The transition metal having highest 3ʳᵈ ionisation enthalpy is :

Solution

Chemistry ma’am · 3 steps

Third ionisation pulls an electron out of the M²⁺ ion, so look at each M²⁺ configuration.

1. Which ion loses. The third ionisation enthalpy is the energy to remove an electron from M²⁺. The two 4s electrons are already gone, so the third electron comes from the 3d set.
2. Stability rule. A half-filled 3d⁵ set is extra stable because of its large exchange energy. Breaking a 3d⁵ set costs a lot of energy. Reaching 3d⁵ costs much less.

   Metal	M²⁺	After losing the third electron	Third IE
   V	3d³	3d²	lowest of the four
   Cr	3d⁴	3d³	moderate
   Mn	3d⁵, half-filled	3d⁴, half-filled set broken	highest
   Fe	3d⁶	3d⁵, half-filled set reached	drops below Mn

3. So. Only Mn²⁺ (3d⁵) has to give up an electron from a stable half-filled set. Mn therefore has the highest third ionisation enthalpy, option (a).

The idea to keep. For the nth ionisation enthalpy, write the configuration of the ion with charge (n-1)+. If that ion is d⁵ or d¹⁰, expect a peak. If losing one electron gives d⁵, expect a dip.

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