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Answer» `B gt Al gt Ga gt In gt Tl`
`B lt Al lt Ga lt In lt Tl`
`B lt Al gt Ga lt In gt Tl`
`B gt Al lt Ga gt In lt Tl` Solution :Ionization enthalpy `(Delta_iH)` decreases down the group as the size of atom increases and screening effect. Al to Ga : Ionization enthalpy increases slightly because both NUCLEAR charge and screening effect increases but due to poor SHIELDING by d-electrons effective nuclear charge on valence electron is more as a RESULT ionization enthalpy increases. On MOVING Ga to In again ionization enthalpy decreases due to shielding effect of d-electrons. On moving from In to Tl ionization enthalpy again increases because of poor shielding effect of 4F electrons. That is why effective nuclear charge increases and ionization enthalpy increases.
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