The elements of Group 13 with highest and lowest first ionisation enthalpies are respectively :

Solution

Chemistry ma’am · 3 steps

Boron is the highest, but the downward trend fails at the bottom: indium is lowest, not thallium.

1. Expected trend. Down a group, atoms get bigger and the outer electron is held more loosely. So you would expect ionisation enthalpy to fall steadily from B to Tl.
2. Poor shielding. Ga comes after the 3d series, and Tl comes after the 4f and 5d series. These d and f electrons shield the nucleus poorly, so the outer electron feels a higher effective nuclear charge. That pushes the ionisation enthalpy of Ga and Tl back up.

   Element	First ionisation enthalpy (kJ mol⁻¹)	Reason
   B	801	smallest atom, highest
   Al	577	larger atom, sharp drop
   Ga	579	poor shielding by 3d, slightly above Al
   In	558	larger atom, lowest
   Tl	589	poor shielding by 4f and 5d, rises again

3. So. The highest is B and the lowest is In, which is option (c).

The idea to keep. In Group 13, the d and f electrons shield poorly, so Ga and Tl break the downward trend. The order is B > Tl > Ga > Al > In.

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