The correct order of the first ionization enthalpy is

Solution

Chemistry ma’am · 3 steps

Group 13 does not fall smoothly: poorly shielding d and f electrons push Ga and Tl back up.

1. Expected trend. Down a group, atoms get bigger, so the first ionisation enthalpy should fall. From B to Al it does fall sharply, because Al has a whole new shell.
2. Poor shielding. Ga comes right after the ten 3d elements. Its ten 3d electrons shield the outer 4p electron poorly, so the effective nuclear charge rises and Ga holds its electron slightly more tightly than Al. Tl has fourteen 4f electrons as well as ten 5d electrons. They shield even worse, and the lanthanide contraction adds to this, so Tl's value jumps above Ga, In and Al.

   Element	First ionisation enthalpy (kJ mol⁻¹)
   B	801
   Al	577
   Ga	579
   In	558
   Tl	589

3. So. Among these three, 589 > 579 > 577, so the order is Tl > Ga > Al, which is option (b).

The idea to keep. In group 13, B is far highest. After that, Ga sits just above Al because of the 3d electrons, and Tl rises above both because of the 4f and 5d electrons. Do not assume a smooth fall down a group.

Answer from the NTA final answer key. Solution by PrepWiser.