The first ionization enthalpies of Be, B, N and O follow the order

The ionisation enthalpy increases as we go from left to right in a period, while it decreases as we come down a group. So according to this, the order will be $Be < B < C < N < O$. But there are two exceptions, in the case of $Be$ and $N$ because of the highly stable $2{s}^{2}$ (completely filled orbital)$and 2{s}^{2}2{p}^{3}$ (half-filled orbital $2{p}^{3}$) valence configuration. Due to this, the correct order is:
$B < Be < C < O < N$.