Explain why the substantial decrease in first ionisation enthalpy is observed between Na and K, and Mg and Ca, is not observed between Al and Ga.

Question

Explain why the substantial decrease in first ionization enthalpy is observed between Na and K, and Mg and Ca, is not observed between Al and Ga.

Answer 1

In ${Na}(186 pm)$ and ${K}$ (231 pm), and ${Mg}$ $(160 {pm})$ and ${Ca}(180 {pm})$ there is adequate increase in atomic size which is not observed in case of ${Al}(118 {pm})$ and ${Ga}(135 {pm})$.

As size of the atom increases, ionisation energy decreases thus, there is no substantial decrease in ionisation enthalpy in case of ${Al}$ and ${Ga}$.