Except Sc, the most common oxidation state of the first row transition elements is +2 which arises from the loss of two 4s electrons. The +2 state becomes more and more stable in the first half of first row transition elements with increasing atomic number because 3d orbitals acquire only one electron in each of the five 3d orbitals (i.e., each d-orbital remains half-filled) and inter electronic repulsion is the least and nuclear charge increases.
In second half of first row transition elements, electron starts pairing up in 3d orbitals and hence there is inter electronic repulsion. (Ti2+ to Mn2+ electronic configuration changes from 3d2 to 3d5 but in second half i.e., Fe2+ to Zn2+ it changes from 3d6 to 3d10.)
