[satrohraj]

I was studying a topic called transition elements in my chemistry text.
There is a sentence saying
"All d-block elements are not true transition elements, for example, II B gp elements Zn, Cd, Hg are not considered as transition elements."

But they didn't mention the reason..
Can you help me with that?

[Bronwen Dekker]

I know that wikipedia is not the best source of information, but it does give an explanation for this as part of its definition of transition metals:

http://en.wikipedia.org/wiki/Transition_metal

Basically, oxidation of e.g. zinc results is a loss of s-electrons, not d-electrons, so its behaviour is more similar to the group II metals.

I remember writing an essay on whether or not zinc should be included in the transition metals and ended up being undecided and asking the biology students to vote between the aesthetic appearance of periodic tables where zinc was coloured the same as the transition metals or the group IIs. At the time I thought that I was being very clever.

[Yggdrasil]

Many of the transition metals' properties comes from the fact that their chemistry involves electrons from d-orbitals.  Since Zn, Cd, and Hg have a d¹⁰ configuration, most of their chemistry involves only the electrons in their s-orbitals.  Therefore, these metals do not exhibit some of the same chemical and physical properties associated with transition metals (e.g. colored ions, multiple oxidation states, ease of oxidation/reduction, etc.).