Question

Why are $Zn$ , $Cd$ , and $Hg$ not transition elements?

Answer 1

Transition elements are elements with partially filled d- orbitals (sometimes known as transition metals). Transition elements are defined by IUPAC as elements with a partially filled d subshell or elements that have the ability to generate stable cations with an incompletely filled d- orbital.

Complete answer:
Transition elements are those that have partially filled d - orbitals in their ground state or any of their oxidation states.
Because of their electrical arrangement, elements like $Zn$ , $Cd$ , and $Hg$ are not transition elements ${\text{ }}\left( {n - 1} \right)$ ${d^{10}}n{s^2}$ is a generic representation of the electrical configuration of these three elements. When these elements are in their ground state, as well as in their typical oxidation state, their orbitals are totally filled. As a result, these parts are not transitional.

Additional information:
Zinc, cadmium, and mercury are not considered transition elements since their electrical configurations differ from that of other transition metals, as previously stated. The rest of the d-block elements, on the other hand, have properties that are quite comparable, and this resemblance can be seen down each row of the periodic table.

Note:
Since their d sub-shell is complete, some people do not regard cadmium, zinc, or mercury to be transition metals. This holds true for scandium as well. It doesn't have a complete d subshell, but there are no d electrons at all when it forms cations. As a result, some people don't think of scandium as a transition metal.