Atoms of which element release the most energy when an electron is added to them in the gas phase?
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Explanation
draftElectron affinity (EA) measures the energy released when a neutral gaseous atom gains an electron: , . A more negative (larger magnitude) EA means more energy is released.
Comparing the group: O vs S — oxygen is smaller, so the incoming electron experiences greater electron-electron repulsion in the compact 2p orbital, lowering its EA magnitude relative to what periodic trends alone would predict. Sulfur, being larger, accommodates the extra electron with less repulsion, giving it a more negative EA (~-200 kJ/mol) than oxygen (~-141 kJ/mol).
Comparing C vs Si: carbon's EA (~-122 kJ/mol) is smaller in magnitude than silicon's (~-134 kJ/mol) for a similar reason (smaller atom, more repulsion), but both are less negative than sulfur's.
So the ranking of energy released (most negative EA) is: .
Thus sulfur releases the most energy when gaining an electron.
Carbon has the smallest EA magnitude among these; a student might mistakenly think EA increases up a group without accounting for how small size increases electron-electron repulsion, incorrectly picking the smallest atom as releasing the most energy.
A student may apply the simple periodic trend 'EA increases across a period and up a group' without recognizing the O/S anomaly, wrongly assuming oxygen (smaller, same group as S) releases more energy than sulfur.
A student might confuse electron affinity with ionization energy trends, or think silicon's larger size relative to carbon makes it the best electron acceptor, without comparing it to O and S correctly.
Correct: sulfur's larger atomic size reduces electron-electron repulsion for the incoming electron compared to oxygen, giving it the most negative (largest magnitude) electron affinity among these four elements, releasing the most energy.
USNCO 2018 National Part I exam, question 46. © American Chemical Society; reproduced for non-commercial practice.