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the stavny parts of substance constructed of atoms with identical electric charge of a kernel and electron shells. Elementary, or simple, substance — the substance consisting of atoms only of one element. The ground mass of living material on Earth chemical elements — oxygen make (see), carbon (see), hydrogen (see), nitrogen (see), calcium (see) and phosphorus (see). Treat the chemical elements which are a part of living material also potassium (see), copper (see), iodine (see), fluorine (see), manganese (see), molybdenum (see), cobalt (see), vanadium (see) and selenium is gray (see), chlorine (see), sodium (see), magnesium (see), iron (see), zinc (see), (see). Chemical elements, contents to-rykh in fabrics does not exceed concentration 1:100 000, call microelements (see), from them absolutely necessary for life activity consider cobalt, iron, copper, zinc, manganese, iodine, fluorine and bromine (see); possibly necessary — aluminum (see), strontium (see), molybdenum, selenium, nickel (see), silicon (see), vanadium. A role of bismuth (see), silver (see) and others X. aa., the fabrics which are a part and biol. liquids of the person and animals, it is studied still insufficiently. Many X. aa. are used in medicine as pharmaceuticals (see); content of a row is X. aa. in fabrics, blood, urine, etc. is an important diagnostic character of many diseases of the person and animals. Nek-ry X. aa. can represent professional harm (see) on the productions connected with their receiving and use.

The nuclear charge (see an atomic nucleus) expressed in terms of a charge of a proton defines atomic number of X. aa. Kinds of atoms (see Atom) same X. aa. with various quantity of neutrons in a kernel and, therefore, with various masses (see. Atomic weight) are called isotopes (see). By 80th 20 century the science knows 107 chemical elements, the most part to-rykh has but two and more radioactive or stable isotopes. The greatest number of stable isotopes — 10 — has tin (see).

For designation X. aa. the special symbols representing one or two first letters of the Latin name of an element serve.

First scientific definition of X. aa. as simple indivisible substances R. Boyle in 1661 gave, but this formulation found universal recognition only at the end of 18 century thanks to A. Lavoisier's works, to-ry in 1789 made the first list X. aa. On the basis of such definition J. Dalton suggested about the invisible atoms characterizing X. aa. Kannizzaro's hypothesis (S. Cannizzaro, I860) about communication of weight quantities of chemical elements with their atomic weight (weight) allowed to determine for the first time precisely the mass of atoms various by X. aa. also served as premises for classification of X. aa. on combining weights — the periodic system created by D. I. Mendeleyev in 1869 (see. Periodic system chemical elemen t).

Modern definition of X. aa. it was formulated after opening by J. Chadwick of a neutron (1932) and creation of the neytronnoprotonny theory of a structure of an atomic nucleus.

In the nature 89 chemical elements are revealed, the others are received in the artificial way at nuclear reactions (see). Oceans and oceanic deposits contain the most part of elements I and VII of groups of a periodic system, usually in the form of ionic connections, and also elements II of group, most often in the form of carbonates (see). Other elements exist hl. obr. in the form of oxides and sulfides. In an elementary state only the few X are found. aa. In a part of Earth, available to mankind, oxygen and silicon — 47% and 29,5%, respectively have the greatest distribution. The ground mass of space substance (apprx. 99%) is made by hydrogen and helium. The elementary structure of planets of solar system, apparently, is identical.

In usual conditions 2 chemical elements — mercury (see) and bromine are in liquid state, 11 — in gaseous, the others — in the form of solids. Nek-rye H.E. exist in various allotropic modifications differing in physical and chemical properties, e.g., carbon exists in the form of graphite, diamond and lonsdaleite.

On reactivity of X. aa. divide into metals, atoms to-rykh give external electrons, and the nonmetals attaching electrons to the external cover. On the sequence of filling of electronic orbitals distinguish intransitive and transitional X. aa.: at the first s-and r-, at the second — d-and f-orbitals are completed.

Identification of X. aa. carry out I by chemical and physical methods, i.e. by a research of result of interaction of substances among themselves or on characteristic properties X. aa. The most characteristic properties sufficient for identification, temperature of boiling and melting, and also density are. Spectral methods of definition H.E are eurysynusic. By method a mass spectrometer (see) is possible for them less than for

1 hour of work qualitatively to define all elements in the studied sample and their percentage. Bibliography: Campbell of D. A. Sovre

mine general chemistry, the lane with English, t. 1, M., 1975; F and at r about in with to and y N. A.

Discovery of chemical elements and origin of their names, M., 1970; Chemistry and the periodic table, under the editorship of K. Sai, the lane with yaponsn., M., 1982.

V. A. Pekkel.