JEE Main · 2022 · 29 Jul 2022 · Shift 2 · d and f Block Elements

Enthalpy of hydration depends on the charge density and electronic configuration of the ion. Among the given ions, ${\mathrm{Mn}}^{2+}$ has a half-filled d⁵ configuration (${\mathrm{d}}^5$), which is particularly stable and results in weaker interaction with water molecules.

The hydration enthalpy values (in kJ/mol) are:

• ${\mathrm{Cr}}^{2+}$: -1925
• ${\mathrm{Mn}}^{2+}$: -1862 (least negative, hence lowest)
• ${\mathrm{Fe}}^{2+}$: -1998
• ${\mathrm{Co}}^{2+}$: -2079

Answer: (B) ${\mathrm{Mn}}^{2+}$