What are the characteristics of the transition elements and why are they called transition elements? Which of the d-block elements may not be regarded as the transition elements?
Transition elements exhibit typical metallic properties such as high tensile strength, ductility, malleability, thermal and electrical conductivity and metallic character. Their melting and boiling points are high which are attributed to the involvement of (n-1)d electrons resulting into strong interatomic bonding. The presence of partly filled d or f orbitals in their atoms makes transition elements different from non-transition elements.
They are called transition elements because originally, their chemical properties were transitional between those of s-block and p-block elements. According to IUPAC, transition metals are defined as metals which have incomplete d subshell either in neutral atom or in their ions.
The d-block elements that may not be regarded as transition elements are zinc, cadmium and mercury of group 12. These elements have full d¹⁰ configuration in their ground state as well as in their common oxidation states. Hence, they do not fulfill the IUPAC criterion of having an incomplete d subshell.
However, being the end members of the 3d, 4d and 5d transition series respectively, their chemistry is studied along with the chemistry of transition metals.
Explanation
The answer covers all three parts of the question: (1) Characteristics from the Summary section including metallic properties, high melting/boiling points, and partly filled orbitals; (2) The naming reason from the IUPAC definition section explaining both the original reasoning and modern definition; (3) The exception of Zn, Cd, Hg from Group 12 which have complete d¹⁰ configuration and thus don't meet the IUPAC criterion for transition elements.