In transition elements, there are greater horizontal similarities in the properties in contrast to the main group elements because of similar ns² common configuration of the outermost shell.
An examination of common oxidation states reveals that excepts scandium, the most common oxidation state of first row transition elements is +2 which arises from the loss of two 4s electrons. This means that after scandium, d-orbitals become more stable than the s-orbital.
Further, +2 state becomes more and more stable in the first half of first row transition elements with increasing atomic number because 3d orbitals acquire only one electron in each of five 3d orbitals (i.e. remains half filled) and electronic repulsion is the least and nuclear charge increases. In 2nd half of first row transition elements, electrons starts pairing up in 3d orbitals.
(Ti²⁺ to Mn²⁺ electronic, configuration changes from 3d² to 3d⁵ but in 2nd half i.e. Fe²⁺ to Zn²⁺ it changes from d⁶ to d¹⁰).