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

Except scandium (which shows an oxidation state of +3), all other first row transition elements show an oxidation state of +2. This is due to loss of two 4s electrons. In the first half, as we move from Ti²⁺ to Mn²⁺, the electronic configuration changes from 3d² to 3d⁵, i.e., more and more of d-orbitals are half-filled imparting greater and greater stability to +2 state. In the second half, i.e., Fe²⁺ to Zn²⁺, the electronic configuration changes from 3d⁶ to 3d¹⁰, i.e., electrons in the 3d orbitals pair up and the number of half-filled orbital decreases. Hence, the stability of +2 state decreases.