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 A1
D. Be and B

Answer

B

Explanation

In spite of higher nuclear charge the ionization enthalpy of O is lower than that of N because N has exactly half- filled more stable electronic configuration but O does not. Similarly in spite of higher nuclear charge, ionization enthalpy of A1 is lower than that of Mg because in case of A1 a less strongly attracted 3p- electron is to be removed 3s- electron is to be removed . Likewise, in spite of higher nuclear charge , the ionization enthalpy of B is lower than that of Be because in case of B , a less strongly held 2p- electron is to be removed while in case of Mg a more strongly held 2s- electron is to be removed.
However in case of Na and Mg ionization enthalpy increases as teh nuclear charge increases. Therefore option (b ) is correct.