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 < Z .The lower IE of B than that of Be is because in B $( 1s^2, \, 2s^2, \, 2p^1 )$, 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 $( 1s^2, \, 2s^2, \, 2p^3$)