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Is there a limit of electrons a single hydrogen atom can have? If so what is it? why? Is the the answer to why scalable to helium?

Emilio Pisanty
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hoboBob
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2 Answers2

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By definition, "hydrogen atom" refers to the neutral system with one proton and one electron, so it cannot hold any extra electrons.

However, protons can hold more than one electron, in which case the system is termed a hydrogen anion. This is a stable, bound system, and the reaction $$ \mathrm{H}+e^- \to \mathrm{H}^- \tag 1 $$ releases about $0.75\:\rm eV$ (an energy known as the electron affinity of the hydrogen atom), plus whatever kinetic energy the electron came in with, through the emission of a photon.

(As an aside, the hydrogen anion, and particularly the reaction $(1)$ above together with its converse in the form of photodetachment, is incredibly important ─ this is the reason why the Sun's spectrum is continuous.)

Free atoms of most elements tend to have positive electron affinities, which means that their singly-charged negative anions are stable systems, and they release energy when they capture their first extra electron. There's a few exceptions, though, starting with helium: atoms which have stable closed shells can 'reject' that extra electron, as it's forbidden from sitting in the closed valence shells and it's forced to sit at higher-energy shells that are too far uphill in energy to be stable.

If you want to up the game and go to a second extra electron, though, to get to $\rm H^{2-}$, the game runs out, and indeed it runs out for every element ─ all the second electron affinities are negative. That is, it takes work to cram a second extra electron in, and the resulting dianion will at best be in a metastable state that's ready and jumping to give that energy back out by dissociating into the single anion and a free electron. It's just too hard to try and hold two extra electrons (and their resulting mutual electrostatic repulsion) within the confines of an atomic system.

Emilio Pisanty
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Your question is about the Hydrogen ion, when it gains electrons. Normally, the Hydrogen ion (we usually call the single proton without electron the Hydrogen ion) when it gains an electron, will have a negative charge.

enter image description here

Now these negative ions (with two or more extra electrons) are unstable. You are basically asking if you can bind a proton with more then two electrons.

Though, you can try to use an external magnetic field to keep it stable.

https://link.springer.com/article/10.1007/s00601-009-0018-7

Árpád Szendrei
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  • This answer is dead wrong ─ there is basically nothing right about it other than the platitudes in the first paragraph. Please do your due diligence and research before answering. – Emilio Pisanty Oct 21 '19 at 14:35
  • @EmilioPisanty what exactly is wrong about it? I thought he was asking whether you can bind a proton with more than one electron. – Árpád Szendrei Oct 21 '19 at 14:53
  • Precisely. You claim that it is not possible to do this in a stable manner, i.e. that the hydrogen anion (one proton, two electrons) is unstable. That's incorrect. Please do your research before continuing this conversation; I would recommend Wikipedia (together with the rest of the links in my answer) as a starting point. – Emilio Pisanty Oct 21 '19 at 15:18
  • @EmilioPisanty I couldn't agree more with you. I have not found other then wiki and whether it is stable or not. Are you saying it is stable with two or more electrons? Though you say "It's just too hard to try and hold two extra electrons (and their resulting mutual electrostatic repulsion) within the confines of an atomic system." – Árpád Szendrei Oct 21 '19 at 15:28
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    Two extra electrons (i.e. one proton, three electrons) is indeed unstable. Your answer goes beyond that and claims that one extra electron (i.e. one proton, two electrons) is also unstable. What is it about the Wikipedia page of the hydrogen anion that gives you even the remotest impression that it's unstable? – Emilio Pisanty Oct 21 '19 at 15:46
  • @EmilioPisanty I edited. The idea was that these Hydrides usually are stable in covalent bonds in molecules, but do they really exist as single atoms? https://en.wikipedia.org/wiki/Hydride – Árpád Szendrei Oct 21 '19 at 17:31
  • What part of the Wikipedia page about the hydrogen anion is unclear? What part of that Wikipedia page gives you any grounds to doubt that it exists as a stable system in vacuum? What does it take for you to understand that you're out of your depth here, that your answer is wrong, and that you've got some serious in-depth research to do before you can fix the problems here? – Emilio Pisanty Oct 21 '19 at 17:37
  • As for your edit: it does make your answer less wrong, in that it makes it quite a bit of a non-answer. There is nothing about the OP that even hints at "You are basically asking if you can bind a proton with more then three electrons" ─ your edit tries desperately to make your initial answer non-wrong, despite the mountain of evidence to the contrary. Why is it so hard for you to say "the $\rm H^-$ anion, listed in the table in the picture, is a stable system"? – Emilio Pisanty Oct 21 '19 at 17:39
  • @EmilioPisanty i agree with you. All I am asking is how is one proton and two electrons stable (in isolation) considering EM forces? And why does the third electron change that? https://physics.stackexchange.com/a/390836/132371 – Árpád Szendrei Oct 21 '19 at 19:05
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    If you don't understand the material, ask separately instead of posting misinformation. The comment thread of a disputed answer is nowhere close to the right place to be asking that. – Emilio Pisanty Oct 21 '19 at 21:03