ATOM, when ordinarily used in chemistry and physics, refers to the smallest particle of an element which can exist either alone or in combination with similar particles of the same or of a differ ent element. The atom also refers to a quantity proportional to the atomic weight of an element. According to the theory of atomism, which dates from pre-Socratic times, the atom is one of the minute indivisible particles of which the whole universe is composed. (X.) Through the important experimental discoveries of the second half of the 19th century it became gradually clear that the atoms of the elements, far from being indivisible entities, had to be thought of as aggregates built up of separate particles. Thus from experiments on electrical discharges in rarefied gases, and es pecially from a closer study of the so-called cathode rays, the existence of small negatively charged particles—the mass of which was found to be about 2,000 times as small as the mass of the lightest atom, the hydrogen atom—was recognized. These small particles, which may be regarded as atoms of negative electricity, are now, following Johnstone Stoney, generally called electrons. Through the investigations of J. J. Thomson and others convinc ing evidence was obtained that these electrons are a constituent of every atom. On this basis a number of the general properties of matter, especially as regards the interaction between matter and radiation, receive a probable explanation.
In fact, the assumption that electrons are vibrating around positions of stable equilibrium in the atom offered a simple pic ture of the origin of spectral lines, which allowed the phenomena of selective absorption and dispersion to be accounted for in a natural way. Even the characteristic effect of magnetic fields on spectral lines discovered by Zeeman could, as was shown by Lorentz, be simply understood on this assumption. The origin of the forces which kept the electrons in their positions remained for a time unknown, as well as the way in which the positive electri fication was distributed within the atom. From experiments on the passage through matter of the high speed particles expelled from radioactive substances, however, Rutherford was in 1911 led to the so-called nuclear model of the atom. According to this the positive electricity is concentrated within a nucleus of dimen sions very small compared with the total space occupied by an atom. This nucleus is also responsible for practically the whole of the atomic mass.
Properties of the Elements.—The nuclear theory of the atom has afforded a new insight into the origin of the properties of the elements. These properties can be divided into two sharply dis tinguished classes.
1. To the first class belong most of the ordinary physical and chemical properties. These depend on the constitution of the electron cluster round the nucleus and on the way in which it is influenced by external agencies. This, however, will depend on the attractive force due to the nucleus which keeps the cluster together. On account of the small size of the nucleus compared with the distance apart of the electrons in the cluster, this force will to a high approximation be determined solely by the total elec tric charge of the nucleus. The mass of the nucleus and the way in which the charges and masses are distributed among the particles making up the nucleus itself will only have an exceedingly small influence on the behaviour of the electronic cluster.
2. To the second class belong such properties as the radioac tivity of the substance. These are determined by the actual in ternal structure of the nucleus. In the radioactive processes we witness, in fact, explosions of the nucleus in which positive or negative particles, the so-called a and 13 particles, are expelled with very great velocities.
The complete independence of the two classes of properties is most strikingly shown by the existence of substances which are indistinguishable from one another by any of the ordinary physi cal and chemical tests, but of which the atomic weights are not the same, and whose radioactive properties are completely different. Any group of two or more such substances are called isotopes (q.v.), since they occupy the same position in the classification of the elements according to ordinary physical and chemical proper ties. The first evidence of their existence was found in the work of Soddy and other investigators on the chemical properties of the radioactive elements. It has been shown that isotopes are found not only among the radioactive elements, but that many of the ordinary stable elements consist of isotopes, for a large number of the latter that were previously supposed to consist of atoms all alike have been shown by Aston's investigations to be a mixture of isotopes with different atomic weights. Moreover the atomic weights of these isotopes are whole numbers, and it is because the so-called chemically pure substances are really mixtures of iso topes, that the atomic weights are not integers.
The inner structure of the nucleus is still but little understood, although a method of attack is afforded by Rutherford's experi ments on the disintegration of atomic nuclei by bombardment with a particles. Indeed, these experiments may be said to have started a new epoch in natural philosophy in that for the first time the artificial transformation of one element into another has been accomplished (see TRANSMUTATION OF ELEMENTS). In what fol lows, however, we shall confine ourselves to a consideration of the ordinary physical and chemical properties of the elements and the attempts which have been made to explain them on the basis of the concepts just outlined.
It was recognized by Mendelejeff that when the elements are arranged in an order which is practically that of their atomic weights, their chemical and physical properties show a pronounced periodicity. A diagrammatic representation of this so-called periodic table is given in Table I., which represents in a slightly modified form an arrangement first proposed by Julius Thomsen. In the table the elements are denoted by their usual chemical symbols, and the different vertical columns indicate the so-called periods. The elements in successive columns which possess homologous chemical and physical properties are connected by lines. The meaning of the square brackets around certain series of elements in the later periods, the properties of which exhibit typi cal deviations from the simple periodicity in the first periods, will be mentioned below.
and that of another line by where R is again the Rydberg constant and N the ordinal num ber of the element in the periodic table. The extreme simplicity of these formulae enabled Moseley to settle any previous un certainty as to the order of the elements in the periodic table, and also to state definitely the empty places in the table to be filled up by elements not yet discovered.
In the nuclear model of the atom, the ordinal number of an element in the periodic table receives an extraordinarily simple interpretation. In fact, if the numerical value of the charge on an electron is taken as unity, this ordinal number, which is often called the "atomic number" (q.v.), can simply be identified with the magnitude of the nuclear charge. This law which was fore shadowed by J. J. Thomson's investigations of the number of electrons in the atom, as well as by Rutherford's original esti mate of the charge on the atomic nucleus, was first suggested by van den Broek. It has since been established by refined measure ments of the nuclear charge, and it has proved itself an unerring guide in the study of the relationship between the physical and chemical properties of the elements. This law also offers an im mediate explanation of the simple rules governing the changes in the chemical properties of radioactive elements following the expulsion of a or # particles.