A characteristic property of d-block elements is their ability to exhibit a variety of oxidation states in their compounds. This is due to the fact that for bonding, in addition to ns electrons, these elements can use inner (n-1)d electrons as well because of very small difference in their energies. Thus, depending upon the number of d electrons involved in bonding, different oxidation states arise.
Except for scandium, the most common oxidation state of 3d elements is +2 which arises from the loss of two 4s electrons. This means that after scandium, d orbitals become more stable than s orbital. Compounds having oxidation states +2 and +3 of these elements have ionic bonds whereas bonds are essentially covalent in higher oxidation states.
Since transition metals exhibit multiple oxidation states, their compounds in the higher oxidation states are strong oxidizing agents as they tend to accept electrons and come to
stable lower oxidation states.