Why are zinc group elements sometimes included among the transition metals?

The zinc group elements—zinc, cadmium, and mercury—are sometimes included among the transition metals because they occupy the final position in the three main transition series of the periodic table. By one common definition, the transition metals are considered to span Groups 3 through 12, which encompasses the elements from scandium to zinc, yttrium to cadmium, and hafnium to mercury.123

However, it is important to note that this classification is not universal. In many chemical contexts, zinc, cadmium, and mercury are treated separately from the other transition metals. This is because they exhibit few of the properties characteristic of the other transition metals, such as the formation of stable compounds in multiple formal oxidation states. While they are often grouped with transition metals based on their location in the d-block of the periodic table, their distinct chemical behavior frequently leads scientists to categorize them as a separate zinc group element classification.14