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

Zinc (Zn), cadmium (Cd), and mercury (Hg) are not considered transition elements because they do not meet the criteria typically associated with transition metals.

Transition metals are elements found in the d-block of the periodic table, which includes elements from scandium (Sc) to zinc (Zn), yttrium (Y), and cadmium (Cd) to mercury (Hg). These elements are characterized by the filling of their d orbitals in the electron configuration. Transition metals typically exhibit a range of oxidation states, have a tendency to form colorful compounds, and often show catalytic activity due to their partially filled d orbitals.

However, zinc (Zn), cadmium (Cd), and mercury (Hg) do not exhibit the typical characteristics of transition metals for the following reasons:

Electron Configuration: Transition metals have partially filled d orbitals in their electron configurations, but Zn, Cd, and Hg have completely filled d orbitals in their ground state configurations. For example:

Zn: [Ar] 3d^10 4s^2
Cd: [Kr] 4d^10 5s^2
Hg: [Xe] 4f^14 5d^10 6s^2
Oxidation States: Transition metals often exhibit multiple oxidation states, whereas Zn, Cd, and Hg typically exhibit only one stable oxidation state:

Zn: Usually exists in the +2 oxidation state.
Cd: Usually exists in the +2 oxidation state.
Hg: Usually exists in the +1 oxidation state for Hg(I) and +2 oxidation state for Hg(II).
Lack of Colorful Compounds: Transition metals are known for forming colorful compounds due to the presence of partially filled d orbitals, which can absorb and emit visible light. In contrast, Zn, Cd, and Hg compounds are often colorless.

Catalytic Activity: Transition metals are often used as catalysts in various chemical reactions due to their ability to change oxidation states easily. Zn, Cd, and Hg are not typically used as catalysts in the same way as transition metals.

In summary, while Zn, Cd, and Hg are located in the d-block of the periodic table, they do not exhibit the typical characteristics of transition metals, primarily because their electron configurations, oxidation states, and chemical behaviors differ significantly from those of typical transition elements.