 Transition metal. In chemistry, the term transition metal or transition element has three possible meanings. The IUPAC definition defines a transition metal as an element whose atom has a partially filled D sub-shell, or which can give rise to cations with an incomplete D sub-shell. Many scientists describe a transition metal as any element in the D block of the periodic table, which includes groups 3 to 12 on the periodic table. In actual practice, the F block lanthanide and actinide series are also considered transition metals and are called inner transition metals. Cotton and Wilkinson expand the brief IUPAC definition see above by specifying which elements are included. As well as the elements of groups 4 to 11, they act scandium and yttrium in group 3 which have a partially filled D sub-shell in the metallic state. Lanthanum and actinium in group 3 are however classified as lanthanides and actinides respectively. English chemist Charles Barry 1890-1968 first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons. For example, lan equals 3 in the fourth row of the periodic table from a stable group of 8 to 1 of 18 or from 18 to 32. These elements are now known as the D block.