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Question 1F.11 asks you to pick which element within each pair has the higher electron affinity: a) tellurium or iodine, b) beryllium or magnesium, c) oxygen or sulfur, d) gallium or indium. I know that electron affinity is highest to the right of the periodic table because those elements (groups 16 and 17) readily accept electrons in order to have an octet. But why does beryllium/magnesium and gallium/indium have equal electron affinities, yet sulfur has a higher electron affinity than oxygen? What's the reasoning behind the trends for electron affinities across the periodic table?
Sulfur has higher electron affinity because oxygen electrons experience more repulsion between each other, since the valence shell is held closer to the nucleus. The repulsion causes more energy to be consumed when an electron is introduced than with sulfur due to the repulsion of electrons being imbalanced. I hope I explained this right :).
Although I do not know if this is definitively why... I imagine that Be and Mg have equal ionization energies because both of their s shells are filled fully with 2 electrons. However this would not apply to Ga and In, but maybe because they only have 1 electron each in their valence shell (p), so it is relatively easy to remove an electron.
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