The atomic numbers of vanadium (V), Chromium (Cr), manganese (Mn) and iron (Fe) are respectively 23, 24, 25 and 26. Which one of these may be expected to have the highest second ionization enthalpy?

The atomic numbers of vanadium, chromium, manganese, and iron are respectively 23, 24, 25, 26. Chromium may be expected to have the highest second ionisation energy.

The electronic configuration of chromium is [Ar]3d⁵4s¹
It loses one electron to form [Ar]3d⁵4s⁰ in which 3d subshell is half-filled and stable. When another electron is removed, it is removed from half-filled 3d subshell and the stability of 3d subshell is lost. This requires higher energy.