$Zn,Cd$ and $Hg$are not regarded as transition elements. Give reason.

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Hint:At first think about the plane of zinc, cadmium, and mercury in the periodic table so that you can get an idea about these three elements. The d-block elements are called transition elements in periodic tables.

Complete step by step answer:
Zinc, cadmium, and mercury are group $12$ elements in the periodic table. They are placed in d-block elements as their valence electron enters into the d-subshell.
The d-block elements are called transition elements because the transition elements are defined as the elements whose atoms or simple ions contain partially filled d –orbitals. The main reason for not considering transition elements is zinc, cadmium, and mercury has fully filled d-orbitals.
Additional Information:- $Zn,Cd$ and $Hg$ have low values of enthalpies of atomization. They all are soft metals as their all electrons are paired and metallic bonds are not strong enough. Zinc and cadmium shows $ + 2$ oxidation state whereas mercury shows $ + 1, + 2$ oxidation states. Zinc is diamagnetic in nature. All three elements occur naturally. They are widely used in electric and electronic applications as well as in various alloys. Zinc and cadmium show similar properties as they are solid metals at standard conditions. Mercury is the only metal that is a liquid at room temperature. Cadmium and mercury both are highly toxic but zinc is very important in the biochemistry of living organisms.
Don’t get confused with the transition and inner transition elements. Transition elements are d-block elements due to which their valence electron enters into d-orbital. Inner-transition elements are f-block elements due to which their valence electron enters into f-orbital.
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