What limits electrons in shells

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What limits electrons in shells

#1  Postby Wenlok » Jul 18, 2018 2:52 am

:think: So i was posed with this question today.

What ‘rule’ limits electrons in shells? For instance, why cant there be more than 2 electrons in the first shell?

For the life of me, i cant help explain to this person why it isnt a ‘rule’ by God, but he doesn’t seem to grasp quantum mechanics.

How would you answer it?
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Re: What limits electrons in shells

#2  Postby Fenrir » Jul 18, 2018 3:00 am

It is a "rule" by god, and that god is Pauli, if you don't know Pauli then you may be excluded. :)
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Re: What limits electrons in shells

#3  Postby SafeAsMilk » Jul 18, 2018 3:14 am

Like water going down a drain, it's just following the path of least resistance according to its physical properties.
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Re: What limits electrons in shells

#4  Postby Matt_B » Jul 18, 2018 6:47 am

Electron degeneracy pressure.

It's also worth remembering that "shells" are just an abstraction that's a part of the Bohr model of the atom, and they have no physical significance.
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Re: What limits electrons in shells

#5  Postby Scot Dutchy » Jul 18, 2018 8:04 am

Matt_B wrote:Electron degeneracy pressure.

It's also worth remembering that "shells" are just an abstraction that's a part of the Bohr model of the atom, and they have no physical significance.


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Re: What limits electrons in shells

#6  Postby Cito di Pense » Jul 18, 2018 10:48 am

Matt_B wrote:Electron degeneracy pressure.

It's also worth remembering that "shells" are just an abstraction that's a part of the Bohr model of the atom, and they have no physical significance.


PEP is general, but the question in the OP, involving 'shells', refers to bound electrons, that is, bound to an atomic nucleus. In general, bound electrons are only one way we find electrons -- see conduction band in metals, in plasmas, and so on. The 'shell', that is, unique set of quantum numbers (except for spin ±1/2 -- which is where the number 2 comes from) is not even relevant. More generally, electrons are fermions, described by Fermi-Dirac statistics. The question asked in the OP does not even address these features of microphysics. The OP is essentially a "why" question, or a philosophical one, but the description we have is just a description of what we observe.
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Re: What limits electrons in shells

#7  Postby DavidMcC » Jul 18, 2018 11:28 am

Matt_B wrote:Electron degeneracy pressure.

It's also worth remembering that "shells" are just an abstraction that's a part of the Bohr model of the atom, and they have no physical significance.

There is a physical signifciance, just not the literal one.
EDIT: The shells represent the size of the electron eigenstates involved.
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Re: What limits electrons in shells

#8  Postby Matt_B » Jul 18, 2018 12:44 pm

If anyone wants a philosophical justification for the axioms of QM, we should just tell them that this is the wrong forum. :lol:

Seriously, the PEP can be mathematically justified using the spin-statistics theorem, so it's not a fundamental axiom in itself. It's not one of the simpler QM proofs out there, although there are some more accessible if hand-wavy variants such as the topological treatment in Feynmann's 1986 Dirac lecture.
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