Berrilium
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Electron Affinity
Why does an atom release energy when an electron is gained?
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Chemundergrad
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If you had an atom that you were trying to Ionise, remove an electron instead of add one. You would have to pull that electron away from the nucleus,
until it reached a point where it was no longer held onto by the nucleus and could break free. There is an electrostatic attraction between the
positively charged nucleus and the negatively charged electron which holds the electrons in place, to pull that electron away from the nucleus you
would need a force to do work on it so it could overcome the attraction it has to the nucleus. Coulombs law describes the electrostatic attraction
between particles, Here.
So you could say the energy that is binding that electron to the nucleus is negative because it is created by a force acting in the opposite direction
to the force needed to ionise the electron from the atom, this is a completely made up value but lets say that the electron is bound to the nucleus by
-10 Joules of energy, to over come that binding energy you would need to provide the same amount of energy in the opposite direction.
So if you reverse this, you need to put energy into pull the electron out. To push an electron in you would get energy given out, because the energy
binding the electron to the nucleus decreases and the whole system becomes more energetically stable.
Sorry this is probably a very bad explanation, if you don't understand what I have said, please ask again, and I'm sure there is someone more
knoweledgable than me who can explain it better.
In addition if you are someone else and have noticed I have made a mistake, or said a load of rubbish please also point that out.
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triplepoint
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You may find this link useful. It is a chemwiki from ucdavis: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_...
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