In the simple and light hydrogen atom, there is one electron revolving around a proton. In the helium atom, there are two electrons revolving around a nucleus having a charge of + 2. The atomic nuclei are, in turn, complex systems, consisting of protons and neutrons, as it was eventually found after the discovery of the neutron and after theoretical analysis. In the atom of uranium, which is element No. 92 in the periodic table, 92 electrons revolve around a nucleus of charge + 92, consisting of 92 protons and 146 neutrons (the principal uranium isotope For a long time after Mendeleev the list of known elements did not go beyond uranium. In the 1930's and 1940's, the elements missing from the periodic table, Nos. 61, 73, and 75, were obtained by nuclear bombardment. In the same way, the use of nuclear reactions finally provided the means of going beyond the limits of the old system of elements and of obtaining one after another an ever-increasing group of transuranic elements, beginning with neptunium—Np (No. 93), and plutonium—Pu (No. 94), which is of importance in nuclear engineering, through the subsequently discovered elements americium Am (No. 95), curium—Cm (No. 96), berkelium—Bk (No. 97), californium-Cf (No. 98), einsteinium—Es (No. 99), fermium—Fm (No. 100), and up to mendelevium—Md (No. 101). At the present time Swedish, Soviet, and American physicists are trying to determine the property of element No.102, No (nobelium), whose name has not yet been definitely decided upon. Quite recently element No. 103, Lw (lawrencium), was discovered. As we see, the new transuranic elements are named not only after geographical places but also after great scientists whose work has been associated with the discovery of new elements, with studies on the atomic nucleus, and with the construction of particle accelerators, such as Curie, Einstein, Fermi, Mendeleev, and Lawrence.
Without going now into the details of atomic structure, the arrangement of electrons into shells, or the interesting properties of the transuranic elements, let us just point out that the question naturally arising as to the top limit of the periodic system of elements, i. e. , the largest possible number of nuclei which are stable to any appreciable extent, has not yet been finally settled. It is well known that, without exception, all the nuclei of isotopes from No. 84 on are radioactive, i. e. , they disintegrate spon taneously with the emission of either helium nuclei (designated as a particles) or p particles (electrons) and of y rays. Many nuclei of the heavier ele ments spontaneously decay into two or more fragments of approximately equal size. This effect was discovered by G. N. Flerov and K. A. Petrzhak shortly after Hahn had demonstrated the fission of uranium by neutron bombardment.
Recently a detailed study was made of the spontaneous fission of element No. 102 into 3 fragments. The rough theoretical estimates worked out by Wheeler yielded at first the rather exaggerated value of Z, 200, and later Z, 140 for the possible number of the significantly stable elements. The calculations we performed together with N. N. Kolesnikov gave the more "pessimistic" estimate of about Z, 110-120 for the number of possible elements.
The latest data seem to support the more conservative estimates. This means that the nuclei of the elements with Z, 110-120 ought to disintegrate within a very short time, mainly by spontaneous fission, and thus the existence of atoms with such nuclei (or with any heavier ones) must be impossible. Alpha decay by spontaneous fission is caused by the fact
that when there is a large number of protons in the nucleus, i. e. , in Zz elements with a high Z, number, where A 40, the electrical forces of repulsion become so considerable that they are no longer balanced by the nuclear forces of attraction between the protons and neutrons. A recent discovery was that of proton radioactivity (G. N. Flerov, V. I. GoPdanskii).
Let us now turn to the atomic nucleus. After the discovery of the neutron by Chadwick, and our preliminary investigations started in collaboration with V.A. Ambartsumyan, we established the fact that the atomic nucleus can consist only of heavy particles, viz., protons and neutrons (nucleons), and does not contain either electrons or any other leptons (light particles), i. e., positrons or mesons.
It was later found that some newly discovered super-heavy particles, such as the lambda hyperon (A), may also be included in nuclei, though not of the ordinary kind, but in hypernuclei that are occasionally formed in certain types of reactions.
The hyperons, as we shall subsequently see, may be considered in many respects as excited states of nucleons, and thus the currently accepted model of the nucleus, a somewhat generalized form of the older version, has it that all nuclei consist of heavy particles, or baryons (nucleons and hyperons), held together by short-range forces, essentially of the it-meson type.
The type of forces involved must be specified, since there is evidence that a very short-lived, metastable system can be formed by nucleons, such as the proton and antiproton, which is held together by electrical rather than nuclear forces. An example is the positronium atom, formed by an electron and a positron.
Our proton-neutron model, supported primarily by W. Heisenberg and E. N. Capon, quickly came to be ranked with the other existing models of Auger, Perrin, and Dirac, which postulated nuclei containing protons, neutrons, and electrons. It left, however, one basic question to be answered: what is the explanation for the emission of electrons, and of what was later recognized as antineutrinos, in the natural beta-decay of nuclei, or in artificially induced beta-decay later discovered by Fermi? Also, what is the explanation for the corresponding emission of positrons and neutrinos in positron decay, discovered by the Joliot-Curie team in 1934? The answer, as it turned out, gave rise to animated controversy. It states that the protons and neutrons composing the nucleus are elementary particles, not consisting of any other actual particles. Beta-decay is a generative action, involving the emission of new particles (electrons, neutrinos), in the same way as in the emission of protons by atomic electrons or of gamma protons by nuclear particles, new particles are produced which were not initially present in the radiating system. This interpretation of beta-decay was also adopted by Blackett and Occhialini, who discovered together with Anderson the positron and the production and annihilation of electron-positron pairs. In this respect their work may be considered as definite proof of the proton-neutron model of the nucleus which may be said to have been universally accepted at the First Soviet Nuclear Conference and at the International Conference on Nuclear Science held at Solvay in 1933.