Although thallium (Z = 81) has only slightly higher atomic radius (170 pm)than that ofindium (167 pm)butits ionizationenthalpy (589 kJ "mol"^(-1))is much higherthan that ofindium (558 kJ "mol"^(-1)) . Explainwhy /

Although thallium (Z = 81) has only slightly higher atomic radius (170

1.	Although thallium (Z = 81) has only slightly higher atomic radius (170 pm)than that ofindium (167 pm)butits ionizationenthalpy (589 kJ "mol"^(-1))is much higherthan that ofindium (558 kJ "mol"^(-1)) . Explainwhy /
Answer» Solution :Althrough on movingfrom `In (Z = 49)` to `TL (Z = 81)`, the nuclearincreases by 32units `(81-49)`yet due to poor SHIELDING effect on intervening 4f- and5d-electrons,effectivenuclear chargeaction on Tl is much higherthan that on Inand hence `Delta_(i)H_(1)`of `Tl`is much higherthan that of `In`.