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You probably know that all stuff[br]is made up of atoms

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and that an atom

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is a really, really, really,[br]really tiny particle.

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Every atom has a core,

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which is made up of at least one

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positively charged particle

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called a proton,

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and in most cases,

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some number of neutral[br]particles called neutrons.

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That core is surrounded

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by negatively charged[br]particles called electrons.

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The identity of an atom is determined

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only by the number[br]of protons in its nucleus.

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Hydrogen is hydrogen because it[br]has just one proton,

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carbon is carbon because it has six,

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gold is gold because it has 79,

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and so on.

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Indulge me in a momentary tangent.

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How do we know about atomic structure?

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We can't see protons,[br]neutrons, or electrons.

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So, we do a bunch of experiments

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and develop a model[br]for what we think is there.

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Then we do some more experiments

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and see if they agree with the model.

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If they do, great.

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If they don't, it might[br]be time for a new model.

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We've had lots of very[br]different models for atoms

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since Democritus in 400 BC,

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and there will almost certainly

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be many more to come.

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Okay, tangent over.

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The cores of atoms tend to stick together,

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but electrons are free to move,

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and this is why chemists love electrons.

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If we could marry them,

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we probably would.

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But electrons are weird.

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They appear to behave either as particles,

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like little baseballs,

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or as waves, like water waves,

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depending on the experiment[br]that we perform.

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One of the weirdest things about electrons

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is that we can't exactly[br]say where they are.

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It's not that we don't have the equipment,

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it's that this uncertainty

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is part of our model of the electron.

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So, we can't pinpoint them, fine.

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But we can say[br]there's a certain probability

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of finding an electron in a given space

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around the nucleus.

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And that means that we can[br]ask the following question:

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If we drew a shape around the nucleus

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such that we would be 95% sure

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of finding a given electron[br]within that shape,

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what would it look like?

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Here are a few of these shapes.

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Chemists call them orbitals,

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and what each one looks like

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depends on, among other things,

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how much energy it has.

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The more energy an orbital has,

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the farther most of its density is

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from the nucleus.

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By they way, why did we pick 95%

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and not 100%?

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Well, that's another quirk

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of our model of the electron.

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Past a certain distance from the nucleus,

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the probability of finding an electron

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starts to decrease

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more or less exponentially,

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which means that while it[br]will approach zero,

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it'll never actually hit zero.

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So, in every atom,

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there is some small,[br]but non-zero, probability

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that for a very, very[br]short period of time,

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one of its electrons

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is at the other end of the known universe.

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But mostly electrons stay[br]close to their nucleus

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as clouds of negative charged density

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that shift and move with time.

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How electrons from one atom

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interact with electrons from another

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determines almost everything.

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Atoms can give up their electrons,

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surrendering them to other atoms,

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or they can share electrons.

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And the dynamics of this social network

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are what make chemistry interesting.

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From plain old rocks

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to the beautiful complexity of life,

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the nature of everything we see,

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hear,

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smell, taste, touch, and even feel

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is determined at the atomic level.