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(B) The first ionization energy generally increases across a period from left to right due to an increase in effective nuclear charge. Exceptions occu

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$Na$ and $Mg$

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(B) The first ionization energy generally increases across a period from left to right due to an increase in effective nuclear charge. Exceptions occur when a more stable electronic configuration (like fully filled or half-filled orbitals) is present in an element to the left of another. For $N$ $(2s^2 2p^3)$ and $O$ $(2s^2 2p^4)$,$N$ has a higher $IE$ than $O$ due to its stable half-filled $p$-orbital. For $Be$ $(2s^2)$ and $B$ $(2s^2 2p^1)$,$Be$ has a higher $IE$ than $B$ due to its stable fully filled $s$-orbital. For $Mg$ $(3s^2)$ and $Al$ $(3s^2 3p^1)$,$Mg$ has a higher $IE$ than $Al$ due to its stable fully filled $s$-orbital. For $Na$ $(3s^1)$ and $Mg$ $(3s^2)$,the trend follows the general rule where $IE$ of $Mg > IE$ of $Na$,so this is not an exception.

Generally,the first ionization energy increases along a period. But there are some exceptions. One which is not an exception is

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