Arrange the following elements in increasing order of first ionization enthalpy: $\text{Li, Be, B, C, N}$.

Ionization enthalpy generally increases across a period from left to right.However, there are two exceptions in this series:$\text{Be}$ vs $\text{B}$: $\text{Be}$ ($1s^{2}2s^2$) has a fully-filled subshell, whereas $\text{B}$ ($1s^{2}2s^{2}2p^1$) has one electron in $2p$. It is harder to remove an electron from a stable $2s^2$ subshell. Thus, $\text{IE}(\text{Be})>\text{IE}(\text{B})$.$\text{N}$ vs $\text{O}$ (not in this list): $\text{N}$ is higher due to half-filled $p^3$.The correct order is: $\text{Li}<\text{B}<\text{Be}<\text{C}<\text{N}$.