>> host: Remember the fireworks
we looked at earlier?
We were trying to figure out
how those fireworks
got their colors.
Now we're going to see
that fireworks get their colors
because of the movement
of a single electron.
In order to understand this
we have to know the properties
of electrons, their locations,
and how they move.
Let's rejoin our students
to see how they're doing
with these topics.
>> professor: Remember
JJ Thompson
and his model of the atom?
He's credited with the discovery
of the electron.
An electron is a tiny, tiny
particle with a negative charge
found outside of the nucleus
of an atom.
Electrons are really important
to properties of the elements,
and the bonding,
but we'll cover that
in a future unit.
Let's look at Bohr models
of several elements.
Please have your
periodic table nearby.
Remember Bohr is the guy
who determined the energy
levels of the atoms.
I created Bohr model of lithium
and beryllium
using these model kits,
but I could've drawn them too,
like you see on the monitor.
First, look at your
periodic table.
What patterns do you see
between the location
on the periodic table
and the models
of these different atoms?
Talk to each other
about what you see.
>> student: Lithium, beryllium,
and fluorine
all have two energy levels
and are on the second row
of the periodic table.
Is a row a periodic
or group?
>> student: I think the rows
are also called periods.
Sodium has three energy levels
and is on the third row.
>> student: Yeah,
but sodium and lithium
have one outside electron,
and they're in the same column.
>> student: They're both
in the first column,
And beryllium
has two outside electrons
and is in the second column.
>> professor: So, what did you
guys notice
about the periodic table
and the Bohr models?
>> student: Well,
lithium and sodium
both have one electron
on the outside energy level.
>> professor: Did you notice
anything else about
lithium and sodium?
>> student: I was going to say
they're both in the same column.
>> student: Yeah, they're both
in the same column, column one.
And they both only have one
electron on the outside,
but beryllium has two
electrons on the outside
and it's in column two.
>> professor: Good job
of linking the periodic table
to the Bohr models, Kat.
The outer-most electrons
are known as valence electrons.
Does the valence electron
pattern hold for fluorine?
>> student: Well, fluorine
has seven valence electrons
and is in group 17,
so the pattern doesn't work
for everything.
>> professor: But, group 17
can also be known as group 7A.
If you consider that,
does your pattern work?
>> student: Yes, it does.
If you ignore
the 10 transition metals,
then group 17
makes sense as seven.
>> professor: What other
patterns did you guys notice?
>> student: I see that
beryllium, fluorine, and lithium
only have two energy levels,
and they're in row two.
>> professor: So what
is the pattern?
>> student: Well, the row number
is the same as the number
of energy levels.
>> professor: Very good.
Use your model kits to create
a Bohr model of neon,
and you guys use your kits
to create a model of fluorine.
Start by looking
at your periodic table
to determine the number
of protons there,
and that will be the same
as the number of electrons.
Now that you have
your models built,
how many electrons
does neon need?
>> student: Neon has
ten electrons
because its atomic number is 10.
>> professor: And how many
of those electrons go into
the first energy level of neon?
>> student: Um, only two
of those electrons
are in the first energy level.
>> professor: How many electrons
will go into the first
energy level of fluorine?
>> student: Fluorine also has
two electrons on the first
energy level.
>> professor: What about
the second energy level?
>> student: Fluorine has nine
electrons and two go on
the first energy level,
and the seven left over
go on the second energy level.
>> professor: 'Kay, what about
neon's second energy level?
>> student: Neon has eight
electrons left
for the second energy level.
>> professor: Perfect.
Remember fluorine is in group
17, or 7A,
and has seven valence electrons.
Neon is in group 18, or 8A,
and has eight valence electrons.
And of course when you don't
have model kits like these,
it's really helpful
to still draw the models out.
Do you guys remember
that fluorine is a halogen,
and halogens are very reactive?
Neon is next to fluorine
and is a noble gas
and not reactive at all.
Look at these Bohr models
for fluorine and neon.
What differences do you see?
>> student: Fluorine and neon
have the same number
of energy levels and they
both have two electrons
in the first energy level.
>> student: But we're looking
for differences.
Fluorine has seven valence
electrons, and neon has 8.
>> professor: Why do you think
that difference makes neon
non-reactive,
but fluorine very reactive?
>> student: Wait,
I have a question.
What if fluorine
got one more electron?
Would be it be non-reactive?
>> professor: Good question.
No, it wouldn't.
Now let's talk about why.
What else would change
about fluorine if it got
another electron?
>> student: Fluorine would get
another proton, too.
>> professor: Actually,
and this is very,
very important,
atoms can gain and lose
electrons, but not protons.
If fluorine got another proton,
it wouldn't be fluorine anymore.
>> student: Aren't protons
positive and electrons negative?
If fluorine got another
electron then it would
have more negatives.
Can it do that?
>> professor: Another good
question!
And yes, it can.
This Bohr model of fluorine
that I added the electron to.
It would now have
a negative charge
and would be called an anion.
Atoms are generally neutral.
But sometimes an atom can gain
an electron and become
a negative ion.
Some atoms can lose an electron
and become a positive ion.
An ion is an atom with
a positive or negative charge.
Positive ions
are called cations.
You can remember
they're positive
because the T in cat
looks like a plus sign.
The negative charged ions
are called anions.
Ions have different properties
than their neutral atom,
like how easily they react
with other chemicals
or how easily they dissolve.
Fluoride, is still reactive
once it becomes an anion.
>> student: Why would fluorine
do that?
Isn't it better to be neutral?
>> professor: Let's look back
at our Bohr model
to answer your question.
Now, how are these two models
alike?
>> student: They both have eight
electrons in the outer-most
energy level.
>> professor: Yes, they do.
Eight electrons in that
energy level is a full
valence shell,
which is very stable.
This is why neon atoms
are non-reactive.
Noble gases all have
full valence shells.
Non-noble gases can gain
or lose electrons
to have full valence shells
because it's more stable.
>> student: But why would atoms
lose or gain electrons?
>> professor: Atoms that don't
have a full valence shell
are reactive,
which means they're likely
to change by gaining
or losing electrons
or by bonding with other atoms.
Only atoms that have a full
valence shell are stable
and not likely to change.
Let's take a look at magnesium
to see an example
of this change.
How many valence electrons
do you see for magnesium?
>> student: Two
valence electrons
because it's in group two.
>> professor: And what would
have to happen
for it to have a full
valence shell?
>> student: It would need
six more electrons.
>> student: Or lose two.
>> professor: Okay, nature takes
the easiest, lowest energy path.
So which of those options
do you think is more likely?
>> student: It would be easier
to lose two electrons
than gain six.
>> professor: Okay,
so what would the charge be?
>> student: It would be
positive.
>> professor: That's right,
but how much positive
if two electrons are removed.
>> student: It would have
two extra positives.
>> professor: So would that
make it a cation or an anion?
>> student: It would be a cation
since it's a positive charge.
>> professor: Nice work.
>> host: Electrons can do more
than move into
and out of an atom.
The movement that creates ions
is not what gives fireworks
their color.
Electrons are not stuck
in a specific energy level,
and they're not orbiting
around the nucleus
like planets orbit the sun.
Electrons are usually found
in their lowest energy state,
which is called
the ground state.
Remember what
our students learned.
nature takes the lowest energy
path, which is why electrons
are normally in their
ground state.
But electrons can gain
and give off specific amounts
of energy, and if electrons
give off enough energy,
they can move
to a higher energy level,
called an excited state.
these units of energy
are called quanta,
the singular form of which
is quantum.
When electrons absorb a quantum
of energy, they can make
what's called, "a quantum leap,"
to an excited state
where they're farther away
from the nucleus.
Then the electrons can give off
that same amount of energy
as they fall back down
to the ground state,
closer to the nucleus
where they are more stable.
When we light fireworks
we're adding energy
to the fireworks.
Some of that energy
causes the electrons
to jump to an excited state.
Higher energy states
are not very stable,
so the electrons fall back
to the ground state,
giving off the quantum of energy
as the colored light we see.
Each element gives off
a different range of colors
because each element has
a different number of electrons.
Calculations can be performed
to determine the amount
of energy and the type of light
that's given off.
If you'd like more help
with this, check out
our Closer Look video,
"Calculating light and waves."
We can observe
the different colors of light
that are given off
through a flame test
where you put various metal
salts into a flame
and observe the different colors
of light.
In a flame test,
what you're actually observing
is the electron falling
from the excited state
to the ground state,
and these different colors
of light are what we see
when we're watching fireworks.
So let's get back
to our classroom to see
how our students do
with their flame tests.
>> professor: We've talked a lot
about electrons,
and this flame test lab
is a great way to see
the effects of electrons moving
among different energy levels.
It'll be kind of like our own,
much quieter, fireworks display.
So, look at the lab supplies
on your table.
You have cotton swabs,
some distilled water,
and spot plates with different
ionic salts in them.
Most of these samples
use nitrate as the anion.
What are some of the cations
you see?
>> student: Copper.
>> student: Lithium.
>> student: Sodium.
>> student: Potassium.
>> professor: Good.
What type of elements
are all of these cations?
>> student: They're all metals.
>> professor: Exactly,
all of your samples
have different metal ions.
What you're going to do
with each sample,
is dip the cotton swab
in the distilled water,
shake it off a little,
and then dip the end into
the salt sample you're testing.
You only need a little bit
of the sample.
If you have too much
on the cotton swab,
it might fall onto the burner
and kinda junks up the burner.
Hold the salt against the side
of the flame
and watch what happens.
Why do you think we're dipping
the cotton swab in water?
Okay.
>> student: So the salt
will stick to it?
>> student: Does it keep
the cotton swab from burning?
>> professor: Both of you
are right.
If you leave the cotton swab
in the flame too long,
or if you stick it too far
in the flame, it'll burn
and it will affect the color
of the flame.
Before you try your first
sample, do a control test
of the wet cotton swab.
Leave it in the flame
until it starts to burn
so you can see what color
the flame is.
Do not turn it into
a mini-tiki torch.
Do not turn any of these
into tiki torches.
Once the sample is tested,
place the hot cotton swab
on the table to cool
before you throw it away.
Make sure you record
your observations
for each sample.
You're recording qualitative
data, so be very descriptive.
Red might not be
descriptive enough.
What if there is more than one
red colored flame?
You're going to use your data
to help you to identify
these unknowns later.
Okay, give it a try.
Be safe,
and I'll be right here
if you have any questions.
>> student: Cool, a red flame.
>> student: I think that's more
like hot pink.
I'll write it down as magenta.
Do you think that'll be
specific enough?
>> student: I think so.
I hope so.
Now let's try copper.
>> student: Wow,
that's bright green.
I wonder if its always
the same color.
Let's do that one again.
>> student: Why would it change
colors?
If the electrons are excited
to the same energy levels
and then fall back,
shouldn't it be the same color?
Because it's the same energy?
>> student: That makes sense,
but let's try it again
and make sure.
Cool, still green.
>> student: I think the sodium
is a yellow/orange color.
Or do you think that's
the cotton swab?
>> student: I think its more
yellow than the cotton swabs.
Let's record it as yellow
and gold.
>> student: Okay,
let's try potassium.
>> student: That looked
like lavender.
But the color disappears
so quickly.
>> student: I saw lavender, too.
>> professor: Now that you've
collected your data.
I want you to use it to identify
these two different unknowns.
Make sure you record
your observations
of the unknowns, too.
Mia, will you get the gas
for me?
So, this is unknown number one.
>> student: A red flame,
but it was a purplish red.
I think its lithium.
>> student: I'd call that
magenta,
but I think it's lithium also.
>> student: There's too much
pinkish-ness
for it to be strontium,
which was the only other
red sample we saw.
>> professor: Okay,
here's unknown number two.
Will you turn the flame off
for me?
What do you guys think?
>> student: Copper is in that
sample,
I saw the bright green flame,
but I also saw red.
>> student: Me, too.
Can there be a mixture
of unknowns?
>> professor: What does
your evidence indicate?
>> student: I think this
is a mixture,
of copper, for sure,
and strontium I think.
>> student: I think so too.
I saw the red and green,
but the red wasn't a purply/red
like the lithium sample.
>> student: Copper
and strontium.
Are we right?
>> student: You're not
going to tell us, are you?
>> professor: No, I'm not.
Does your evidence
support your conclusion?
Did you collect
your data properly?
>> student: Yes and yes.
>> professor: Can you think
of any reasons
why your data might be flawed?
>> student: I wonder about
the mixture.
In art we talked about
how some colors
cancel each other out.
What if the green or the red
canceled something else out?
Or what if what we saw
was a combination
of two different salts,
and we missed them both?
>> professor: Those are
some great points.
And that is a flaw
in using this type of test
to identify mixed samples.
Now, I have a question for you:
why did you see these colors?
Why wasn't copper purple
instead of green?
>> student: Well,
different colors of light
have different amounts
of energy.
>> student: And Bohr's research
said that each energy level
in each different element has
a different amount of energy.
>> student: So, the colors
have to do with
the energy differences
of the different energy levels
in atoms.
>> student: When the electrons
get excited by the flame,
they jump up energy levels
and when they fall back
to the ground state,
the energy is given off.
>> student: So that's why copper
is green.
It's the energy of the electron
moving energy levels,
and that's why copper
is always green,
because that electron
moves between the same levels.
>> professor: Great answers.
If we took the light
from our flame tests
and directed it through a prism,
we'd get a spectrum of lines
that correspond
to that specific element.
When we think of light
going through a prism,
we often think of a rainbow
of color being produced.
But that's not what we see
from the colored light
of a flame test.
This is called a spectroscope
and it provides a way to see
the spectrum of a chemical.
We use it to look at an element,
such as the mercury
in this sealed tube.
Look through it.
What do you see?
>> student: I just see
a bunch of lines
that are different colors.
>> student: Me too.
>> professor: Now look
at this sample.
This is neon.
>> student: Still lines,
but not the same lines
as with mercury.
>> professor: Each element
produces
a characteristic pattern
of lines in its spectrum.
The lines correspond
to certain wavelengths of light,
and therefore certain amounts
of energy.
As the electrons move
from one energy level to another
in each of the samples.
The spectroscope separates
those wavelengths
from each other and gives us
a fingerprint of sorts
for each of the elements.
These are spectre for three
of the different flame test
elements you tested:
Beryllium, copper,
and strontium.
You can see how the spectre
are different for each.
Using the identified samples,
can you identify the unknown?
>> student: That's easy,
it's strontium.
>> professor: Right.
Strontium and the unknown
each have the same
spectral lines.
Good job.
>> host: So, we've learned a lot
about electrons in this lesson.
The periods, or rows
of the periodic table correspond
to the number of energy levels
in an atom.
The family or column
correspond to the number
of valence electrons.
Electrons can be added to atoms
to make anions,
or removed to make cations.
And we learned
that with enough energy,
electrons can move energy levels
within an atom and give us this.
Great work!
There's even more information
waiting for you
on the Periodic Table.
Join us for the next video
in the playlist.
We'll learn even more
about the Periodic Table
when we study periodic trends.
