Why does nitrogen (Z = 7) have a higher first ionization energy than oxygen (Z = 8), despite oxygen having a higher nuclear charge?
Answer and explanation
Correct answer: A. Nitrogen has a stable half-filled 2p3 subshell, whereas oxygen has paired 2p4 electrons with repulsion
Nitrogen has the valence configuration 2s2 2p3, where each 2p orbital contains one unpaired electron (Hund's rule). Oxygen has configuration 2s2 2p4, meaning one 2p orbital contains a paired set of electrons. The mutual electrostatic repulsion between the two paired electrons in the same orbital makes removing one electron easier, lowering oxygen's first ionization energy below that of nitrogen.
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