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

Element (+2 state)`""_(21)Se^(2+) ""_(22)Ti^(2+) ""_(23)V^(2+) ""_(24)Cr^(2+) ""_(25)Mn^(2+)` Electronic configuration `3d^1 3d^2 3d^3 3d^4 3d^5`.
  In all the elements listed, the removal of two 4s-electrons (in `Cr^(2+)`, i.e., from 4s and le from 3d), the 3d-orbitals get gradually occupied. Since, the number of empty d-orbitals increases with increase in atomic number of cations, so the stability of the cations `(M^(2+))` increases from `Sc^(2+)` to `Mn^(2+)`.