Although thallium (Z = 81) has only slig...

Although thallium (Z = 81) has only slightly higher atomic radius (170 pm) than that of indium (167 pm) but its ionization enthalpy `(589 kJ "mol"^(-1))` is much higher than that of indium `(558 kJ "mol"^(-1))` . Explain why ?

Text Solution

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Althrough on moving from `In (Z = 49)` to `Tl (Z = 81)`, the nuclear increases by 32 units `(81-49)` yet due to poor shielding effect on intervening 4f- and 5d-electrons, effective nuclear charge action on Tl is much higher than that on In and hence `Delta_(i)H_(1)` of `Tl` is much higher than that of `In`.

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