130. 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? 

The important characteristics of transition metals are:
(i)    All transition elements are metallic in nature, e.g., all are metals.
(ii)   These metals exhibit variable oxidation states.
(iii)    Transition metal atoms or ions generally form the complexes with neutral, negative and positive ligands.
(iv)    Compounds of transition metals are usually coloured.
(v)    The compounds of these metals are usually paramagnetic in nature.
(vi)    Transition metals and their compounds act as good catalysts, i.e., they show catalytic activities.
(vii)    These metals form various alloys with other metals of the series.
(viii)    These metals form interstitial compounds with C, N, B and H.
The presence of partially filled d-orbitaIs in the electronic configuration of atomic and ionic species of these elements is responsible for the characteristic properties of transition elements. They are called transition elements because of their position in the periodic table. These elements lie in the middle of periodic table between s and p-blocks (i.e., between group 2 and group 13). A transition element may be defined as a element whose atom or at least one of its simple ions contain partially filled d-orbitals, e.g., iron, copper, chromium, nickel etc.
The general characteristic electronic configuration may be written as (n – 1)d^1–10ns^1–2.
The elements of group 12 i.e., Zinc, Cadmium, and Mercury are generally not regarded as transition elements as their atoms and all ions formed have completely filled d-orbitals i.e., these do not have partially filled d-orbitals in atomic state or common oxidation state (Zn²⁺, Cd²⁺, Hg²⁺).
Zn (30) = [Ar] 4s² 3d¹⁰ Zn²⁺ = [Ar] 3d¹⁰4s°
Cd (48) = [Kr] 5s² 4d¹⁰ Cd²⁺ = [Kr] 4d¹⁰5s°
Hg (80) = [Xe] 6s² 5d¹⁰ Hg²⁺ = [Xe] 5d¹⁰6s°