CARBON GROUP ELEMENT

any of the five chemical elements that make up Group IVa of the periodic tablenamely, carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). Elemental carbon is a minor component in the crust of the Earth, but it is among the more plentiful elements in the universe. Only hydrogen, helium, oxygen, neon, and nitrogen are more abundant. Silicon is a basic constituent of the Earth's crust, exceeded only by oxygen in abundance; it is the seventh most plentiful element in the universe. Germanium, tin, and lead, on the other hand, are relatively scarce both in the upper terrestrial layer and in the cosmos. The properties of the carbon group elements are intermediate between those associated with the elements of the adjacent boron and nitrogen groups of the periodic table. In all these groups the metallic properties, which result from the tendency to hold valence (outermost) electrons more loosely, increase with atomic number. This characteristic is most apparent in the case of the carbon group elements. Carbon is a nonmetal and silicon nearly so. Germanium resembles a metal in appearance and in certain other properties. Tin is almost completely metallic, whereas lead is a true metal in every respect. The atoms of the carbon group elements have a similar electronic structure. They all have four electrons in their outermost shells. Every element of the group forms covalent bonds with nonmetallic atoms. Carbon and silicon cannot lose or gain electrons to form free ions, while germanium, tin, and lead do form metallic ions but only with two positive charges. In the solid state, elemental carbon, silicon, germanium, and gray tin (alpha tin) occur as cubic crystals based on a three-dimensional arrangement of bonds. Each atom is covalently bonded to four neighbouring atoms, forming the corners of a tetrahedron. Decreasing melting points, boiling points, and heat energies associated with fusion, sublimation, and vaporization among these four elements with increasing atomic number and atomic size indicate a parallel weakening of the covalent bonds in this kind of structure. Figure 1: Modern version of the periodic table of the elements. To see more information about an any of the five chemical elements that make up Group IVa of the periodic tablenamely, carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). (See Figure.) Except for germanium, all of these elements are familiar in daily life either as the pure element or in the form of compounds, although, except for silicon, none is particularly plentiful in the Earth's crust. Carbon forms an almost infinite variety of compounds, in both the plant and animal kingdoms. Silicon and silicate minerals are fundamental components of the Earth's crust; silica (silicon dioxide) is sand. Tin and lead, with abundances in the crust lower than those of some so-called rare elements, are nevertheless common in everyday life. They occur in highly concentrated mineral deposits, can be obtained easily in the metallic state from those minerals, and are useful as metals and as alloys in many applications. Germanium, on the other hand, forms few characteristic minerals and is most commonly found only in small concentrations in association with the mineral zinc blende and in coals. Although germanium is indeed one of the rarer elements, it assumed importance upon recognition of its properties as a semiconductor (i.e., limited ability to conduct electricity). Additional reading Overviews are provided in Clifford A. Hampel (ed.), The Encyclopedia of the Chemical Elements (1968); and Therald Moeller, Inorganic Chemistry: An Advanced Textbook (1952). Specific coverage of individual carbon group elements can be found in F. Glockling, The Chemistry of Germanium (1969); and P.G. Harrison, The Chemistry of Tin (1989). George S. Hammond and Valerie J. Kuck (eds.), Fullerenes (1992), provides information on a recently discovered form of elemental carbon. The Editors of the Encyclopdia Britannica