Periodic Trends

AP Chemistry· difficulty 2/5

First ionization energies for Li, Be, B, C, N, O, F, Ne are plotted vs Z.

Why does IE₁ drop slightly from N to O even though Z increases?

A
$N has fewer protons than O.$
B
$Pairing in the 2p_x orbital introduces electron-electron repulsion, lowering IE.$
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C
$O has a half-filled p subshell.$
D
$O is smaller than N.$

Explanation

N has half-filled 2p (extra stability). O's fourth 2p electron must double up, raising repulsion and making it easier to remove.

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