TRANSITION ELEMENT

any of various chemical elements that have valence electrons in two shells instead of only one. The elements from scandium to copper (atomic number 2129), yttrium to silver (3947), and lanthanum to gold (5779) are frequently designated as the three main transition series. All of the transition elements are metals. Many of them, such as iron, copper, gold, nickel, silver, titanium, and manganese, are of major economic or technological importance. Most of the transition metals have high melting points, low vapour pressures, and high densities. A large number combine with each other and with other metallic elements to form useful alloys. The majority of the transition elements dissolve in mineral acids, but a few, notably gold, silver, and platinum, are unaffected by nonoxidizing (simple) acids. The atoms of the transition elements use not only the electrons in their outermost shells for bonding but also those of the underlying d orbitals as well. Furthermore, as they are able to accept electrons in unoccupied d orbitals, transition metals and their stable compounds often manifest catalytic properties; i.e., they are able to effect chemical changes without themselves undergoing modification. For most transition metals, the compounds formed in at least one oxidation state are coloured and are paramagnetic (capable of being weakly magnetic). Figure 1: Modern version of the periodic table of the elements. To see more information about an any of various chemical elements that have valence electrons in two shells instead of only one. While the term transition has no particular chemical significance, it is a convenient name by which to distinguish the similarity of the atomic structures and resulting properties of the elements so designated. They occupy the middle portions of the long periods of the periodic table of elements (see Figure) between the groups on the left-hand side and the groups on the right. Specifically, they form groups 3 through 12. Of these 68 transition elements only 24 are treated here, the others being dealt with in other articles. Additional reading Edwin M. Larsen, Transitional Elements (1965), gives a brief and elementary topical discussion. D.L. Kepert, The Early Transition Metals (1972), presents a detailed discussion of the chemistry of the metals in groups 4, 5, and 6. J.P. Candlin, K.A. Taylor, and D.T. Thompson, Reactions of Transition-Metal Complexes (1968), covers its subject matter in great detail. Also useful is Geoffrey Wilkinson, Robert D. Gillard, and Jon A. McCleverty (eds.), Comprehensive Coordination Chemistry (1987). Charles L. Thomas, Catalytic Processes and Proven Catalysts (1970), reviews the actual uses of industrial catalysts by process type. Carl J. Ballhausen, Introduction to Ligand Field Theory (1962), gives a working account of how ligand field theory is applied to the d-block elements. The electronic structures of transition-metal compounds and complexes are covered in Carl J. Ballhausen and Harry B. Gray, Molecular Electronic Structures: An Introduction (1980); Carl J. Ballhausen, Molecular Electronic Structures of Transition Metal Complexes (1979); and D.T. Burns, A. Townshend, and A.G. Catchpole, Inorganic Reaction Chemistry, vol. 2, Reactions of the Elements and Their Compounds, 2 parts (1981). Also of interest are W.P. Griffith, The Chemistry of the Rarer Platinum Metals (1967); R.J.H. Clark, The Chemistry of Titanium and Vanadium (1968); R. Colton, The Chemistry of Rhenium and Technetium (1965); F. Fairbrother, The Chemistry of Niobium and Tantalum (1967); C.T. Horovitz (ed.), Scandium: Its Occurrence, Chemistry, Physics, Metallurgy, Biology, and Technology (1975); Elaine A. Seddon and Kenneth R. Seddon, The Chemistry of Ruthenium (1984); Richard J. Puddephatt, The Chemistry of Gold (1978); and S.A. Cotton and Frank A. Hart, The Heavy Transition Elements (1975). F. Albert Cotton The Editors of the Encyclopdia Britannica