Transition metal is traditionally defined as all chemical elements in the block-d on the periodic table, including zinc (Zn), cadmium (Cd) and mercury (Hg). This means is the group 3 to 12 on the periodic table. IUPAC then defined as all elements of the transition metals which have d orbital electrons that are incomplete or that only can form stable ions with incomplete d orbital. Based on this definition the above three elements (Zn, Cd, and Hg) are not included in the transition metals.

Based on the traditional definition, there are 40 elements including transition metals with atomic numbers 21 to 30, 39 to 48, 71 to 80, and 103 to 112. The name "transition" is obtained based on the position of these elements in the periodic table.

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