The correct order of the first ionization enthalpy is
- Ga > Al > B
- Tl > Ga > Al
- Al > Ga > Tl
- B > Al > Ga
Answer
(2) Tl > Ga > Al
Solution
iH1 (kJ mol-1): B 801, Al 577, Ga 579, In 558, Tl 589
∵ Ga: poor shielding by the 3d10 electrons (d-block contraction) ⇒ Ga > Al
∵ Tl: poor shielding by 4f14 and 5d10 electrons (lanthanoid contraction) ⇒ Tl > Ga
∴ Tl > Ga > Al
∵ B has the highest value and Ga > Al ⇒ (1), (3), (4) are wrong
Answer: (2)