Any chemical element with valence electrons in two shells instead of only one. This structure gives them their outstanding ability to form ions containing more than one atom (complex ions, or coordination compounds), with a central atom or ion (often of a transition metal) surrounded by ligands in a regular arrangement. Theories on the bonding in these ions are still being refined. The elements in the periodic table from scandium to zinc (atomic numbers 21–30), yttrium to cadmium (39–48), and lanthanum to mercury (57–80, including the lanthanide series) are frequently designated the three main transition series. (Those in the actinide series and beyond, actinium to copernicium [89–112], also qualify.) All are metals, many of major economic or industrial importance (e.g., iron, gold, nickel, titanium). Most are dense, hard, and brittle, conduct heat and electricity well, have high melting points, and form alloys with each other and other metals. Their electronic structure lets them form compounds at various valences. Many of these compounds are coloured and paramagnetic (see paramagnetism) and (as do the metals themselves) often act as catalysts. See also rare earth metal.
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