In chemistry, the term transition metal (or transition element) has three possible meanings: In chemistry, the term transition metal (or transition element) has three possible meanings: English chemist Charles Bury (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 n = 3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32. These elements are now known as the d-block. In the d-block the atoms of the elements have between 1 and 10 d electrons.