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

Explanation

Elements (`+2` state) `._(21)Sc^(2+), ._(22)Ti^(2+), ._(23)V^(2+), ._(24)Cr^(2+)` and `._(25)Mn^(2+)` have more stable `+2` oxidation state and the outer electronic configuration are `3d^(1), 3d^(2), 3d^(3), 3d^(4),` and `3d^(5)`, respectively. In all the elements listed, the removal of two `4s` eletrons (in `Cr^(2+), 1e^(-)` from `4s` and `1e^(-)` from `3d`), the `3d`-orbitals get gradually occupied. Since, the numner of empty d-orbitals decreases or the number of unpaired eletrons in `3d`-orbitals increases with increase in atomic number of cations, so the stability of the carions `(M^(2+))` increases from `Sc^(2+)` to `Mn^(2+)`.