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

Explanation

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 `Ti^(2+)` to `Mn^(2+)` , the electronic configuration changes from `3d^(2)` to` 3d^(5)` , 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., electrons in the 3d orbitals pair up and th enumber of half - filled orbital decreases. Hence, the stability of `+2` state decreases.