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

Explanation

The sum `IE_1+IE_2` increase. As a result, the standard reduction potentials `(E^(ɵ))` become less and less negative. Hence, the tendency to form `M^(2+)` ion decrease. The greater Stability of `+2` state for Mn is due to half-filled d-subshell `(d^5)` that for zinc is due to completely filled d-subshell `(d^(10))` and that for nickel is due to highest (negative) enthalpy of hydration.