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We were talking about the
energies of the various states

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in a multi-electron atom.
And the question is,

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how do we know what these
energies are,

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experimentally?
And the technique that we use

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to know those energies is
something called photoelectron

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spectroscopy.
In principle,

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it is the same kind of
spectroscopy as when we talked

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about photoemission from a
solid, the photoelectron effect

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explained by Einstein.
And, by the way,

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I put on the website an article
that is really very clear about

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what Einstein's contributions
were.

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In particular,
in explaining the photoelectric

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effect.
And it has some sociology in

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it, too.
It is a really easy-to-read

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article.
I encourage you to take a look

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at it as a study break sometime.
Anyway.

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The idea here is the same,
the photoelectron spectroscopy

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off of atoms and molecules.
It is the same thing as the

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photoelectron emission from a
solid, in that you send in a

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photon and that causes an
electron to be ejected.

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But, unlike the solid where we
essentially had just one state

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at a particular energy,
in an atom or a molecule,

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we have many different states
with many different energies.

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And what we typically do,
then, is we send in a photon

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that will be able to ionize all
of those electrons.

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That is, it is a photon with
enough energy to rip off even

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the most strongly bound
electron.

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And so, that photon is
typically an X-ray photon.

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For example,
we take this unsuspecting neon

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atom, and we bring in an X-ray
photon and make neon plus.

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And one of the electrons that

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can come off is,
of course, the 2p electron,

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here, so that the ion
configuration is

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1s 2 2s 2 2p 5.
And then, of course,

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another possible electron to be
pulled off is that 2s electron.

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The electron configuration is
1s 2 2s 1 2p 6.

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And then, finally,

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if the photon has enough
energy, we can pull off the 1s

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electron, to make the 1s 1 2s 2
configuration,

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2s 2p6 configuration.

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And what we then do in this
technique is we measure the

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kinetic energies of all the
electrons that come off.

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We measure the kinetic energies
and disperse them so that we get

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a plot of the number of
electrons with a particular

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kinetic energy.
The plot is what I show you

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here, number of electrons versus
the kinetic energy.

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And what you see is that we
have three kinetic energies that

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show up.
We have electrons with three

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different kinetic energies.
We have three different kinetic

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energies because in neon,
we had three states with

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different energies.
So, for example,

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this energy,
384 eV, that corresponds to the

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electrons that were pulled off,
electrons that were in that 1s

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state.
That is the most strongly bound

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state.
For example,

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if this is the energy of our
incident photon,

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and then this is the ionization
energy, which is going to be

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largest for the 1s electron
because it is most strongly

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bound, well, then,
the energy difference between

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the incident energy and that
ionization energy is the kinetic

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energy of the electron.
That is the leftover energy

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that goes into the kinetic
energy of the electron.

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That is going to be smallest
because that 1s electron is most

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strongly bound.
Then we have this kinetic

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energy here, electrons with that
kinetic energy were electrons

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sitting in the 2s state.
That has higher kinetic energy

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because they are less strongly
bound.

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This is the energy of the
incident photon.

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This is the ionization energy
of that 2s electron.

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That leftover energy,
then, is that kinetic energy.

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And then, finally,
this is the feature that

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represents the electrons in the
2p state.

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They are least strongly bound.
If this is the incident energy,

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this is that ionization energy
for the 2p state.

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All of this energy is leftover
to go into kinetic energy of

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that 2p electron moving away
from the atom.

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And so, in general,
as the kinetic energy goes up,

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the binding energy here is
getting less and less negative.

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It is getting weaker and
weaker.

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Or, in other words,
as the kinetic energy goes up,

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the binding energy is getting
more negative in this direction.

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Very quickly,
if you look at a photoelectron

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spectrum and you see three lines
like this, you know you have

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three states at three different
energies.

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And you always know that the
highest kinetic energy means the

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least strongly bound electron,
the lowest kinetic energy means

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the most strongly bound
electron.

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And then, we can use these
results to actually calculate

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the binding energies of the
electrons by conservation of

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energy.
And this you have to know for

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the exam.
Incident energy is that

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ionization energy or the work
function, in the case of the

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solid, plus the kinetic energy.
Or, I can turn this around.

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I can solve for the ionization
energy.

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That will be the incident
energy, which is right here.

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This is the energy of that
X-ray photon,

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1,253 eV, minus the kinetic
energies that we actually

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measure, which are going to give
us those ionization energies for

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each one of the electrons in
their corresponding state.

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And then, once we have those
ionization energies,

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it is easy to turn it into a
binding energy because that

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ionization energy is equal to
minus that binding energy.

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For example,
since the ionization energy,

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the 1s electron,
is 870 eV, the binding energy

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of that 1s electron is minus
eV.

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And I also want you to notice
just how much more strongly

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bound that electron in the 1s
state is compared to the

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electrons in the n equals 2
shell.

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That is because that n equals 1
shell is much closer in to the

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nucleus, where that attractive
interaction is much stronger.

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And, therefore,
this electron is much more

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strongly bound than those
electrons in the n equals 2

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state.
Of course, 2s is lower in

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energy than 2p,
and that is important.

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This is because in the 2s
state, you have a finite

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probability of being really
close to the nucleus.

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That is what makes that 2s a
little bit lower in energy than

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the 2p.
Now, what we are going to do is

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I am going to start lecturing
about trends in the Periodic

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Table, and this is where the
questions come for your section.

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Oh, okay.
Do you have any extra paper?

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Not much.
Well, the way you were going to

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be able to answer a question for
your recitation was by making a

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paper airplane and launching it
to the blackboard after I say

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launch.
And the first paper airplane to

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hit the blackboard,
well, you will have the

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opportunity to answer the
question.

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Your recitation will have that
opportunity to answer the

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question.
And we will keep score.

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Now, we are kind of running out
of extra paper.

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You can take the one page that
asks for the seating plan that

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you will not need again,
as a piece of paper to make an

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airplane.
Do you have any other paper?

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Here are a couple of extra
pages.

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Also, if you need some,
there are some paper airplanes

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up here.
You are welcome to take those.

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Okay.
Some of you are more

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strategically placed than
others, and so you may want to

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designate someone in your
recitation section as a launcher

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and send them down here so that
everybody gets an equal chance.

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That is fine with me.
And, as I said,

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if anybody wants a paper
airplane.

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Oh, also the other thing,
write your recitation section

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number on the airplane so that I
can identify which airplane and

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what section it comes from.
And they have to be airplanes.

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They cannot be wads.
And here are some extra

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airplanes if you want.

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You can take some and pass them
back if you want.

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While you are doing that,
I am going to start lecturing

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here about trends in the
Periodic Table.

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And, of course,
it is these electron

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configurations that allow us to
understand the trends in the

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Periodic Table.
But you should realize that the

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Periodic Table was initially put
together by Mendeleev and others

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in 1850-1870.
And it was put together on the

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basis of the similarity of the
chemical and physical properties

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of the elements.
For example,

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lithium, sodium,
potassium, they were put in the

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same column because those
elements are all soft,

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malleable elements that were
very reactive.

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Helium, neon,
argon were put in the same

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column because they are all
atoms that are very unreactive.

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And, of course,
today, we understand why their

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properties are what they are.
And we understand it on the

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basis of the electron
configurations.

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Lithium, sodium,
potassium, we have this extra

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valence electron in the s state
that makes it very reactive.

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Helium, neon,
argon are very unreactive

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because they have this closed
shell, this inner gas

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configuration.
But really, by the late 1800s

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and early 1900s,
the similar chemical

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properties, as you go down a
column of the Periodic Table,

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was really very firmly believed
to an extreme extent.

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So strongly,
that it was known that,

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and, of course,
human beings ate salt and that

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the body had sodium ions in it
and potassium ions in it,

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if that is the case,
if you can consume sodium and

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potassium, why not a little
lithium?

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In 1925 or so,
there was marketed a soft

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drink, and this soft drink
wanted a lemon-lime flavor to

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it.
To get that lemon-lime flavor

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they needed a little bit of
citric acid, but that is not

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very soluble in water.
To make it soluble you make it

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a salt.
And, for whatever reason,

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they used lithium citrate.
Why not?

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Let's use some lithium citrate.
And that soft drink was none

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other than 7-Up.
Honestly.

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And, in fact,
the benefits of lithium were

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touted.
Lithium was claimed to give the

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beverage healthful benefits,
an abundance of energy,

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enthusiasm, clear complexion,
lustrous hair,

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lustrous eyes,
shinning eyes,

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and you have got to drink this
stuff.

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It was the market for 25 years
until the early 1950s,

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when the antipsychotic
properties of lithium were

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beginning to be noticed,
along with the severe side

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effects of lithium.
And it was then taken off the

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market, or at least taken off
the market with the citrate as

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the lithium salt in it.
And it was really only another

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20 years, 1970,
before lithium was actually

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00:14:32,000 --> 00:14:36,000
approved as a drug for the
treatment of antipsychotic

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behavior.
I don't know how many

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individuals suffered the side
effects of lithium from drinking

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too much 7-Up,
but there is a moral to this

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story.
And the moral,

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00:14:49,000 --> 00:14:53,000
of course, is that even though
we are about to talk about the

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general trends along a column of
a Periodic Table,

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00:14:56,000 --> 00:15:00,000
don't put any element in your
mouth just because it is in the

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00:15:00,000 --> 00:15:06,000
same column of the Period Table
as some element you do eat.

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00:15:06,000 --> 00:15:11,000
This is a for real story.
One of the properties we are

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00:15:11,000 --> 00:15:15,000
going to talk about is the
ionization energy.

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00:15:15,000 --> 00:15:21,000
And we have already used the
word ionization energy quite a

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00:15:21,000 --> 00:15:27,000
lot, but let me just formalize
the definitions here.

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00:15:27,000 --> 00:15:30,000
For example,
we have a boron atom being

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00:15:30,000 --> 00:15:32,000
ionized.
The energy difference between

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00:15:32,000 --> 00:15:37,000
the products and the reactions
here is that ionization energy.

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00:15:37,000 --> 00:15:42,000
But this ionization energy is
minus the binding energy of the

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00:15:42,000 --> 00:15:46,000
2p electron in boron.
That is what that ionization

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00:15:46,000 --> 00:15:49,000
energy is.
We are ripping off the least

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00:15:49,000 --> 00:15:53,000
strongly bound electron.
And when we rip off the least

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00:15:53,000 --> 00:15:57,000
strongly bound electron,
we call that the first

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00:15:57,000 --> 00:16:02,000
ionization energy.
That is the energy to remove

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00:16:02,000 --> 00:16:06,000
the electron from the highest
occupied atomic orbital.

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00:16:06,000 --> 00:16:10,000
That is the same idea,
the highest occupied atomic

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00:16:10,000 --> 00:16:14,000
orbital, that is the same idea
as the least strongly bound

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00:16:14,000 --> 00:16:17,000
electron.
However, most of the time,

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00:16:17,000 --> 00:16:22,000
when we talk about ionization
energy, we don't use the word

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00:16:22,000 --> 00:16:25,000
first.
If you see a list of ionization

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00:16:25,000 --> 00:16:29,000
energies, if you see a table of
ionization energies,

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00:16:29,000 --> 00:16:33,000
they don't say first.
That first is implied.

235
00:16:33,000 --> 00:16:36,000
Now, of course,
there are other kinds of

236
00:16:36,000 --> 00:16:39,000
ionization energies.
For example,

237
00:16:39,000 --> 00:16:43,000
the second ionization energy.
We can keep going here.

238
00:16:43,000 --> 00:16:47,000
We can take boron plus,
rip off the next least strongly

239
00:16:47,000 --> 00:16:51,000
bound electron to make boron
plus 2.

240
00:16:51,000 --> 00:16:55,000
That energy change is what we
define as a second ionization

241
00:16:55,000 --> 00:16:58,000
energy.
That second ionization energy

242
00:16:58,000 --> 00:17:02,000
is minus the binding energy of
the 2s electron in boron plus.

243
00:17:02,000 --> 00:17:04,000
And we can keep going.

244
00:17:04,000 --> 00:17:08,000
There is a third ionization
energy, taking another electron

245
00:17:08,000 --> 00:17:11,000
from boron plus 2 to
boron plus 3.

246
00:17:11,000 --> 00:17:14,000
And there is a fourth
ionization energy,

247
00:17:14,000 --> 00:17:17,000
boron plus 3 to boron
plus 4.

248
00:17:17,000 --> 00:17:20,000
And a fifth ionization energy,
boron plus 4,

249
00:17:20,000 --> 00:17:23,000
boron plus 5,
all the way down.

250
00:17:23,000 --> 00:17:25,000
And now we are the bare
nucleus.

251
00:17:25,000 --> 00:17:28,000
That is our definition of
ionization energy,

252
00:17:28,000 --> 00:17:32,000
second, third,
fourth, and fifth.

253
00:17:32,000 --> 00:17:38,000
Now comes the first question,
and the question is going to be

254
00:17:38,000 --> 00:17:43,000
on the side boards here.
We have this reaction.

255
00:17:43,000 --> 00:17:50,000
This is boron plus
going to boron plus 2.

256
00:17:50,000 --> 00:17:55,000
What we are doing here is
removing a 2s electron.

257
00:17:55,000 --> 00:18:02,000
[LAUGHTER] Remember about the

258
00:18:02,000 --> 00:18:08,000
fool aiming for their chemistry
professor?

259
00:18:16,000 --> 00:18:20,000
Look at the second reaction
here, boron to boron plus.

260
00:18:20,000 --> 00:18:23,000
Again, we are pulling off a 2s

261
00:18:23,000 --> 00:18:26,000
electron.
And the question is,

262
00:18:26,000 --> 00:18:30,000
are these two energies equal?
Now, the way we are going to

263
00:18:30,000 --> 00:18:34,000
answer it is after I get out of
the way, I am going to say

264
00:18:34,000 --> 00:18:36,000
launch.
You are going to launch.

265
00:18:36,000 --> 00:18:39,000
And the first paper airplane to
hit the blackboard,

266
00:18:39,000 --> 00:18:42,000
we are going to look at that
recitation number and you are

267
00:18:42,000 --> 00:18:44,000
going to have a chance to
answer.

268
00:18:44,000 --> 00:18:47,000
Are you ready?
Launch.

269
00:18:55,000 --> 00:19:00,000
Recitation four.
Are these two energies equal?

270
00:19:00,000 --> 00:19:02,000
That is you.

271
00:19:08,000 --> 00:19:10,000
Are these two energies equal?
No.

272
00:19:10,000 --> 00:19:13,000
Final answer?
Yes, they are right.

273
00:19:13,000 --> 00:19:19,000
One point for recitation four.
These energies are not equal.

274
00:19:19,000 --> 00:19:22,000
Why?
Well, because this ionization

275
00:19:22,000 --> 00:19:25,000
energy is the second ionization
energy. It is

276
00:19:25,000 --> 00:19:31,000
minus the binding energy of
the 2s electron in boron plus.

277
00:19:31,000 --> 00:19:36,000
This energy is the ionization

278
00:19:36,000 --> 00:19:39,000
energy of the 2s electron in
boron.

279
00:19:39,000 --> 00:19:46,000
It is minus the binding energy
of the 2s electron in boron.

280
00:19:46,000 --> 00:19:49,000
They are not equal.
Next question.

281
00:19:49,000 --> 00:19:54,000
Which one of these binding
energies is greater?

282
00:19:54,000 --> 00:19:58,000
Wait.
I am getting out of the way.

283
00:19:58,000 --> 00:20:02,000
Are you ready?
In position.

284
00:20:02,000 --> 00:20:04,000
Launch.

285
00:20:09,000 --> 00:20:12,000
Recitation eight.
Where are you,

286
00:20:12,000 --> 00:20:16,000
eight?
You are over here.

287
00:20:16,000 --> 00:20:20,000
Eight does not want to identify
itself.

288
00:20:20,000 --> 00:20:24,000
All right.
Which one is greater,

289
00:20:24,000 --> 00:20:27,000
top or bottom?
Top.

290
00:20:27,000 --> 00:20:32,000
Final answer?
Boy, you are right.

291
00:20:32,000 --> 00:20:36,000
Top one is greater.
Recitation eight.

292
00:20:36,000 --> 00:20:43,000
It is greater because the
effective charge in boron plus

293
00:20:43,000 --> 00:20:50,000
is larger than it is in boron.
There are fewer electrons for

294
00:20:50,000 --> 00:20:55,000
shielding.
The effective charge is larger.

295
00:20:55,000 --> 00:21:02,000
That binding energy is greater.
Or, that ionization energy

296
00:21:02,000 --> 00:21:06,000
here, the way I write it,
is greater.

297
00:21:06,000 --> 00:21:10,000
Fantastic.
Now, we are going to look at

298
00:21:10,000 --> 00:21:17,000
some trends in the ionization
energy along the Periodic Table.

299
00:21:17,000 --> 00:21:22,000
The first question is,
here is the Periodic Table,

300
00:21:22,000 --> 00:21:28,000
as we go across it,
what happens to that ionization

301
00:21:28,000 --> 00:21:31,000
energy?
Wait.

302
00:21:31,000 --> 00:21:36,000
Are we in position?
Ready?

303
00:21:36,000 --> 00:21:39,000
Launch.

304
00:21:45,000 --> 00:21:49,000
Recitation five.
What happens to the ionization

305
00:21:49,000 --> 00:21:54,000
energy as you go across the
Periodic Table?

306
00:21:54,000 --> 00:21:57,000
Is that five?
Is that your answer,

307
00:21:57,000 --> 00:21:59,000
five?

308
00:22:05,000 --> 00:22:11,000
What happens to the ionization
energy as you go across the

309
00:22:11,000 --> 00:22:15,000
Periodic Table from here to
here?

310
00:22:15,000 --> 00:22:18,000
It increases.
You are right,

311
00:22:18,000 --> 00:22:22,000
recitation five.
Here is the trend.

312
00:22:22,000 --> 00:22:26,000
Here is row one,
from hydrogen to helium.

313
00:22:26,000 --> 00:22:32,000
Here is row two,
from lithium to neon.

314
00:22:32,000 --> 00:22:35,000
It increases.
It increases because,

315
00:22:35,000 --> 00:22:41,000
as you go across the Periodic
Table here, Z increases.

316
00:22:41,000 --> 00:22:45,000
The nuclear charge increases.
But, of course,

317
00:22:45,000 --> 00:22:50,000
there is another parameter
here, and that is r.

318
00:22:50,000 --> 00:22:55,000
But, as you go across the
Periodic Table right here,

319
00:22:55,000 --> 00:23:00,000
you are putting electrons into
the same shell,

320
00:23:00,000 --> 00:23:05,000
essentially.
You are putting those electrons

321
00:23:05,000 --> 00:23:09,000
into shells that have,
roughly speaking,

322
00:23:09,000 --> 00:23:12,000
the same distance of r from the
nucleus.

323
00:23:12,000 --> 00:23:18,000
And so the factor that wins out
is Z, the effective charge here.

324
00:23:18,000 --> 00:23:23,000
And the ionization energy as
you go across then increases.

325
00:23:23,000 --> 00:23:27,000
But you also see that there are
some glitches.

326
00:23:27,000 --> 00:23:33,000
You see that boron is a little
bit lower energy than --

327
00:23:38,000 --> 00:23:42,000
What did I want to say here?
I'm sorry.

328
00:23:42,000 --> 00:23:46,000
I am looking at it from the
side.

329
00:23:46,000 --> 00:23:51,000
Here is the electron
configuration of beryllium.

330
00:23:51,000 --> 00:23:56,000
Here it is, boron,
Z equals 5.

331
00:24:02,000 --> 00:24:04,000
What you see,
here, is that in boron,

332
00:24:04,000 --> 00:24:09,000
you have to put that extra
electron into the 2p state,

333
00:24:09,000 --> 00:24:11,000
which requires some more
energy.

334
00:24:11,000 --> 00:24:16,000
The bottom line is that the
nuclear charge does not increase

335
00:24:16,000 --> 00:24:20,000
fast enough to compensate
completely for the extra energy

336
00:24:20,000 --> 00:24:24,000
that you need to have to get to
this 2p state.

337
00:24:24,000 --> 00:24:28,000
Boron has a little bit lower
ionization energy than

338
00:24:28,000 --> 00:24:32,000
beryllium.
And to see another glitch,

339
00:24:32,000 --> 00:24:37,000
that other glitch is between
nitrogen and oxygen.

340
00:24:37,000 --> 00:24:43,000
Again, we can understand that
glitch by the electron

341
00:24:43,000 --> 00:24:46,000
configuration.
Here is nitrogen,

342
00:24:46,000 --> 00:24:49,000
Z equals 7.
Here is oxygen,

343
00:24:49,000 --> 00:24:53,000
Z equals 8.
And the bottom line here is

344
00:24:53,000 --> 00:24:57,000
that in oxygen,
you have to put an electron in

345
00:24:57,000 --> 00:25:04,000
a state in which there already
is an electron.

346
00:25:04,000 --> 00:25:06,000
That is a repulsive
interaction.

347
00:25:06,000 --> 00:25:11,000
Again, the increase in the
nuclear charge just is not large

348
00:25:11,000 --> 00:25:16,000
enough to fully compensate for
that repulsive interaction.

349
00:25:16,000 --> 00:25:21,000
We can understand those little
glitches in terms of these

350
00:25:21,000 --> 00:25:24,000
electron configurations.
And, in general,

351
00:25:24,000 --> 00:25:29,000
here, that ionization increases
as we go across the Periodic

352
00:25:29,000 --> 00:25:32,000
Table.
Here is the third row.

353
00:25:32,000 --> 00:25:36,000
You also see there are glitches
in the third row.

354
00:25:36,000 --> 00:25:41,000
You see there is a glitch
between magnesium and aluminum.

355
00:25:41,000 --> 00:25:45,000
They are right underneath
beryllium and boron.

356
00:25:45,000 --> 00:25:48,000
A glitch between phosphorous
and sulfur.

357
00:25:48,000 --> 00:25:51,000
They are right underneath
nitrogen and oxygen.

358
00:25:51,000 --> 00:25:55,000
For the same reason,
there is a glitch at those

359
00:25:55,000 --> 00:26:01,000
elements in the second row.
Now, the next question.

360
00:26:01,000 --> 00:26:09,000
As we go down a column of the
Periodic Table,

361
00:26:09,000 --> 00:26:16,000
what happens to the ionization
energy?

362
00:26:16,000 --> 00:26:18,000
Wait.
Ready?

363
00:26:18,000 --> 00:26:20,000
Set?
Launch.

364
00:26:20,000 --> 00:26:26,000
Five.
What happens to the ionization

365
00:26:26,000 --> 00:26:30,000
energy?
What?

366
00:26:30,000 --> 00:26:33,000
Decreases.
You are right,

367
00:26:33,000 --> 00:26:37,000
recitation five.
Why does it decrease?

368
00:26:37,000 --> 00:26:43,000
It decreases because?
Well, it decreases because you

369
00:26:43,000 --> 00:26:50,000
are putting electrons into
shells that are farther and

370
00:26:50,000 --> 00:26:57,000
farther away from the nucleus.
Yes, as you go down the column,

371
00:26:57,000 --> 00:27:02,000
Z increases.
But it does not increase fast

372
00:27:02,000 --> 00:27:07,000
enough as r increases.
Remember there are the two

373
00:27:07,000 --> 00:27:11,000
factors, Z and r.
Although Z increases,

374
00:27:11,000 --> 00:27:17,000
you are putting the electrons
into shells that are farther

375
00:27:17,000 --> 00:27:21,000
from the nucleus.
Therefore, that attractive

376
00:27:21,000 --> 00:27:26,000
interaction does go down,
and the ionization energy goes

377
00:27:26,000 --> 00:27:29,000
down.
Great.

378
00:27:29,000 --> 00:27:33,000
Now, we are going to talk about
another property.

379
00:27:33,000 --> 00:27:38,000
That other property is called
the electron affinity.

380
00:27:38,000 --> 00:27:42,000
For example,
if I take chlorine and add an

381
00:27:42,000 --> 00:27:46,000
electron to it to make chlorine
minus.

382
00:27:46,000 --> 00:27:51,000
There is an energy change here.
That energy change,

383
00:27:51,000 --> 00:27:56,000
delta E, is equal to minus
kilojoules per mole.

384
00:27:56,000 --> 00:28:02,000
That is, the fact that this

385
00:28:02,000 --> 00:28:08,000
energy change is negative,
this tells you that the

386
00:28:08,000 --> 00:28:13,000
chlorine ion is more stable than
the neutral atom.

387
00:28:13,000 --> 00:28:19,000
Now, in terms of an equation,
what is the definition here of

388
00:28:19,000 --> 00:28:24,000
the electron affinity?
That is the next question.

389
00:28:24,000 --> 00:28:29,000
Are you ready to launch?
Launch.

390
00:28:34,000 --> 00:28:36,000
Recitation eight.
Where are you?

391
00:28:36,000 --> 00:28:39,000
You are right here.
No, you are right there.

392
00:28:39,000 --> 00:28:43,000
Recitation eight,
what is the definition of the

393
00:28:43,000 --> 00:28:45,000
electron affinity?

394
00:28:50,000 --> 00:28:55,000
TAs cannot help.
Energy required to add an

395
00:28:55,000 --> 00:28:59,000
electron, that is --

396
00:29:09,000 --> 00:29:12,000
Recitation eight,
you are right.

397
00:29:12,000 --> 00:29:16,000
It is minus delta E.
Fantastic.

398
00:29:16,000 --> 00:29:20,000
This is the delta E for the
reason.

399
00:29:20,000 --> 00:29:25,000
The electron affinity is minus
that quantity.

400
00:29:25,000 --> 00:29:30,000
The electron affinity,
then, of chlorine is

401
00:29:30,000 --> 00:29:36,000
kilojoules per mole.
Now, unlike the ionization

402
00:29:36,000 --> 00:29:42,000
energy, the electron affinity
can be positive or negative.

403
00:29:42,000 --> 00:29:47,000
For example,
if you try to stick an electron

404
00:29:47,000 --> 00:29:52,000
on a nitrogen atom to make N
minus,

405
00:29:52,000 --> 00:29:56,000
delta E for that reaction is
positive.

406
00:29:56,000 --> 00:30:02,000
It is 7 kilojoules per mole.
And so that electron is not

407
00:30:02,000 --> 00:30:06,000
going to stick onto the
nitrogen.

408
00:30:06,000 --> 00:30:11,000
The electron affinity in that
case is negative.

409
00:30:11,000 --> 00:30:14,000
It is minus 7 kilojoules per
mole.

410
00:30:14,000 --> 00:30:20,000
Again, you can understand that
in terms of the electron

411
00:30:20,000 --> 00:30:25,000
configuration for the nitrogen.
Here is the electron

412
00:30:25,000 --> 00:30:32,000
configuration for nitrogen.
If you go and you want to add

413
00:30:32,000 --> 00:30:35,000
in another electron,
like right there,

414
00:30:35,000 --> 00:30:37,000
there is a repulsive
interaction.

415
00:30:37,000 --> 00:30:39,000
And Z equals 7.
In this case,

416
00:30:39,000 --> 00:30:44,000
that nuclear charge is not
large enough to compensate for

417
00:30:44,000 --> 00:30:48,000
this repulsive interaction,
so the electron affinity,

418
00:30:48,000 --> 00:30:51,000
here, is negative.
Noble gases have negative

419
00:30:51,000 --> 00:30:55,000
electron affinities.
You cannot add an electron to

420
00:30:55,000 --> 00:30:57,000
that.
It is not stable.

421
00:30:57,000 --> 00:31:02,000
Why?
Because you are destroying,

422
00:31:02,000 --> 00:31:10,000
in effect, the electronic
configuration of the rare gas.

423
00:31:10,000 --> 00:31:17,000
With that, now we want to ask
is how does the electron

424
00:31:17,000 --> 00:31:23,000
affinity change as you go across
the periodic table?

425
00:31:23,000 --> 00:31:28,000
That is our next question.
Wait.

426
00:31:28,000 --> 00:31:31,000
Okay.
Ready?

427
00:31:31,000 --> 00:31:34,000
Set?
Launch.

428
00:31:41,000 --> 00:31:47,000
Six, where are you?
What happens to the electron

429
00:31:47,000 --> 00:31:52,000
affinity as you go across the
Periodic Table,

430
00:31:52,000 --> 00:31:56,000
from left to right?
Decreases.

431
00:31:56,000 --> 00:32:01,000
Is that your final answer?
Yes?

432
00:32:01,000 --> 00:32:05,000
You are wrong.
The electron affinity,

433
00:32:05,000 --> 00:32:08,000
as you go across,
increases.

434
00:32:08,000 --> 00:32:12,000
The halogens,
fluorine, chlorine have very

435
00:32:12,000 --> 00:32:17,000
large electron affinities,
because adding an electron

436
00:32:17,000 --> 00:32:21,000
gives you that noble gas
configuration.

437
00:32:21,000 --> 00:32:28,000
The electron affinity increases
as you go across the Periodic

438
00:32:28,000 --> 00:32:31,000
Table.
Now what happens to the

439
00:32:31,000 --> 00:32:35,000
electron affinity as we go down
the Periodic Table?

440
00:32:35,000 --> 00:32:39,000
You guys, do you have any paper
airplanes?

441
00:32:39,000 --> 00:32:42,000
We have to give you some.
Are you okay?

442
00:32:42,000 --> 00:32:46,000
That is our next question.
What happens to the electron

443
00:32:46,000 --> 00:32:49,000
affinity as we go down the
Periodic Table?

444
00:32:49,000 --> 00:32:50,000
Ready?
Set?

445
00:32:50,000 --> 00:32:52,000
Launch.

446
00:33:00,000 --> 00:33:03,000
Recitation four.
What does it do?

447
00:33:03,000 --> 00:33:06,000
Does it go down?
Decreases.

448
00:33:06,000 --> 00:33:09,000
It absolutely does.

449
00:33:14,000 --> 00:33:17,000
The electron affinity,
as you go down,

450
00:33:17,000 --> 00:33:21,000
decreases.
It decreases because the

451
00:33:21,000 --> 00:33:25,000
nuclear charge,
here, is increasing,

452
00:33:25,000 --> 00:33:31,000
but you are putting electrons
in shells that are farther away

453
00:33:31,000 --> 00:33:37,000
from the nucleus.
The overall effect is that that

454
00:33:37,000 --> 00:33:42,000
r dependence dictates,
and the electron affinity goes

455
00:33:42,000 --> 00:33:43,000
down.

456
00:33:48,000 --> 00:33:54,000
Let me do one more here.
Or, maybe a couple more.

457
00:33:54,000 --> 00:34:01,000
Now, what I want to ask is
about the atomic radius.

458
00:34:07,000 --> 00:34:15,000
And what I want to know is what
happens to r as you go across

459
00:34:15,000 --> 00:34:21,000
the Periodic Table.
I am getting out of the way.

460
00:34:21,000 --> 00:34:22,000
Wait.
Ready?

461
00:34:22,000 --> 00:34:26,000
Set?
I haven't said launch yet.

462
00:34:26,000 --> 00:34:28,000
Ready?
Set?

463
00:34:28,000 --> 00:34:32,000
Go.
Recitation one.

464
00:34:32,000 --> 00:34:36,000
What happens to r?
Decreases.

465
00:34:36,000 --> 00:34:40,000
It decreases.
It goes down.

466
00:34:40,000 --> 00:34:44,000
This is important.
It goes down,

467
00:34:44,000 --> 00:34:51,000
here, because Z is increasing.
Z is increasing,

468
00:34:51,000 --> 00:35:00,000
and you are putting electrons
into the same shell.

469
00:35:00,000 --> 00:35:07,000
And so the same shell means the
same distance from the nucleus.

470
00:35:07,000 --> 00:35:13,000
Larger Z, the radius actually
goes down, the size goes down.

471
00:35:13,000 --> 00:35:20,000
And then, one more question.
What happens as you go down the

472
00:35:20,000 --> 00:35:25,000
Periodic Table?
I am going to get out of the

473
00:35:25,000 --> 00:35:26,000
way.
Ready?

474
00:35:26,000 --> 00:35:29,000
Set?
Go.

475
00:35:34,000 --> 00:35:45,000
One.
It decreases?

476
00:35:45,000 --> 00:36:02,000
What did you say?
Increases.

477
00:36:02,000 --> 00:36:37,000
It does increase because you
are putting electrons into

478
00:36:37,000 --> 00:37:10,000
shells farther and farther away.
What is the score,

479
00:37:10,158 --> 00:37:13,000
Christine?
Fantastic.