Question

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

Answer 1

$+2$ state becomes more and more stable in the first half of the first row transition elements with increasing atomic number as 3d orbitals acquire only one electron in each of five 3d orbitals (remain half-filled) and the electronic repulsion is least and the nuclear charge increases. In the second half of the first row transition series, electrons pair up in 3d orbitals. This increases the electronic repulsion.