Question

Generally, the first ionization enthalpy increases along a period with few exceptions. Choose the correct option which is not an exception.

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

Answer 1

The correct option is B Na and Mg

In spite of higher nuclear charge, the ionization enthalpy of O $<$ N , Al $<$ Mg and B $<$ Be. The reason for all these is the half-filled electronic configuration.

However, in case of Na and Mg, ionization enthalpy increases as the nuclear charge increases and therefore, is not an exception.