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Which of the following is the BEST explanation for the unusual decrease in first ionization energy moving from nitrogen ($N$) to oxygen ($O$)?

A

The oxygen atomic radius is smaller than nitrogen, making it easier to remove an electron from oxygen as compared to nitrogen.

B

The electron being removed from nitrogen is a member of a half-filled sublevel, which is more stable than the partially filled oxygen sublevel, making it easier to remove an electron from oxygen as compared to nitrogen.

C

The electron being removed from nitrogen is one that occupies a 2p orbital singularly, whereas the electron being removed from oxygen is one of a pair of electrons within a 2p orbital of oxygen. Removing the electron from the paired orbital reduces electron-electron repulsions and is easier to remove compared to the electron of nitrogen.

D

Oxygen has a greater atomic number than nitrogen and thus a greater effective nuclear charge ($Z_{eff}$), which will result in a greater hold on its electrons, thereby requiring more effort to remove the electron from oxygen as compared to nitrogen.

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