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

Answer

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-orbital` are half-filled imparting greater and greater stability to `+2` state. in the second half,i.e,. `Fe^(2+)` to `Zn^(2+)` , the electronic configuration changes from `3d^(6)` to `3d^(10)` ,i.e., electron in the 3d orbitals pair up and the number of half-filled orbital decrease . Hence , the stability of +2 state decreases.