The atomic numbers of vanadium (M, chromium 21 (Cr), manganese (Mn) and iron (Fe) are, respectively 23, 24, 25 and 26. The one with highest second ionisation enthalpy has atomic number?

Fe+(26)=[Ar]3d6,4s1Mn+(25)=[Ar]3d5,4s1V+(23)=[Ar]3d2,4s2Cr+(24)=[Ar]3d5,4s0[ By first IP]The electronic configuration of Cr+ is most stable, hence formation of Cr2+ by second IP requires maximum enthalpy.