What are the exceptions to the general periodic trend in first ionization energy within a period?

Explanation:

Ionization energy is the least energy needed to remove the most loosely bound electrons in an atom. Ionization energy increases across the periodic table and decrease down the group as expected because of increase in number of shell, which increases atomic radius.

As seen in the ionization energy table (check periodic table for ionization energy) there are few exceptions.

The ionization energy of group III elements is lower than that of group II and that of group VI is lower than that of group V.

In the first ionization energy, the ionization energy of Boron is lower than that of Beryllium and that if Oxygen is lower than that of Nitrogen.

This is because the 2S² outermost sublevel of beryllium is full while the 2P¹ outermost sublevel of Boron is almost empty. Less ionization energy will be required to remove the one electron in the outermost 2P sublevel.

Nitrogen has a higher ionization energy than oxygen because the outermost 2P sublevel is half filled.

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