[TED解説]確率の雲として存在する電子

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You probably know that all stuff is made up of atoms and that an atom is a really, really, really, really tiny particle.

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Every atom has a core, which is made up of at least one positively charged particle called a proton,

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and in most cases, some number of neutral particles called neutrons.

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That core is surrounded by negatively charged particles called electrons.

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The identity of an atom is determined only by the number of protons in its nucleus.

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Hydrogen is hydrogen because it has just one proton, carbon is carbon because it has six, 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, neutrons, or electrons.

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So, we do a bunch of experiments and develop a model for what we think is there.

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Then we do some more experiments 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 be time for a new model.

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We’ve had lots of very different models for atoms since Democritus in 400 BC, and there will almost certainly be many more to come.

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Okay, tangent over.

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The cores of atoms tend to stick together, but electrons are free to move, and this is why chemists love electrons.

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If we could marry them, we probably would.

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But electrons are weird.

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They appear to behave either as particles, like little baseballs, or as waves, like water waves, depending on the experiment that we perform.

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One of the weirdest things about electrons is that we can’t exactly say where they are.

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It’s not that we don’t have the equipment, it’s that this uncertainty 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 there’s a certain probability of finding an electron in a given space around the nucleus.

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And that means that we can ask the following question:

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If we drew a shape around the nucleus such that we would be 95% sure of finding a given electron 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, and what each one looks like depends on, among other things, how much energy it has.

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The more energy an orbital has, the farther most of its density is from the nucleus.

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By they way, why did we pick 95% and not 100%?

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Well, that’s another quirk of our model of the electron.

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Past a certain distance from the nucleus, the probability of finding an electron starts to decrease more or less exponentially, which means that while it will approach zero,

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it’ll never actually hit zero.

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So, in every atom, there is some small, but non-zero, probability that for a very, very short period of time,

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one of its electrons is at the other end of the known universe.

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But mostly electrons stay close to their nucleus as clouds of negative charged density that shift and move with time.

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How electrons from one atom interact with electrons from another determines almost everything.

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Atoms can give up their electrons, surrendering them to other atoms, or they can share electrons.

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And the dynamics of this social network are what make chemistry interesting.

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From plain old rocks to the beautiful complexity of life, the nature of everything we see, hear, smell, taste, touch, and even feel is determined at the atomic level.