The second ionisation enthalpies of chromium and copper are unusually…

The correct answer is Option (4) → The Cr⁺ and Cu⁺ ions have the stable d⁵ and d¹⁰ configurations, respectively.

The second ionisation enthalpy is the energy needed to remove an electron from the M⁺ ion. It is unusually high when M⁺ has a half-filled or completely filled d subshell, because these configurations are especially stable.

• Cr (Z = 24): 3d⁵ 4s¹; the first electron leaves 4s, so Cr⁺ is 3d⁵ (half-filled).

• Cu (Z = 29): 3d¹⁰ 4s¹; the first electron leaves 4s, so Cu⁺ is 3d¹⁰ (completely filled).

• The second electron must therefore come out of a stable d⁵ or d¹⁰ set, which needs extra energy.

Option (1) Chromium and copper have the highest enthalpies of atomisation in the series. — Incorrect.

• Their values (Cr 397, Cu 339 kJ mol⁻¹) are not the highest; vanadium, for example, has 515 kJ mol⁻¹.

• In any case, atomisation enthalpy is not part of the ionisation enthalpy of a gaseous ion.

Option (2) In both atoms, the second electron is removed from the 4s orbital. — Incorrect. Each atom has only one 4s electron, and it is lost in the first ionisation. The second electron comes from the 3d subshell.

Option (3) The Cr²⁺ and Cu²⁺ ions have half-filled d subshells. — Incorrect. Cr²⁺ is 3d⁴ and Cu²⁺ is 3d⁹; neither is half-filled. The second ionisation enthalpy depends on the stability of the ion that loses the electron, M⁺.

Option (4) The Cr⁺ and Cu⁺ ions have the stable d⁵ and d¹⁰ configurations, respectively. — Correct. Removing an electron from these stable configurations is difficult, so the second ionisation enthalpy is unusually high.

NCERT (2026-27 edition):
4.3.3 Ionisation Enthalpies – “…The dominant term is the second ionisation enthalpy which shows unusually high values for Cr and Cu where M⁺ ions have the d⁵ and d¹⁰ configurations respectively. The value for Zn is correspondingly low as the ionisation causes the removal of one 4s electron which results in the formation of stable d¹⁰ configuration.”