Generally, the first ionisation energy increases along a period. But there are some exceptions one which is not an exception is

Options

A. N and O
B. Na and Mg
C. Mg and Al
D. Be and B

Answer

B

Explanation

Inspite of higher nuclear charge , the ionisation enthlpy of O is lower than that of N because N has exactly half filled more stable electronic configuration but O does not. Similarly , inspite of higher nuclear charge, ionisation enthalpy of Al is lowr than that of Mg because in case of Aluminium a less strongly attached 3p electron is to be removed. like wise, inspite of higher nuclear charge the ionisation enthalpy of B is lower than that of Be because in case of B, a less strongly held 2p electrons is to be removed, while is to be removed. however , in cases of Na and Mg, ionisation enthlpy increases as the nuclear charge increases.