.
And let's put some
values on here.
Let's put some values.
For the majority of elements in
the Periodic Table, we get
values of average valence
electron energy less than 11
electron volts.
And when we have such values,
we have the following
properties: the valence
electrons are weakly held.
The valence electrons
are weakly held.
How do we know this?
Because the binding energy
isn't so strong.
That means the electronic
are weakly held.
So we say that the element
is a good electron donor.
It's a good electron donor
and we term this a metal.
The property that makes an
element a metal is that it has
a low value of average valence
electron energy and therefore,
it is a good electron donor.
And it turns out about
75% of the Periodic
Table is made of metals.
At the other extreme we have
high values of average valence
electron energy.
High values.
So when the average valence
electron energy is high, we
have the complementary
set of properties.
So that means the valence
electrons are tightly held.
Valence electrons
tightly held.
Tightly held.
And so the element is a
poor electron donor.
But complementary fashion, it
is a good electron acceptor
because when it gets near
electrons it tends to grab
them and hold on to them.
So this is a good electron
acceptor.
And we term such a element
a nonmetal.
And so roughly, about 25%
of the Periodic Table is
non-metallic.
And I think I've got
that shown here.
And you see just a
few elements off
to the upper right.
And then in the middle,
we've got this--
about a half a dozen elements
and they have values of
average valence electron energy
intermediate between 11
and 13 electron volts.
And so, they can behave either
as electron donors or electron
acceptors depending on who
else is in the room.
So if you put an element like
silicon with a very strong
metal, silicon will act as
an electron acceptor.
If you put silicon in the
presence of something that's a
very strong nonmetal, silicon
can act as an electron donor.
It has that dual property.
And these elements are called
semimetals or metalloids.
And I think I've got some
images to portray that.
There's the semimetals.
And if you look carefully at
the Periodic Table that you
have, you'll see that there's
this red staircase here.
So there's silicon, arsenic,
tellurium, antimony and so on
that act as semimetals.
They can behave in
both fashions.
All right, so now with this
classification scheme, let's
start thinking about chemical
reactivity.
And seeing if we can, on the
basis of where the elements
are found on the Periodic
Table, start
to make some judgments.
Well I know one thing right
off the bat that along the
right-hand column, the noble
gases are chemically inert.
That much we know.
Noble gases are chemically
inert.
I know there's a Nobel Prize out
there for getting xenon to
combine with fluorine, and the
chemistry textbooks love to
point out these exceptions.
But by and large, the group 8
elements are chemically inert.
So what do we know about their
electronic structure?
They all have the
same electronic
structure ns 2, np 6.
With the exception of helium.
Helium, because it's 1s 2.
There's no p.
So beside helium, starting with
neon, we've got 2s2, 2p6.
Argon is 3s2, 3p6, and so on.
n2 p6, it has 8 p electrons.
It has a full valence shell.
And because the last electrons
are p electrons, this is
termed octet stability.
There's something about having
a full shell that renders the
system satisfied
energetically.
And it doesn't want
to react anymore.
So we say octet stability, and
if you want to be a wise guy,
parenthetically you whisper
helium is due at stability.
But you know, whatever.
All right, so let's make this
octet stability a hypothesis
and see how far we
can go with it.
So the next one I want
to look at is sodium.
Now sodium, it's got the
electronic structure 1s 2,
2s2, 2p6, 3s1.
Now, I really don't need to talk
about anything up to n
equals 3 because this is
not valence electrons.
I just put it up there for
completeness, but this is all
we have to worry about.
Now we know that sodium
is a metal.
It's a metal.
It's got an average valence
electron energy of about 5.2
electron volts, which happens
to be the ionization energy.
Because that's trivial; it's
only got one electron, so the
average valence electron
energy is equal to the
ionization energy.
So it's a metal.
So it's a good electron donor.
But let's think about that.
If we could get rid of that
electron in sodium, we could
then turn it into something that
has the same electronic
structure as neon.
And why do we want to do that?
Well because there seems to
be an energy well there.
If we can make something
neon-like, then
it's going to be happy.
So let's do it.
Let's write a reaction sodium to
lose an electron and become
sodium plus.
And sodium plus now
is this minus 3s1.
So it's isoelectronic
with neon, and neon
has the octet stability.
But charge neutrality
forbids me to
say, OK, lose an electron.
It won't.
The only way for sodium
to lose an electron
is to find an acceptor.
So if you put a good electron
donor in the presence of a
good electron acceptor, then the
donor can give an electron
to the acceptor and both profit
from the transaction.
So, where are we going to find
a good electron acceptor?
Well I just told you, the
nonmetals are the really good
electron acceptors.
So let's go zooming over to the
right-hand edge of that
row and look at chlorine.
We need an electron acceptor.
So choose just for argument's
sake, chlorine.
It's got an average
valence electron
energy way, way up there.
So let's go.
Chlorine, it has the electronic
structure of neon
plus 3s2, 3p5.
So chlorine, if it could acquire
an electron, would
then become isoelectronic
with argon.
So imagine chlorine plus
electron becomes Cl minus and
Cl minus is isoelectronic
with argon.
So now I've got two elements.
One that's one electron richer
than inert gas structure, and
one that's one electron poorer
than inert gas structure.
If I put them together, put them
both together in the same
reactor, they will react and
electron transfer occurs via
electron transfer.
Chemical reaction occurs.
Chemical reaction occurs through
electron transfer.
And what's the purpose
of electron transfer?
It's to achieve full
valence shell.
Achieve full valence shell
occupancy, if you like.
Achieve full valence
shell occupancy.
So, now we've got it.
But there's more.
There's more.
What happens after the electron
transfer step?
We don't have neutral
species anymore.
The sodiums have given up their
electrons to become
sodium ions.
And the chlorines have acquired
electrons to become
chloride ions.
And these are free.
Let's make this simple.
Let's make it a gas
phase reaction.
So sodium vapor and chlorine
gas have become sodium ion
vapor and chloride ion vapor.
What do you know happens when
you have charges of opposite
value, opposite polarity?
There's a coulombic force
of attraction.
So the sodium will attract
the chlorine.
So let's put the two together.
So sodium attracts chlorine,
but it's not over.
There's more sodiums, there's
more chlorines.
So chlorine attracts sodium,
which attracts sodium.
Which could attract
more chlorines.
Which could attract
more sodiums.
And what do you see
happening here?
Well we're now starting with a
gas phase and we're forming
some giant atomic aggregate.
This is a giant ion aggregate.
What's the state of matter going
to be if this thing's
honking big?
It's going to be a solid.
We're going to form a solid
starting with those two gases:
sodium vapor and
chlorine vapor.
And what about the solid?
What do we see about the atomic
arrangement here?
All the sodiums are the same
size and all the chlorides are
the same size.
They have the same coulombic
forces of attraction in
between them.
Can you see that they're going
to form not just a solid, but
they're going to form a solid
consisting of atoms in a
regular array?
This is going to be
an ordered solid.
And there's a plain Anglo-Saxon
word for an
ordered solid, it's
called a crystal.
Now look at how far
we've come.
Look at how far we've come.
10 minutes ago we hadn't done
anything about chemical
reactivity.
We made one observation.
We had the tools of course.
We were ready.
We're ready for discovery.
We had the tools.
We had the average valence
electron energy.
We knew that helium, neon,
argon, krypton, xenon, radon,
et cetera, all have
this inertness.
We postulated that the octet
stability was some--
as a sweet spot.
And then we went with it.
And operating with that one
postulate, we get to electron
transfer and we're concluding
that if I take a really,
really good metal--
see, I can generalize
this now.
There's nothing peculiar about
sodium that makes it react
this way and no other element
will react this way.
I could replace sodium
with any other metal.
I could say any metal, any
metal will react with any
nonmetal in order to achieve
octet stability.
So now you can take big pieces
of the Periodic Table and
conclude that entire sets of
elements will act in this way
to form ionic compounds,
crystals, atoms of regular
periodicity.
That's a nice pun
there, right?
We started with the Periodic
Table and now we have periodic
spacing of atoms. I think
that's beautiful.
Obviously this view
is not shared by
the rest of the audience.
This crystal consists of ions.
It's held together by
what chemists like
to refer to as bonds.
This is called an ionic bond.
And what you're witnessing here
is ironic bonding via
electron transfer.
So we've now categorized the
first form of primary bonding.
Sodium chloride, magnesium
oxide--
ionic bonding.
And ionic bonding must give us
solids at room temperature.
If you've got ionic bonds and
you don't have a solid,
clearly you are at elevated
temperature where at elevated
temperature, the thermal energy
is disruptive enough to
break those bonds.
Those are very, very
strong bonds.
And what we're going to do next
day is look at the nature
of those bonds.
So I think I'm going
to stop the formal
lesson at this point.
And by the way, perhaps I failed
to mention on the first
day of the class.
What I'm doing is I'm lecturing
for, out of the 50
minutes, about 45 on
hard core topics.
And then, about the last 5,
7 minutes, I want to go to
something related to these--
chemistry in the
world around us.
That's why you see this
little break.
But it doesn't mean that
class is dismissed.
There's a difference between
changing topics
and dismissing class.
I don't know how people
confuse the two.
But somehow I've inadvertently
communicated to you evidently.
So we're not dismissing yet.
We're going to try to apply
this knowledge somehow.
So what I wanted to do was to
show you what happens with
these ionic solids
in commerce.
And in particular, I wanted to
talk about metallurgy and the
best form of metallurgy,
which is of course,
electrometallurgy.
Which is the kind of research
that I'm involved in.
And electrometallurgy
is quite pervasive.
The aluminum beverage can is
made by electrometallurgy, as
is magnesium.
So I'm going to talk to you a
little bit about magnesium.
This is a bar of magnesium.
Obviously, it's magnesium
because if it were steel, it
would be so massive that I
wouldn't be able to fling it
around like this.
The density of iron is 7.87,
the density of magnesium is
about 1.76.
It's less than 2.
It's 2/3 of the density
of aluminum.
It's lighter than aluminum.
Now perhaps some of
you've been told
that magnesium burns.
Well, let's see what happens.
There's a billet of magnesium
and this is
the key made of steel.
We're OK.
We're OK.
It's not going to burn.
That's a myth.
Magnesium in powder
form, magnesium in
ribbon form will burn.
The surface to volume ratio is
so high that the oxidation
generates a lot of heat.
But here the surface to
volume ratio is so
low that it's fine.
In fact, I went to school at
the University of Toronto.
And at the time the department
head was a man by the name of
Pigeon who invented a process
during World War II to make
magnesium cheaply.
And when he came into the
classroom to lecture on the
day that he would talk about
magnesium, in those days they
had lab benches at the front
of the class with sinks and
gas outlets.
And he would put the magnesium
billet on a stand and light a
Bunsen burner under it.
And of course, people in the
class sort of leaned back, and
he would lecture for 45 five
minutes with the Bunsen flame
burning the whole time.
And then towards the end he
would extinguish the flame,
let the billet cool.
And then pick it up and go back
to his office with it to
make the point.
All right, so how do we
make these metals?
Well, what we do is we reverse
the electron transfer step.
And we start with the cations
and anions, and we force the
electrons back onto the
cations anions.
Thereby, creating the
neutrals again.
This is called electrolysis.
So cation plus electron
gives us neutral.
Anion minus electron
gives us neutral.
So in the case of magnesium,
we start with magnesium
chloride, which dissolves and
forms magnesium cations and
chloride anions.
So, at elevated temperature--
in this case, at about 700
degrees Celsius, the ionic
solid becomes an ionic liquid.
And it's clear, colorless, and
has the fluidity of water.
And this is a very simple
schematic, so we've made the
cathode of steel negative and
on the negative electrode,
magnesium ions are reduced to
magnesium, which is a liquid
at this temperature.
And these are little bubbles
of magnesium
and they pool here.
And we collect them and
syphon them off.
And on the other electrode,
we're
making bubbles of chlorine.
And they rise and
we collect them.
So we reverse nature.
If you put chlorine in the
presence of magnesium it wants
to form magnesium chloride.
Here we do the reverse.
So electrolysis undoes the
spontaneous electron transfer
of ion formations.
So I decided I better get
something more authoritative
than my little cartoon.
So first I looked on the library
website and I found
that there was an article in
this volume of Advances in
Molten Salt Chemistry.
And there's this article here,
an authoritative article about
chemistry and electrochemistry
of magnesium production.
And so I decided to turn to the
article and let me read
the first paragraph of this
authoritative article.
A cubic kilometer--
what is a cubic kilometer?
It's a cube, 1 kilometer
on edge.
A cubic kilometer of sea water
contains approximately 1
million tons of magnesium.
1 million tons of magnesium as
the salt magnesium chloride
decahydrate dissolved
in sea water.
More than has ever been produced
in one year by all
the magnesium plants
in the world.
The world production of
magnesium right now is about
600 million tons.
No, furthermore, sea water
contains only 3.7% of the
total magnesium present in the
earth's crust. Clearly
magnesium resources are
ubiquitous and virtually
inexhaustible.
So when people tell you we've
got to recycle because we're
running out of resources,
I read this--
I don't know.
You might want to recycle for
other reasons, but you can't
make the case it's because
of scarcity.
This is magnesium that
has been made by this
electrolytic root.
And what's the value of this?
It can substitute for steel
in automobiles.
It's density is less than 2.
Steel is about 8.
Factor of 4 you lightweight the
vehicle, which means per
unit distance traveled,
less fuel consumed.
Fewer emissions.
And less fuel, which means less
dependence on imported petroleum.
So why aren't we doing this?
Why aren't we lightweighting our
vehicles with magnesium.
Because the bonds are so strong
the energy required to
make this metal is so high
that it's very costly.
If I give you a light bulb that
costs three times what
the other light bulb does and
burns only twice as long, you
won't buy it.
If it costs three times as much
and burns 10 times as
long, then you're willing
to pay the premium.
So what we do in my research
group, among other things, is
study the electrochemical
processes by which we make
these metals in order to make
the process more efficient.
Thereby reducing the cost,
making these lighter materials
more competitive.
Thereby, making the world a
better place by electron
transfer in service
of humanity.
And I invite you
to do the same.
But in order to do so,
you have to learn
the lessons of 3091.
Because after all, this is the
most important subject you
will take at MIT.
All right, we'll see
you on Friday.
