Match the properties given in list-I with the metals given in list-II…

The correct answer is Option (2) → (A)-(II), (B)-(I), (C)-(IV), (D)-(III)

Ionisation enthalpies are unusually high when the electron has to be removed from a stable (half-filled or completely filled) d subshell, and the enthalpy of atomisation is high when the atom has many unpaired electrons for metal–metal bonding.

• (A) Element with highest second ionization enthalpy — This matches (II) Cu. Cu (Z = 29) is [Ar]3d¹⁰4s¹; the first electron leaves 4s and gives \( \ce{Cu^{+}} \): 3d¹⁰. The second electron must come out of this completely filled 3d¹⁰ set, so Δ_iH₂ of Cu (1958 kJ mol⁻¹) is the highest.

• (B) Element with highest heat of atomisation — This matches (I) Co. Δ_aH°: Co 425, Cr 397, Cu 339, Zn 126 kJ mol⁻¹; Zn has no unpaired d electrons for bonding and is the lowest.

• (C) M in M(CO)₆ — This matches (IV) Cr. Hexacarbonylchromium(0), \( \ce{Cr(CO)6} \), is the octahedral homoleptic carbonyl; Ni forms \( \ce{Ni(CO)4} \) and Fe forms \( \ce{Fe(CO)5} \).

• (D) Element with highest third ionization enthalpy — This matches (III) Zn. Zn (Z = 30) is [Ar]3d¹⁰4s²; removing the two 4s electrons gives \( \ce{Zn^{2+}} \): 3d¹⁰. The third electron must break the full 3d¹⁰ set, so Δ_iH₃ of Zn (3837 kJ mol⁻¹) is the highest.

Shortcut: (A)-(II) appears in Options (1) and (2), and (B)-(I) leaves only Option (2).

Note: The same/similar question has been asked in different years by NTA as per the details given below: