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

Answer: (C)

B < Be < O < N

Answer 2

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 $\left(1 s^{2}, 2 s^{2}, 2 p^{2}\right)$, electron is to be removed from $2 p$ which is easy while in $Be \left(1 s ^{2}, 2 s ^{2}\right)$, electron is to be removed from $2 s$ which is difficult. The low IE of $O$ than that of $N$ is because of the half-filled $2 p$ orbitals in $N\left(1 s^{2}, 2 s^{2}, 2 p^{3}\right)$