How can an atom have a negative charge?

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  • #1
erocored
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I have a neutral charged atom. When I bring an electron to this atom what the force will hold this electron with neutral atom?
 

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  • #2
erocored
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What kind of force hold extra electrons with neutral charged atom?
 
  • #3
anuttarasammyak
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If an atom were a neutral particle, no force would work on the electron. An atom is made of nucleus with positive charge and electrons of negative charge. The nucleus attracts and the electrons repulse the additional electron. By the balance of these forces the atom may hold the additional electron. Quantum mechanics gives more details.
 
  • #4
erocored
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If an atom were a neutral particle, no force would work on the electron. An atom is made of nucleus with positive charge and electrons of negative charge. The nucleus attracts and the electrons repulse the additional electron. By the balance of these forces the atom may hold the additional electron. Quantum mechanics gives more details.
Does the resulting force that act on the electron from the protons and electrons of the atom is zero?
 
  • #5
Drakkith
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The electric force from the nucleus. The very short, very abridged explanation is that the positive charge of the nucleus is, at very close ranges, only partially shielded by the electrons. So an electron can still feel an attraction at very close range to a neutral atom.
 
  • #6
erocored
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the positive charge of the nucleus is, at very close ranges, only partially shielded by the electrons.
How much partially shielded?
 
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  • #7
anuttarasammyak
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No, because they are distributed in different positions in the atom, their forces on the additional electron do not cancel.
 
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  • #8
sophiecentaur
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Good question - you'd think that 10 protons would only hold 10 electrons BUT a very simple model of electrons buzzing round a positive nucleus would suggest that the 'extra' electron you refer to is nearer to the attractive force from the positive nucleus than to the repulsive force of the electrons on the other side - so overall it is attracted.
Note - negative ions are not as stable as neutron atoms so they react easily with any positive ions that happen to be around or even just lose that electron.

Basically it's all harder than it may seem at first so you can't rely on initial and intuitive conclusions about this. (Or any Science, for that matter.
 
  • #9
erocored
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or even just lose that electron.
Do you mean that one of the additional electrons can be knocked out by the others in any point of time?
 
  • #10
sophiecentaur
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I think that's taking an already much-too mechanical model one step too far. Of course, electrons don't 'knock into' each other because the repulsive force between two charges would be too great for them to get that close.

Edit - but bound electrons really don't behave like particles anyway.
 
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  • #11
snorkack
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If an atom were a neutral particle, no force would work on the electron. An atom is made of nucleus with positive charge and electrons of negative charge. The nucleus attracts and the electrons repulse the additional electron. By the balance of these forces the atom may hold the additional electron. Quantum mechanics gives more details.
Most simple details:
Most dielectrics are polarizable and form a dipole moment that attracts a charge when influenced by the charge.
Indeed, consider nucleus, electron and the extra electron.
In absence of the extra electron, the first electron would be located on the average at the same place as the nucleus. The first electron would orbit away from the nucleus, but if, say, second electron is very far away and has negligible effect then the first electron is equally likely to be in any direction from nucleus.
If, however, the second electron is closer then the second electron attracts the nucleus and repels the first electron. The result is that the first electron is more distant from the second electron than the nucleus is.
Therefore while the charges of first electron and nucleus exactly cancel, their forces to the second electron do not - the repulsion of first electron to second electron is necessarily weaker than the attraction to nucleus, because the second electron is farther, because the force to second electron is repulsion.

Now if the second electron and the atom were not quantum mechanical particles, or had a large mass, they would always be bound by the induced dipole attraction, no matter how feeble.
Since electrons are quantum mechanical and have a small mass, they can be bound to neutral atoms when the attraction is strong enough, but not when it is weaker.
 
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  • #12
Drakkith
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How much partially shielded?

I don't know to be honest. I think the answer is very complicated and requires large amounts of computational power to model accurately.
 
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  • #13
sophiecentaur
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I don't know to be honest. I think the answer is very complicated and requires large amounts of computational power to model accurately.
On that scale there is no satisfactory alternative to a QM approach. I could mention that considering the Potential Energy in the system is probably the better approach over dealing with the forces. Only at large distances is the Electric Potential of the nucleus equal to the EP of the electrons so there is no simple ‘cancellation’ near the atom.
 
  • #14
Doc Al
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Moderator's Note: Two nearly identical threads have been merged. (For better or worse.)
 
  • #15
Drakkith
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On that scale there is no satisfactory alternative to a QM approach.

I was under the impression that even a QM approach still required large amounts of computational power to accurately model anything other than a hydrogen atom.
 
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  • #16
Vanadium 50
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You can do two electrons (e.g. H-, He, Li+) without too much trouble.
 
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  • #17
sophiecentaur
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I was under the impression that even a QM approach still required large amounts of computational power to accurately model anything other than a hydrogen atom.
The H atom is the only one with a 'closed' solution, afaik. Beyond that involves numerical solutions. There would seem to be very little point, though, in spending massive effort on a quasi mechanical calculation, which is what my earlier post didn't make clear. The concept of 'shielding' doesn't fit into it except as an arm waving justification that the two ideas don' actually clash too much.
 
  • #18
Vanadium 50
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The H atom is the only one with a 'closed' solution, afaik

That is correct. Note that He (or H-) is a three body problem and there doesn't even exist a classical solution.

That said, one can get pretty close - qualitatively correct and quantitative to 5 or so percent - using only grad school level methods. Qualitatively, you have an electron orbiting a dipole. Unfortunately, that doesn't have a closed form solution either.
 
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  • #19
shjacks45
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As a chemist, an atom may add an extra electron(s) to fill its valence shell. False question as naked ions only exist in very high vacuum.
 
  • #20
shjacks45
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I think that's taking an already much-too mechanical model one step too far. Of course, electrons don't 'knock into' each other because the repulsive force between two charges would be too great for them to get that close.

Edit - but bound electrons really don't behave like particles anyway.
Electrons, according to quantum chemistry, have wave function related to their atomic orbitals. Actually the electrons largely overlap each other when in nuclear orbitals. There is no repulsive force between electrons, other than two electrons with identical spin can't occupy the same orbital. Atoms with partially filled shell/orbitals can accept an electron in that orbital.
 
  • #21
Drakkith
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There is no repulsive force between electrons, other than two electrons with identical spin can't occupy the same orbital.

I'm not sure this is correct. My understanding was that the atomic orbital changes when you add another electron due to their mutual repulsion, which is why an exact solution to the Schrodinger equation only exists for atoms with a single electron.
 
  • #22
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There is no repulsive force between electrons
There is, just like between all other particles with the same type of electric charge.
False question as naked ions only exist in very high vacuum.
I'm not sure what you mean by "naked ions" but free ions (not bound in molecules) exist in high density places, too.
 
  • #23
sophiecentaur
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There is no repulsive force between electrons,
I have to disagree with that, too. All the fields around all the particles in an atom are equally relevant. surely the idea of the so-called 'shielding' of the outer electrons cn be explained in terms of 'repulsive forces'.
It is very important not to get too mechanical about how the atom works, though. Bound electrons do not behave as if they have 'motion', as in a low density beam.
 

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