IUPAC defines transition metals as metals having an incomplete d subshell either in the neutral atom or in their ions. Zinc (Zn), cadmium (Cd), and mercury (Hg) are not considered transition metals because they have full d¹⁰ configuration in their ground state as well as in their common oxidation states. Specifically, in the case of group 12 elements like zinc, cadmium, and mercury, the d orbitals are completely filled, making them exceptions to the typical definition of transition metals. Their electronic configurations do not align with the characteristic incompletely filled d subshells associated with transition elements as defined by IUPAC.