Transition elements are distinguished by the presence of partly filled d or f orbitals in their atoms. This characteristic sets them apart from non-transition elements, allowing for different chemical behaviors.

IUPAC defines transition metals as metals with an incomplete d subshell either in neutral atoms or ions. Zinc, cadmium, and mercury are excluded due to their full d 10 configuration in both ground state and common oxidation states.

The d-block elements from groups 3-12 are known as transition metals, while the f-block elements, specifically 4f and 5f orbitals, are referred to as inner transition metals.

The two series are lanthanoids and actinoids. The 4f series comprises elements from Cerium (Ce) to Lutetium (Lu).

The 3d series includes elements from Scandium (Sc) to Zinc (Zn).