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 $\mathrm{Be}<\mathrm{B}<\mathrm{C}<\mathrm{N}<\mathrm{O}$. But there are two exceptions, in the case of $\mathrm{Be}$ and $\mathrm{N}$ because of the highly stable $2{\mathrm{s}}^2$ (completely filled orbital) and $2{\mathrm{s}}^{2}2{\mathrm{p}}^3$ (half-filled orbital $2{\mathrm{p}}^3$) valence configuration. Due to this, the correct order is:
$\mathrm{B}<\mathrm{Be}<\mathrm{C}<\mathrm{O}<\mathrm{N}$.