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)

Li < B < Be < C < O < N < F < Ne

Answer 2

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