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