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In the Periodic Table, the elements are classified into four blocks namely; s, p, d and f after the type of the atomic orbital which receives the last electron. The d-b lock elements are called tra from fourth period and onwards. The d-block contains the elements of the groups 3-12. These elements possess properties that are transitional between the s- and p –block elements. In other words, they represent transition or change in properties from s-block to p-block and are, therefore, called transition elements.

The transition elements may be defined as the elements whose atoms or simple ions contain partially filled d-orbitals.

This definition of transition elements does not cover zinc, cadmium and mercury because they have completely filled d-subshell in their atoms and common oxidation states. As a result Zn, Cd, Hg do not exhibit characteristic properties of transition elements. However, they are studied along with other d-block elements.

All the elements belonging to this section are metals. Most of the transition metals have immense industrial importance. For example, the precious metals, gold, platinum, palladium, silver, etc., and commonly used metals such as iron, copper, mercury, etc., all belong to transition elements. In this Unit, we shall discuss the general characteristics of transition elements.