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

Except Sc, the most common oxidation state of the first row transition elements is +2 which arises from the loss of two 4s electrons. The +2 state becomes more and more stable in the first half of first transition elements with increasing atomic number because 3d orbitals acquires only one electron in each of the five 3d orbitals.