Generally, the first ionization enthalpy increases along a period. But there are some exceptions. One which is NOT an expection is:

Options

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

Answer

B

Explanation

Be, N and Mg have higher values than expected, this is due ro stable configurations of these elements. Be and Mg have fully - filled orbitals, in nitrogen 2p-orbitals are half-filled.