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 ?

Explanation

General characteristics of transition elements :
1. All transition elements except mercuty (which is a liquid) have typical metallic structure and show typical metallic properties, such as thermal and electrical conductivity, malleability, ductillity, metallic lustre, high tensile strength, etc.
2. They have melting and boiling points, high enthalpies of sublimation (or atomisation) and high enthalpies of vaporisation. These properties depend upon the strength of metallic bond in them.
3. They have high densities as compared to s - block metals.
4. Their first ionisation enthalpies are higher than those of s - block elements and less than those of p - block elements.
5. These are electropositive in nature.
6. They exhibit variable oxidation states.
7. Transition metal atoms or ions generally form complexes with suitable ligands.
8. Compounds of transition metals are usually coloured.
9. Their compounds are usually paramagnetic in nature.
10. A number of transition metals and their compounds exhibit catalytic properties.
11. These metals form alloys with other metals of the series.
12. They form intersitial compounds with C, N, B and H. The presence of incompletely filled d - orbitals in the atomic and ionic species of the transition elements is responsible for their characteristic properties. They are placed in between highly electropositive metals of s - block on one side, and less electropositive metals and electronegative non - metals on the another side. Hence, they are termed as 'transition elements.' The elements Zn, Cd and Hg belonging to d - block are not regarded as transition elements, because their atoms in the ground and ions in common oxidation states do not have partially filled d - orbitals.