Explain why is there a phenomenal decrease in ionisation enthalpy from carbon to silicon?

Explanation

This is because of large atomic size of silicon (atomic radius 118pm) than that of carbon (atomic radius=77pm). As a result, the electrons are less closely attracted to the nucleus in silicon atom as compared in carbon atom. The ionization enthalpy of silicon is, therefore, less, for exmple, `Delta_(i)H_(I)` of silicon is 786kJ `"mol"^(-1)` while that of carbn is 1086kJ `"mol"^(-1)`.