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The electron affinity of a neutral oxygen atom is −142 kJ (it releases this energy). The electron affinity for the now double anion $\ce{O^2-}$ is 710 kJ (work must be done on the atom).

My question is, why the large jump in electron affinity? We are taught that the electrons do not interact much with each other in the way of shielding. I think this to be incorrect, because otherwise, the effective nuclear charge on all of the valence electrons should be about the same, including the new ones added to ionize the atom. This would make the electron affinity about the same for each electron. Plus, doesn’t the oxygen atom want to complete its shell? So why is it putting up a fight?

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The addition of an electron to any negatively charged ion will need to overcome the repulsive force of the negative ion. This is generally less favorable than adding an electron to a neutral atom, and the electron affinity will be positive. Even though the additional electron will complete the octet, as in the case of an ion where the addition of electron will complete the subshell, the electron affinity is a positive value.

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Nothing really likes to be a double anion. But there are mitigating factors.

First -- getting more charges onto the electronegative atom gives the opportunity for greater electrostatic energy in an (or a predominantly) ionic lattice.

Second -- with the specific electron configuration of oxygen or sulfur, the work required to get the second electron is less than what would be required, for instance, to get a chloride di-anion.

Add this up and the result is that the lattice energy is enough to make the di-anion in the condensed, generally solid compounds where we usually see it.

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