Question

Which one of the following order is correct for the first ionisation energies of the elements?

• A. $B<Be<N<O$
• B. $Be<B<N<O$
• C. $B<Be<O<N$
• D. $B<O<Be<N$

Answer 1

The correct option is C $B<Be<O<N$

First ionisation energy increases in a period.
Thus, the first $IE$ of the elements of the second period should be as follows $Be<B<N<O$
But in practice, the elements do not follow the above order. The first $IE$ of these elements is $B<Be<O<N$
The lower $IE$ of $B$ than that of $Be$ is because in $B(1s^2,2s^2,2p^2)$, electron is to be removed from $2p$ which is easy while in $Be(1s^2,2s^2)$, electron is to be removed from $2s$ which is difficult. The low IE of $O$ than that of $N$ is because of the half-filled $2p$ orbitals in $N(1s^2,2s^2,2p^3)$.