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To clarify: My question is not Why are atoms empty?, my question is Why are they empty so much?

The classical orbit of an atom, roundly speaking, is where the probability to find an electron is highest (or higher than a given number). Here I quote one paragraph from the answer of user ACuriousMind about the emptiness of an atom:

The idea that atoms are mostly "empty space" is, from a quantum viewpoint, nonsense. The volume of an atom is filled by the wave functions of its electrons, or, from a QFT viewpoint, there is a localized excitation of the electron field in that region of space, which are both very different from the "empty" vacuum state.

But this doesn't explain why 99% of an atom is "empty". I mean, why is the distance from the nucleus to the classical orbit (or orbital) so large, 99 times of the radius of its nucleus? Is it because of the mathematical result from the Schrodinger equation?

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Ooker
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    Something doesn't make sense to me here. You quote a passage saying that atoms are not empty, then you go on to ask why they are so empty. The question seems to be self-contradictory as you've written it. Could you clarify that? – David Z Dec 06 '14 at 11:03
  • I have edited my question. Is it better? – Ooker Dec 06 '14 at 11:12
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    But 99% of an atom isn't empty. As ACuriousMind says it's filled with (at least one) delocalised electron. – John Rennie Dec 06 '14 at 11:14
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    @Ooker I don't think so. Now it's confusing in two ways: you're still asking why an atom is so empty right after quoting a statement that it's not empty, and you're also using a quite different definition of the word "empty" than everyone else. Plus, you say the orbital radius is far; well, far relative to what? Why shouldn't it be as large as it is? – David Z Dec 06 '14 at 11:15
  • @DavidZ I have edited it again. Does it still make you confusing? – Ooker Dec 06 '14 at 11:45
  • Yes, it still seems confusing, though I guess this latest edit helps a little bit. I have a pretty good feeling any version of the question that still contains the sentence "But this doesn't explain why 99% of an atom is 'empty'" will also seem confusing. (Just for the record, I'm not saying that simply removing that sentence will make the question clear.) – David Z Dec 06 '14 at 11:48
  • @DavidZ I think that sentence is understandable. If you need a concise question, "why is the distance from the nucleus to the classical orbit (or orbital) so large, 99 times of the radius of its nucleus?" is the one for you. – Ooker Dec 06 '14 at 15:50
  • It is meaningless to look at the "distance" from the nucleus to the classical orbit, because the classical orbit isn't realized in nature. The electron in the lowest lying orbital state even has a non-zero probability to be inside the nucleus! The main point of my answer is that classical thinking doesn't apply here, so you should abandon the notion that the electron is a point-like classical object, and jointly abandon the notion that there is an "orbit" for it. – ACuriousMind Dec 06 '14 at 16:27
  • @ACuriousMind My up-to-now answer is because it is the mathematical result. That "distance" doesn't contain any physical meaning. – Ooker Dec 06 '14 at 17:01

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As soon as you lose the idea that only "mass" can legitimately fill something, then there is no "empty" anywhere. Even a vacuum is not empty.

  • I have to say I think this is a misleading analogy. Planets aren't delocalised while electrons are. You risk leading the OP down a path to increased misunderstanding. – John Rennie Dec 06 '14 at 11:16
  • I agree - changed –  Dec 06 '14 at 11:21
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Imagine you have a house, and you have a dog in that house, and you left in the morning. You are now standing in front of that house. Is the house empty? From one point, you could say Yes, the house is 99% empty, only 1% of it is filled with dog, and not air. But then, give me an example of a place with no dog. Alright, you say the bathroom. Bzzt. There is still a chance that the dog is in the bathroom, so you can't say it's EMPTY. Following this logic, the dog could really be anywhere in the house, and the house isn't really empty at all.

Could be completely wrong, but that's what I think after reading ACuriousMind's answer.

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  • That's really answer the Why are atoms empty? question, not the Why are they empty so much? one – Ooker Dec 06 '14 at 11:13
  • oops ok just read your edit – QCD_IS_GOOD Dec 06 '14 at 11:14
  • After reading a bit, it seems that the electron, as it can no longer be considered as a classical particle and more of an electron cloud, as usual wants to get to the most stable energy state, which turns out to be a battle between the potential energy it has from being near the nucleus and the kinetic energy it has. Through some maths you can work out the size and shape of the cloud. I think. – QCD_IS_GOOD Dec 06 '14 at 11:27