Atom

ATOM (from the Greek atomos-indivisible). The word A. is used in modern science in different senses. In most cases, A. refers to the ultimate quantity of a chemical element, the further division of which leads to the loss of the element's individuality, i.e., to a sharp change in its physical-chemical properties. Such an atom is fundamentally divisible and changeable. Electrons can be removed from it (or added to it) (ionization), it can be brought into various states (excitation), certain types of A. spontaneously decay, transforming into others (radioactivity). Sometimes it is more rational to call A. the ultimate electrically charged particles, electrons (-) and protons (+), from which all matter is composed. These particles, as far as is known, are truly indivisible, unchangeable, and are elementary.--One can distinguish four stages in the development of atomic theory. I. The idea of the discontinuity of matter, of separate, absolutely indivisible and unchangeable particles, the movement and combination of which gives all the diversity of phenomena, arose in the 5th century B.C. (Leucippus, Democritus). II. In the 17th and 18th centuries, the idea of A. becomes more distinct. A. begin to hypothetically be attributed different mechanical properties (e.g., elastic balls). But A. in this era remains an arbitrary hypothesis. III. The results of quantitative chemical analysis provide a solid experimental basis for atomic doctrine. It becomes necessary to explain the law of multiple proportions by Dalton. Conversely, on the basis of this law, the relative masses of atoms of different elements (atomic weights) are determined. The strengthened concept of A. serves as the basic assumption of the mechanical theory of heat and the theory of gases. The concept of a molecule as a specific group of A., connected by chemical forces, is clarified. Mendeleev's periodic law establishes a close relationship between different types of A. IV. The study of the passage of electricity through gases, the discovery of radioactivity and other facts clarified the variability, divisibility and, consequently, the complex structure of the chemical A. On the other hand, charged A. of helium (alpha-particles) and electrons, flying out of radioactive elements, possess such great speeds and, consequently, energy that the action of each A. can be detected separately, by flashes of phosphorescence (scintillations), by electrical and photographic effects. Thanks to this, the path of each fast charged A. can become visibly observable. Thus, the discontinuity of matter becomes obvious. The element of discontinuity is further found in the laws of interaction of particles of matter and in radiation (quantum theory). The application of quantum theory to atomic spectra makes it possible to know the internal structure of A. Thus, from a philosophical conjecture, the atom gradually becomes first a plausible hypothesis, then an inevitable hypothesis, and finally an undeniable reality. The modern doctrine of A. All types of electrically neutral matter are built from elementary particles of two types: negatively charged electrons and positively charged protons. All protons and part of the electrons are concentrated in a small volume-in the center of A., forming a massive compact "nucleus" with a residual positive charge. The remaining electrons are located on the periphery of A., rotating in orbits of larger or smaller sizes. Corresponding to this concept of the structure of A., it is customary to compare it with a planetary system. When electrons and protons combine into atoms, the charges add up strictly additively, the total charge equals the algebraic sum of the charges; therefore, the number of protons of a neutral A. must exactly equal the number of electrons. In a neutral atom, the positive charge of the nucleus must be balanced by the negative charge of the outer electrons; therefore, the number of outer electrons is determined by the charge of the nucleus, and not its mass. The charge of the nucleus or the number of outer electrons will mainly determine the physical-chemical properties of A. By changing the charge of the nucleus, starting with H, successively by 1, we will gradually move from one cell of the periodic system of elements to another until uranium, in which the charge of the nucleus is 92 elementary units. Thus, the "serial number" of an element in the periodic system immediately indicates the charge of its nucleus, or the number of outer electrons. The composition and structure of the nucleus can be different with the same residual positive charge. For this, it is sufficient for the difference in charges between protons and intra-nuclear electrons to remain constant. Thus, in the same cell of the periodic table*, there can be A. with different nuclei, but with the same number of outer electrons (isotopes, see). The laws that determine the interaction of electrons and protons in A. differ from the classical laws of mechanics and electromagnetism in that they include the element of discontinuity and the integer nature of relationships (quantum laws). It was possible to give a complete theory of the structure of A. and its stationary states only in relation to hydrogen (see).