JEE Main 2025 Apr 3 Shift 2, Chemistry Q24: d-Block Elements

Enthalpy of atomisation depends on the number of unpaired d-electrons available for metallic bonding. Approximate values (kJ mol${}^{-1}$): Sc 326, Mn 281 (low because of stable half-filled d${}^5$ s${}^2$ configuration), Co 425, Cu 339. Cobalt has the highest.

Co${}^{2+}$ has the electron configuration [Ar] 3d${}^7$, which in a free-ion picture has three unpaired electrons.

Spin-only magnetic moment: $\mu =\sqrt{n(n+2)}\text{BM}=\sqrt{3\times 5}=\sqrt{15}\approx 3.87$ BM.

$\therefore$ Rounded to the nearest integer, $\mu =4$ BM.