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[SQUEAKING]

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[RUSTLING]

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[CLICKING]

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Now, this gets me
to why this matters,

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and that's how I want
to end today's lecture

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in the last five minutes because
we talked about emission,

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electrons transitioning
from their discrete orbitals

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as a way of emitting
or absorbing.

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And so you if you
have different atoms,

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there's hydrogen. These
are the absorption lines--

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absorption lines.

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Carbon has absorption lines.

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Oxygen, nitrogen,
sulfur, so lots of things

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have absorption lines.

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Why does this matter?

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Well, of course, because
of the refrigerator.

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I love this plot.

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This is a plot of
the refrigerator.

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This is the size
of the refrigerator

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as a function of year.

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The date only goes to 2002,
but it's roughly stable now.

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Does anybody know
what's limiting

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the size of the refrigerator?

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It was going up and up and up
a lot, and then it stopped.

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Does anybody know
what limits that?

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STUDENT: The average
height [INAUDIBLE]..

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PROFESSOR: It's the
width of the doorway.

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It's the width of the doorway.

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That's why we don't-- that's
why people get a second

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refrigerator.

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And there's energy use.

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Energy Star program, big deal.

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Big deal.

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But I'm talking
about refrigerators

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for a different reason.

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I'm talking about refrigerators
because until the 1980s,

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the way we did cooling
was with a chemical called

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chlorofluorocarbon.

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This is a chemistry
class, and we're

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going to be talking about--

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when we talk about
why this matters,

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we connect to the chemistry.

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Well, the chemistry
there was critical.

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There was a molecule there
called chlorofluorocarbon

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or CFC.

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And the thing
about that molecule

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is that its absorption,
the chlorofluorocarbon--

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so let's write it
down here, CCl3F.

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There's a gas phase, so we
put the little lower script g.

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That molecule has
its own absorption.

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It has its own absorption,
and it has its own reaction,

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and so do other things.

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But it has a really
important reaction

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because when it
absorbs UV rays, then

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it doesn't just like then
like electrons get pumped up.

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No, the whole thing reacts.

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And so this thing degrades
into CCl2F plus chlorine gas,

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and that's once it's up maybe
in the atmosphere, which

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it was getting released
all the time because

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of the refrigerators.

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And the thing is that the
chlorine gas would then

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react with ozone, and this
would go to ClO plus O2,

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and here's the problem.

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I mean, we're
already at a problem,

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but now it's a big
problem because--

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you can already tell.

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But I've got ClO, and
that's really reactive.

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So what happens is
ClO, which was a gas--

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I'm just being careful
here with my subjects--

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would react with atomic
oxygen to give you

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O2 and the chlorine atom back.

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And here's the thing.

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Now I've got my
chlorine atom again.

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These are very reactive
atoms in the atmosphere.

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They love ozone.

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And so this cycle
for one CFC molecule

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would happen 100,000
times, roughly,

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before that chlorine
kind of goes

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away and does something else--

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100,000 ozone molecules per CFC.

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Why does that matter?

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Well, it matters because--

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I love this plot.

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This is the energy of light
from the sun hitting our planet.

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So this is the energy that
you get, and look at this.

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This is the sun at the
top of the atmosphere.

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Here it is on Earth, the red.

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Look at these things here.

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If you could only
see in this spectrum,

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the world would be dark because
there's no light on the planet.

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Why?

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Because it's all absorbed in
the upper atmosphere by water.

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Absorption-- it's the
same principle, Bohr.

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Bohr's idea of electrons
getting promoted

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when they absorb
light is happening,

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and it's protecting us because
right here, see, that's the UV.

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Low wavelength, high energy.

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UV, O3.

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Look at that.

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Look at what it's taking out.

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That little sliver of
yellow is critical.

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And ozone was doing the
job, and it still is.

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But if we tear all that
ozone out with these CFCs,

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we have a crisis.

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And that led to what is one of
the greatest policy decisions

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which was made in 1987-- it's
called the Montreal Protocol--

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to essentially globally
ban-- the US led this.

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This is a great example of
how policy is so critical.

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The US led this
effort to ban CFCs,

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and we're almost back
to normal ozone levels,

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and we avoid this.

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And by the way, if
this had happened,

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the prediction at the time
was roughly 280 million

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additional cases of skin
cancer over this generation.

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This is a very big
deal, and it all

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comes back to
absorption of electrons

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by atoms and molecules.

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See you guys on Friday.