OXYGEN GROUP ELEMENT

also called chalcogen any of the five chemical elements comprising Group VIa of the periodic classificationnamely, oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). In cosmic abundance oxygen ranks fourth among the elements, after hydrogen, helium, and neon. In mass, however, it makes up approximately 20 percent of the Earth's lower atmosphere, about 46 percent of its solid crust, and nearly 90 percent of its waters. Sulfur is widely scattered throughout the universe, but is only ninth among all elements in total abundance. It accounts for about 12 percent of the mass of certain meteorites, but constitutes only from 0.03 to 0.06 percent of the Earth's crust. Selenium, tellurium, and polonium are much rarer than sulfur. The chalcogens are distinguished by an electron configuration in which six electrons occupy the outermost shell. Any atom having such an electronic structure tends to form a stable shell of eight electrons by adding two more and producing an ion with a double negative charge. This tendency to form negatively charged ions is quantitatively expressed in terms of two properties: electronegativity (the assumption of a partial negative charge when present in covalent combination) and electron affinity (the ability of a neutral atom to take up an electron and form a negative ion). With the exception of fluorine, oxygen has the highest electronegativity and electron affinity of any known element. Both these properties decrease in intensity among the other chalcogens with increasing atomic number and mass. Another property common to all the chalcogens except polonium is catenationi.e., the bonding of an atom to an identical atom. Sulfur manifests this mode of combination most distinctly. Figure 1: Modern version of the periodic table of the elements. To see more information about an also called chalcogen, any of the five chemical elements comprising Group VIa of the periodic classificationnamely, oxygen, sulfur, selenium, tellurium, and polonium. (See Figure.) A relationship among the first three members of the group was recognized as early as 1829; tellurium was assigned its place by 1865, and the discovery of polonium in 1898 completed the group. By the early 1970s, the possibility of finding element 116, the next member of Group VIa, in nature appeared to be negligible. Estimates of the proportions of the various kinds of atoms in the universe put oxygen fourth in abundance, after hydrogen, helium, and neon, but the importance of such a ranking is slight since hydrogen atoms account for almost 94 percent of the total and helium for most of the rest. About three atoms out of 10,000 are oxygen, but because the mass of an oxygen atom is approximately 16 times that of a hydrogen atom, oxygen constitutes a larger fraction of the mass of the universe, though still only about 0.5 percent. In the regions ordinarily accessible to man, howeveri.e., within a few kilometres of the surface of the Earthoxygen is the most abundant element: in mass, it makes up about 20 percent of the air, about 46 percent of the solid crust of the Earth, and about 89 percent of the water. Oxygen is represented by the chemical symbol O. In the air, oxygen exists mostly as molecules each made up of two atoms (O2), although small amounts of ozone (O3), in which three atoms of oxygen make up each molecule, are present in the atmosphere. Oxygen is a colourless, odourless, tasteless gas essential to living organisms, being taken up by animals, which convert it to carbon dioxide; plants, in turn, utilize carbon dioxide as a source of carbon and return the oxygen to the atmosphere. Oxygen forms compounds by reaction with practically any other element, as well as by reactions that displace elements from their combinations with each other; in many cases, these processes are accompanied by the evolution of heat and light and in such cases are called combustions. In cosmic abundance, sulfur ranks ninth among the elements, accounting for only one atom of every 20,00030,000. Sulfur occurs in the uncombined state as well as in combination with other elements in rocks and minerals that are widely distributed, although it is classified among the minor constituents of the Earth's crust, in which its proportion is estimated to be between 0.03 and 0.06 percent. On the basis of the finding that certain meteorites contain about 12 percent sulfur, it has been suggested that deeper layers of the Earth contain a much larger proportion. Seawater contains about 0.09 percent sulfur in the form of sulfate. The most important source is underground deposits of very pure sulfur present in domelike geologic structures where the sulfur is believed to have been formed by the action of bacteria upon the mineral anhydrite, in which sulfur is combined with oxygen and calcium. Deposits of sulfur in volcanic regions probably originated from gaseous hydrogen sulfide generated below the surface of the Earth and transformed into sulfur by reaction with the oxygen in the air. Sulfur exists under ordinary conditions as a pale yellow, crystalline, nonmetallic solid; it is odourless and tasteless, combustible, and insoluble in water. Its chemical symbol is S. It reacts with all metals except gold and platinum, forming sulfides; it also forms compounds with several of the nonmetallic elements. Several million tons of sulfur are produced each year, mostly for the manufacture of sulfuric acid, which is widely used in industry. The element selenium (symbol Se) is much rarer than oxygen or sulfur, comprising approximately 90 parts per billion of the crust of the Earth. It is occasionally found uncombined, accompanying native sulfur, but is more often found in combination with heavy metals (as copper, mercury, lead, or silver) in a few minerals. The principal commercial source of selenium is as a by-product of copper refining; its major uses are in the manufacture of electronic equipment, in pigments, and in making glass. The gray, metallic form of the element is the most stable under ordinary conditions; this form has the unusual property of greatly increasing in electrical conductivity when exposed to light. Selenium compounds are toxic to animals; plants grown in seleniferous soils may concentrate the element and become poisonous. Tellurium is a silvery-white element (symbol Te) with properties intermediate between those of metals and nonmetals; it makes up approximately one part per billion of the Earth's crust. Like selenium, it is less often found uncombined than as compounds of metals such as copper, lead, silver, or gold, and is obtained chiefly as a by-product of the refining of copper or lead. No large use for tellurium has been found. Polonium (symbol Po) is an extremely rare, radioactive element found in minerals containing uranium. It has some scientific applications as a source of alpha radiation. Additional reading The biosphere and ecosystems related to sulfur and selenium are discussed in A.B. Roy and P.A. Trudinger, The Biochemistry of Inorganic Compounds of Sulphur (1970); William E. Winner, Harold A. Mooney, and Robert A. Goldstein (eds.), Sulfur Dioxide and Vegetation: Physiology, Ecology, and Policy Issues (1985); and Eric S. Saltzman and William J. Cooper (eds.), Biogenic Sulfur in the Environment (1989). Allotropy and structure are presented in an article by L.B. Gittinger, Sulphur, 1970, Engineering and Mining Journal, 172:133136 (1971); and in the following books: K.W. Bagnall, The Chemistry of Selenium, Tellurium, and Polonium (1966); D.M. Chizhikov and V.P. Shchastlivyi, Selenium and Selenides (1968; originally published in Russian, 1964), and Tellurium and the Tellurides (1970; originally published in Russian, 1966); Ralph A. Zingaro and W. Charles Cooper (eds.), Selenium (1974); W. Charles Cooper, Tellurium (1971); F. Tuinstra, Structural Aspects of the Allotropy of Sulfur and the Other Divalent Elements (1967); Arthur V. Tobolsky and William J. MacKnight, Polymeric Sulfur and Related Polymers (1965); and Beat Meyer (ed.), Elemental Sulfur: Chemistry and Physics (1965). Compilations of physical and numerical data are found in Wendell M. Latimer, The Oxidation States of the Elements and Their Potentials in Aqueous Solutions, 2nd ed. (1952). Robert C. Brasted The Editors of the Encyclopdia Britannica