Question

Among second period elements/the correct order for first ionisation enthalpy is

• A. Li < Be < B < C < N < O < F < Ne
• B. Li < B < Be < C < O < N < F < Ne
• C. Li > Be > B > C > N > O > F > Ne
• D. Li > B > C > Be > O > N > F > Ne

Answer 1

Answer: (B)

The fast lonisation Energy $(IE)$ increases in a. period.
Thus, the first IE of the elements of second period should be as follows:
$Li<Be<B<G<N<O<F<Ne$
But in practice, the elements do not follow the above order.
The first IE of these element is
$Li<B<Be<C<O<N<F<Ne$
The lower IE of B than that of Be is because in $B\left(1s^{2}2s^{2}2p^1\right)$, electron is to be
removed from $2p$ which is easy while in $Be\left(1s^2,2s^2\right)$, electron is to be removed from $2s$ which is difficult.
The low IE of $O$ is lower than that of $N$ because of the half- filled $2p$ orbitals in $N\left(1s^2,2s^2,2p^3\right)$.