What are transition elements ? Which of the d block elements are not regarded as transition elements and why ?

Explanation

According to definition, the transition elements are those in which have Partly filled d-subshell in their elementary form or in their commonly occuring Electronic configuration of `Zn, Cd and Hg `and their common oxidation state `Zn^(2+), Cd^(2+), Hg^(+)` and `Hg^(2+)` as follows:
`Zn (Z=30) [Ar]3d^(10) 4s^2`
Cd has electronic configuration :
`[Kr] 4d^10 5s^2`

`Hg(Z = 80) : [Xe] 4f^14 5d^10 6s^2`
`Hg^(+) : [Xe] 4f^(14) 5d^10 6s^(1)`
`Hg^(2+) : [Xe] 4f^(14) 5d^(10)`
`Hg^(2+) : [Xe] 4f^(14) `

Since Zn, Cd.and Hg do not have Partly filled d-orbitals in their elementary states or most common oxidation states, these are not classified as a transition elements.