The first ionization enthalpy values (in kJ mol–1) of group 13 elements are : B Al Ga In Tl 801 577 579 558 589 How would you explain this deviation from the general trend ?

Hello Student
On moving down a group,ionizationenthalpy generally decreases due to an increase in the atomic size and shielding. Thus, on moving down group 13,ionizationenthalpy decreases from B to Al. But, Ga has higherionizationenthalpy than Al. Al follows immediately afters–block elements, whereas Ga follows afterd–block elements. The shielding provided byd-electrons is not very effective. These electrons do not shield the valence electrons very effectively. As a result, the valence electrons of Ga experience a greater effective nuclear charge than those of Al. Further, moving from Ga to In, theionizationenthalpy decreases due to an increase in the atomic size and shielding. But, on moving from In toTl, theionizationenthalpy again increases. In the periodic table,Tlfollows after 4fand5delectrons. The shielding provided by the electrons in both these orbitals is not very effective. Therefore, the valence electron is held quite strongly by the nucleus. Hence, theionizationenergy ofTlis on the higher side.

Dear Student,
Please find below the solution to your problem.

On moving down a group,ionizationenthalpy generally decreases due to an increase in the atomic size and shielding. Thus, on moving down group 13,ionizationenthalpy decreases from B to Al. But, Ga has higherionizationenthalpy than Al. Al follows immediately afters–block elements, whereas Ga follows afterd–block elements. The shielding provided byd-electrons is not very effective. These electrons do not shield the valence electrons very effectively. As a result, the valence electrons of Ga experience a greater effective nuclear charge than those of Al. Further, moving from Ga to In, theionizationenthalpy decreases due to an increase in the atomic size and shielding. But, on moving from In toTl, theionizationenthalpy again increases. In the periodic table,Tlfollows after 4fand5delectrons. The shielding provided by the electrons in both these orbitals is not very effective. Therefore, the valence electron is held quite strongly by the nucleus. Hence, theionizationenergy ofTlis on the higher side.

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