Calculate the number of unpaired electrons in the following gaseous ions: Mn3+, Cr3+, V3+ and Ti3+. Which one of these is the most stable in aqueous solution?

The ions with number of unpaired electrons is as follows:
Ions electronic configuration Number of unpaired electrons
Mn3+ [Ar] 3d4 4
Cr3+ [Ar] 3d3 3
V3+ [Ar] 3d2  2
Ti3+ [Ar] 3d1 1

The most stable ions in an aqueous solution is Cr3+ because in presence of water ligand 3d orbital of Cr gets split into two levels, t2g and eg. Thus, all the three electrons are present in lower level that is t2g and it stabilized the Cr3+ ion.