Generally, the first ionization energy increases along a period. But there are some exceptions. One which is not an exception is (KCET 2010)

As per the trend, the ionization energy increases from left to right in a period. Magnesium has high ionization enthalpy than sodium and it is not a case of exception. Exception Reason for exception N O IP > IP Due to the half-filled stability of nitrogen Mg Al IP > IP Due to the loosely bond 3p electron in aluminum Be B IP > IP Due to the loosely bond 2p electron in boron