Shouldn't electron affinity increase going down the noble gases because they have more electrons? Why does it decrease?

1 Answer
Truong-Son N.
Sep 25, 2017

Electron affinity is not about how well the new electron can be held onto (that's electronegativity and ionization energy), but about the change in energy due to adding the new electron.

For noble gases, the electron affinity is positive, since it requires input of energy to place it into the next energy level. The orbital that gains the new electron is always the #(n+1)s# orbital for the noble gas on the #n#th energy level.

As a result, since orbital angular momentum is not relevant here (all of the #s# orbitals have #l = 0#), we can take the hydrogen atom energy levels by analogy:

http://dev.physicslab.org/

The energy levels of the #s# orbitals converge as #n# increases. So, the energy needed to add a new electron into the #(n+1)s# orbital decreases as #n# increases.

Therefore, electron affinity becomes less positive as we examine larger/heavier noble gases.

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