What are the characterstics 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 ?

Explanation

Characterstics of the transition elements (d-block)
(i) Electronics configuration General configuration of these elements is `(n-1)d^(1-10)ns^(1-2)`.
(ii) Physical properties These elements have metallic properties such as metallic lustre, high tensil strength, ductility, malleabillity, high thermal and electrical conductivity,high melting point (except Zn, Cd, Hg), hardness etc.
(iii) Atomic ionic Size In a Given series, radii decreases with increasing atomic numbers.
(iv) Ionisation enthalpies Due to an increase in nuclear charge which accompanies the filling of inner d-orbitals, there is an increase in ionisation enthalpy along each series of the transition elements from left to right.
(v) Oxidation states These elements exhibit variable oxidation states.
e.g. Cu shows two oxidation states `(+1 "and" +2)` and Mn shows oxidation states from `+72"to" +7`.
(vi) Magnetic properties These elements show diamagnetism and paramagnetism.
(vii) Formation of coloured salts The compounds of transition elements form coloured ions e.g. `Mn^(3+): "violet", Fe^(2+): "green"` etc.
(viii) Complex formation These elements form complex compounds due to their small size, availability of vacant d-orbitals and high charge density. e.g. `[P_(t)Cl_(4)]^(2-)`.
(ix) Catalyst many of these elements are used as catalyst. e.g. `V_(2)O_(5)` is used as a catalyst in contact process for the manufacture of `H_(2)SO_(4)`.
(x) Interstitial compound formation Transition elements form interstitial compounds, in which H, C or N, etc. are trapped inside the crystal lattices of metals.
The elements which have incompletely filled d-orbitals in its ground state or in any of its oxidation state are called (Zn), cadmium (Cd), mercury (Hg) of group `12` have full `d^(10)` configuration in theri ground state as well as in their common oxidation states, thus they are not regarded as transition metals.