JEE Main Chemistry question

The first ionisation energy of oxygen is less than that of nitrogen. Which of the following is the correct reason for this observation? [Kerala CEE 2008]

(A)Lesser effective nuclear charge of oxygen than nitrogen
(B)Lesser atomic size of oxygen than nitrogen
(C)Greater interelectron repulsion between two electrons in the same p-orbital counter balances the increase in effective nuclear charge on moving from nitrogen to oxygenCorrect
(D)Greater effective nuclear charge of oxygen than nitrogen

Answer: (C)

Solution

Nitrogen's configuration has three unpaired electrons in separate orbitals (extra stable, no pairing repulsion), while oxygen's forces two electrons into the same orbital; the resulting electron-electron repulsion in that doubly-occupied orbital makes it easier to remove an electron

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Question ID VK-133187