Explain briefly how + 2 state becomes more and more stable in the half of the first row transition elements with increasing atomic number ?

Explanation

Except scandium (which exhibits only + 3 oxidation state), the most common oxidation state of the first row transition elements is +2 which arises from the loss of two 4s-electrons. In the first half, as we move from `Ti^(2+)` to `Mn^(2+)`, the outer electronic configuration changes from `3d^(2)` to `3d^(5)` i.e., the number of unpaired electrons in 3d - subshell increases with increasing atomic number, thereby imparting greater and greater stability to + 2 state. Increase in exchange energy is responsible for such increasing stability. In the second half i.e., from `Fe^(2+)` to `Zn^(2+)`, the electronic configuration changes from `3d^(6)` to `3d^(10)`. This causes a decrease in the number of unpaired the paired electrons. So the stability of + 2 state decreases successively.