If the first ionisation enthalpy of the elements of the third period (from Z = 11 to Z = 18 ) is plotted against their atomic numbers, the graph generally shows an upward trend. However, there are specific 'dips' where the ionisation enthalpy of an element is lower than that of the immediately preceding element. Identify the pair of atomic numbers corresponding to the elements where these dips occur.

Step-by-step solution

Across the third period (from Na to Ar ), the first ionisation enthalpy generally increases due to an increase in effective nuclear charge. However, exceptions occur due to the extra stability of fully-filled and half-filled subshells: 1. Mg ( Z = 12 ) has a stable 3s^2 configuration, whereas Al ( Z = 13 ) has a 3s^2 3p^1 configuration. It is easier to remove the 3p^1 electron of Al than a 3s electron of Mg . Thus, a dip occurs at Z = 13 . 2. P ( Z = 15 ) has a stable half-filled 3s^2 3p^3 configuration, whereas S