Transition elements are defined as elements that have incompletely filled d-orbitals in their ground state or in any of their common oxidation states.

• Not all d-block elements are transition elements. Zinc (Zn), Cadmium (Cd), and Mercury (Hg) are not considered transition elements because they have a completely filled d¹⁰ configuration in their ground state and their common oxidation state (+2). Their general electronic configuration is (n-1)d¹⁰ns².
• For instance, Scandium (Sc) is a transition element as it has an incompletely filled d-orbital in its +3 oxidation state.