CHEMICAL ELEMENT

also called element, any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Elements are the fundamental materials of which all matter is composed. This article considers the origin of the elements and their abundances throughout the universe. The geochemical distribution of these elementary substances in the Earth's crust and interior is treated in some detail, as is their occurrence in the hydrosphere and atmosphere. The article also discusses the periodic law and the tabular arrangement of the elements based on it. For detailed information about the compounds of the elements, see chemical compound. The Editors of the Encyclopdia Britannica also called Element, any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Chemical elements constitute the fundamental materials of which all matter consists. By 1996, 112 elements were known. Of these, about 90 are found in nature either chemically free or in combination with other elements. The others are artificially produced. See the accompanying Table of chemical elements. Each element consists of atoms of only one type. The distinguishing mark of an element and its atoms is the number of protons in each atom's nucleus, which is designated by the term "atomic number." For example, the lightest element, hydrogen, has only one proton in its nucleus, and hence has the atomic number 1. The number of electrons surrounding the nucleus is also equal to the atomic number, and the sum of the number of protons and neutrons making up the nucleus is the "mass number," an integer approximately equal to the atomic weight. Atoms of the same atomic number that have differing numbers of neutrons in their nuclei are called isotopes. Roughly one-third of the elements found in nature occur in a chemically free state on Earth. These elements, which obviously are not very active chemically, include nitrogen, gold, platinum, copper, and the noble gases. The five most abundant elements in the Earth's crust are oxygen (461,000 parts per 1,000,000, or 46.1 percent), silicon (28.2 percent), aluminum (8.23 percent), iron (5.63 percent), and calcium (4.15 percent). Hydrogen is by far the most abundant element in the universe, accounting for more than 90 percent of the total number of atoms and for about three-fourths of the mass. Helium is next in abundance, constituting about 7 percent of the number of atoms and nearly one-fourth of the total mass. The properties of the elements are to a large degree attributable to the electronic structure and size of their atoms. Accordingly, they are extremely diverse. Helium, for example, has the lowest known melting point (below -272.2 C at 26 atmospheres pressure) and boiling point (-268.93 C at 1 atmosphere) of any of the elements, while tungsten has the highest known melting point (3,422 C at 1 atmosphere) and boiling point (5,555 C at 1 atmosphere). Densities at room temperature range between 0.089 grams per litre for hydrogen (a gas at that temperature) and about 22.6 g/cm3 for iridium and osmium. Most properties show fairly regular gradations within the main groups of the periodic table, whereas in the periods there is considerable divergence. Only 10 of the elements-carbon, sulfur, copper, antimony, iron, tin, gold, silver, mercury, and lead-were known in the uncombined state in ancient times. Although they were recognized as distinct varieties of matter, they were not classified as elements. In ancient and medieval philosophy, the elements were earth, air, fire, and water, the four simple substances of which all material bodies were thought to be compounded. The modern use of the concept dates from the early 1660s, when the English chemist Robert Boyle described elements as primitive and simple, or perfectly unmixed, bodies that are not made of any other bodies or of one another. From this time the term "element" was reserved for material substances, but, as early chemistry was chiefly preparative and descriptive, no criteria were immediately available for determining whether substances were elements or compounds. By the middle of the 18th century, however, chemistry became a quantitative science based on the use of the analytic balance, and the required chemical criteria were established. In 1789 Antoine-Laurent Lavoisier of France published the first scientific list of elements in his book Trait lmentaire de chimie (Elements of Chemistry). On the basis of data obtained in experimental studies of chemical reactions, he included 23 elements in his list. The formulation of the periodic law (q.v.) in 1869 by Dmitry I. Mendeleyev of Russia provided further stimulus to the search for new elements by indicating not only their existence but also their probable properties. Early in the 20th century two presumably different radioactive elements were discovered that had identical chemical properties and spectra but different densities and radioactive properties. The two specimens were found to be variants of the element thorium and were called isotopes by the English chemist Frederick Soddy upon suggestion from Margaret Todd. They have the same atomic number and thus occupy the same position (i.e., that of the element) in the periodic table but differ in the number of neutrons in the nuclei and thus have different mass numbers. In 1912 the English physicist Sir J.J. Thomson showed that the stable element neon, atomic number 10 and atomic weight 20.180, is a mixture of isotopes by separating the isotopes neon-20 and neon-22. Later, a third isotope, neon-21, was separated. Every element has at least one radioactive isotope. See also isotope. Additional reading General works Overviews are provided by Mary Elvira Weeks, Discovery of the Elements, 7th ed. rev. by H.J. Leicester (1968), a description of the events, both human and technical, surrounding the discovery of each of the elements and the implications on the ideas of the time; D.N. Trifonov and V.D. Trifonov, Chemical Elements: How They Were Discovered, trans. from Russian (1982); Esmarch S. Gilreath, Fundamental Concepts of Inorganic Chemistry (1958), a short, technical description of ideas concerning atomic structure, periodic relationships, and radioactivity; and Eduard Farber, The Evolution of Chemistry, 2nd ed. (1969), a history of ideas, methods, and materials from a chemist's viewpoint.The following works may be consulted for information on individual elements and groups of elements: P.A. Cox, The Elements (1989); R.T. Sanderson, Chemical Periodicity (1960), chapter 5, which provides a clearly written, nontechnical discussion of the physical properties of the elements individually up to xenon, followed by a discussion of the various groups and the trends to be observed in these groups; John Emsley, The Elements, 2nd ed. (1991); Clifford A. Hampel (ed.), The Encyclopedia of the Chemical Elements (1968); Allen J. Bard (ed.), Encyclopedia of Electrochemistry of the Elements (1973- ); E.I. Hamilton, The Chemical Elements and Man: Measurements, Perspectives, Applications (1979); N.N. Greenwood and A. Earnshaw, Chemistry of the Elements (1984); J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 16 vol. (1922-37), and supplements; Therald Moeller, Inorganic Chemistry: An Advanced Textbook (1952), with discussions of the physical forms and properties of the elements in the various periodic groups, as well as comments on their preparation; M. Cannon Sneed, J. Lewis Maynard, and Robert C. Brasted (eds.), Comprehensive Inorganic Chemistry, 8 vol. (1953-61); John C. Bailar, Jr., et al. (eds.), Comprehensive Inorganic Chemistry, 5 vol. (1973); F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. (1988); and A.G. Massey, Main Group Chemistry (1990).Two important comprehensive reference works are Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978-84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991, covering commercial preparations of elements and important compounds; and Gmelins Handbuch der anorganischen Chemie, 8th ed. (1924- ), arranged by element, with articles in German and English-since 1981 most of the articles have appeared in English, and the volumes now have English titles: Gmelin Handbook of Inorganic Chemistry (1981-89) and Gmelin Handbook of Inorganic and Organometallic Chemistry (1990- ). J.J. Lagowski The Editors of the Encyclopdia Britannica Origin of the elements Studies on the topic include Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, reprinted 1983); Claus E. Rolfs and William S. Rodney, Cauldrons in the Cosmos (1988); Virginia Trimble, "The Origin and Abundances of the Chemical Elements," Reviews of Modern Physics, 47(4):877-976 (October 1975), and "The Origin and Abundances of the Chemical Elements Revisited," The Astronomy and Astrophysics Review, 3(1):1-46 (1991); G.J. Mathews (ed.), Origin and Distribution of the Elements (1988), a collection of symposium papers; G.J. Wasserburg, "Short-Lived Nuclei in the Early Solar System," in David C. Black and Mildred Shapley Matthews (eds.), Protostars & Planets II (1985), pp. 703-754; D.L. Lambert, "The p-Nuclei: Abundances and Origins," The Astronomy and Astrophysics Review, 3:201-256 (1992). Geochemical distribution of the elements Introductory works include V.M. Goldschmidt, Geochemistry, ed. by Alex Muir (1954, reissued 1970), an authoritative, comprehensive account of the whole field of geochemistry; Brian Mason and Carleton B. Moore, Principles of Geochemistry, 4th ed. (1982), a standard introductory text; K.H. Wedepohl (ed.), Handbook of Geochemistry, 2 vol. in 6 (1969-78), reviewing each element in detail; Paul Henderson, Inorganic Geochemistry (1982); and John A. Tossell and David J. Vaughan, Theoretical Geochemistry: Applications of Quantum Mechanics in the Earth and Mineral Sciences (1992). The following works provide more specialized coverage on the Earth and solar system: Heinrich D. Holland, The Chemistry of the Atmosphere and Oceans (1978); Stuart Ross Taylor and Scott M. McLennan, The Continental Crust: Its Composition and Evolution (1985), which provides data on the oceanic crust as well; H. Wnke, G. Dreibus, and E. Jagoutz, "Mantle Chemistry and Accretion History of the Earth," in A. Krner, G.N. Hanson, and A.M. Goodwin (eds.), Archaean Geochemistry (1984), pp. 1-24; Horton E. Newsom and John H. Jones (eds.), Origin of the Earth (1990); Grant Heiken, David Vaniman, and Bevan M. French (eds.), Lunar Sourcebook (1991); John F. Kerridge and Mildred Shapley Matthews (eds.), Meteorites and the Early Solar System (1988); Stuart Ross Taylor, Solar System Evolution: A New Perspective (1992); and E. Anders and N. Grevesse, "Abundances of the Elements: Meteoritic and Solar," Geochimica et cosmochimica acta, 53(1):197-214 (January 1989). The Editors of the Encyclopdia Britannica Geochemical distribution of the elements Knowledge of the geochemical distribution of elements involves elucidation of the relative and absolute abundances of the chemical elements in the Earth and in its various parts-the crust, interior, atmosphere, and hydrosphere. This comprises a major part of the science of geochemistry, which is the study of the distribution of the chemical elements in space and time and the laws governing this distribution. Basic knowledge in this area was largely accumulated during the 19th century. As noted above, the concept of a limited number of chemical elements had been established by 1800, and the appearance of the periodic table, in 1869, provided a new insight into the limitations on the number of elements. Concurrent with these advances in chemical understanding, from about 1850 onward there was a steadily increasing output of analytical data on the Earth's rocks, minerals, and waters, mainly from laboratories in Europe and North America. The output from North America was materially increased following the establishment of the United States Geological Survey in 1879 and the appointment of Frank W. Clarke as chief chemist in 1884. Clarke's name will always be linked with the study of the geochemical distribution of the elements-indeed, the term clarke was proposed as the unit for the average percentage of an element in the Earth's crust by Soviet scientists and has been generally adopted. In 1889 Clarke wrote the first of his many publications on the geochemical distribution of the elements. He assembled many chemical analyses of rocks from different continents, calculated average values, and showed that the overall chemical compositions of continental areas are remarkably similar. By combining these averages he obtained values for the abundances of the commoner elements in the continental crust of the Earth, values that have not been materially changed in spite of the vast increase of available data since that time. He also estimated abundances for many of the less common elements; these estimates were based in many instances on very limited and imprecise data and subsequently have been improved. A further development of great significance was the assemblage of comprehensive data on the abundances of individual elements in terrestrial materials and in the Cosmos (based on solar and meteorite abundances) by the Norwegian geochemist Victor Moritz Goldschmidt during the 1930s. Goldschmidt's tables provided the basis for modern research on the geochemical distribution of the elements, and his compilation of data on cosmic abundances was the key to later theories on element synthesis in stars and supernovae. Goldschmidt also contributed to the understanding of elemental distribution within the Earth through his geochemical classification of the elements into lithophile, siderophile, chalcophile, and atmophile. Lithophile elements are those with a strong affinity for oxygen; they are concentrated in the crust or lithosphere as silicate and oxide minerals. Siderophile elements are principally metals that alloy readily with iron; Goldschmidt explained their scarcity in the Earth's crust by their concentration in the nickel-iron core (the siderosphere). Chalcophile elements are those with strong affinity for sulfur; they occur mainly as sulfides. And atmophile elements are gases, such as nitrogen, argon, and other rare gases, which are unreactive and hence accumulate in the atmosphere. (Goldschmidt also proposed a group of biophile elements, for those that concentrate in living matter-essentially carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus.) Terrestrial distribution The study of earthquake waves passing through the body of the Earth has shown that the interior is not uniform; it consists of distinct shells separated by concentric discontinuities at which the velocities of the passing waves change. The two major discontinuities that are universally recognized are the Mohorovicic Discontinuity, which divides the Earth's crust from its underlying mantle, and the Wiechert-Gutenberg Discontinuity, which separates the mantle from the core. The latter discontinuity exists at a depth of 2,900 kilometres (1,800 miles); it is marked by a sudden increase in density, from about 5.7 at the base of the mantle to 9.7 at the top of the core. The only reasonable interpretation of this discontinuity is that the mantle consists of silicates and oxides of the common elements (largely magnesium and iron), and the core consists of metallic iron alloyed with minor amounts of other elements (analogous to the nickel-iron in meteorites). The Mohorovicic Discontinuity varies in depth from place to place; it averages about 33 kilometres (20 miles) below the continents and about 8 kilometres (5 miles) below the bottom of the deep oceans. It too is marked by a density increase from crust to mantle-a comparatively small one, from about 3 to 3.3. To the three spherical divisions-crust, mantle, and core-two more should be added: the hydrosphere, which is the discontinuous shell of fresh and salt water, on and within the crust; and the atmosphere, the ocean of air that surrounds the Earth, gradually thinning into the vacuum of outer space. Origin of the elements The fundamental reaction that produces the huge amounts of energy radiated by the Sun and most other stars is the fusion of the lightest element, hydrogen, its nucleus having a single proton, into helium, the second lightest and second most abundant, with a nucleus consisting of two protons and two neutrons. In many stars the production of helium is followed by the fusion of helium into heavier elements, up to iron. The still heavier elements cannot be made in energy-releasing fusion reactions; an input of energy is required to produce them. The proportion of different elements within a star-i.e., its chemical composition-is gradually changed by nuclear fusion reactions. This change is initially concentrated in the central regions of the star where it cannot be directly observed, but it alters some observable properties of the star, such as brightness and surface temperature, and these alterations are taken as evidence of what is going on in the interior. Some stars become unstable and discharge some transmuted matter into interstellar space; this leads to a change in the chemical composition of the interstellar medium and of any stars subsequently formed. The main problem concerned with the origin of the chemical elements is to decide to what extent the chemical composition of the stars seen today differs from the initial chemical composition of the universe and to determine where the change in chemical composition has been produced. Reference is made in this article to the chemical composition of the universe, but most of the observations refer to our own and neighbouring galaxies. Cosmic abundances of the elements The relative numbers of atoms of the various elements are usually described as the abundances of the elements. The chief sources of data from which information is gained about present-day abundances of the elements are observations of the chemical composition of stars and gas clouds in the Galaxy, which contains the solar system and part of which is visible to the naked eye as the Milky Way; of neighbouring galaxies; of the Earth, Moon, and meteorites; and of the cosmic rays.