JEE Main · 2024 · 04 Apr 2024 · Shift 2 · Classification of Elements and Periodicity

The correct order of the first ionization enthalpy is

(A) $Al > Ga > Tl$
(B) $Ga > Al > B$
(C) $B > Al > Ga$
(D) $Tl > Ga > Al$ ✓ Correct

Answer: (D)

Solution

Down group 13, the first ionization enthalpy does not fall monotonically because of poor shielding by intervening filled d- and f-orbitals: Ga's IE exceeds Al's due to poor shielding by the filled 3d subshell ("d-block contraction"), and Tl's IE exceeds Ga's due to poor shielding by the filled 4f subshell (lanthanide contraction, plus relativistic effects).
Net order: Tl > Ga > Al.

JEE Main · 2024 · 04 Apr 2024 · Shift 2 · Single correct option

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