Isotopes

ISOTOPES, chemical elements located in the same cell of the periodic system and, consequently, possessing the same atomic number or ordinal number. In this case, isotopes do not, generally speaking, have to possess the same atomic weight. Different isotopes of the same element possess identical electron shells and all properties (so-called peripheral properties) associated with the electron shell: chemical, electrochemical, spectral, etc. Therefore, they are inseparable from one another by ordinary methods used for the separation of elements. Isotopes differ only in the structure of the nucleus, namely in the number of positive elements, protons, contained within it, upon which the mass of the atom depends, and in the number of intranuclear electrons. A different number of protons results in differences in mass, i.e., in the "atomic weight" of the isotopes, and the methods for their separation are based on this: 1) diffusion through fine-pored partitions makes it possible to separate heavy particles from light ones. By this means, Harkins succeeded in separating HCl into two fractions that differed in density by 0.004; 2) isothermal evaporation at low temperature and pressure allowed Hevesy to separate mercury into two fractions, the densities of which were equal to 0.99974 and 1.00023, respectively (if the density of ordinary mercury is taken as unity). Due to the relatively small difference in the atomic weights of isotopes, these methods do not allow for the complete separation of isotopes, but only for the enrichment of each fraction with lighter or heavier isotopes. Complete separation of isotopes, but only in infinitesimally small quantities, immeasurable by ordinary laboratory methods, is achieved by 3) the method of Aston, which represents a modification of the method of J. J. Thomson. Positively charged atoms of elements, moving in an electric field in the form of so-called canal rays, are deflected in an electric and magnetic field and are brought to a focus in such a way that particles possessing the same mass land at the same point on a photographic plate. By this means, it was proven that more than half of the studied elements consist of a mixture of isotopes, with some, such as Hg, Xe, Sn, Pb, consisting of many isotopes (up to 8). All radioactive elements have isotopes. Chlorine consists of a mixture of two isotopes with atomic weights of 35 and 37, with a predominance of the former. Having increased the accuracy of his method to 0.1%, Aston established an important law of whole numbers: the atomic weight of each isotope is a whole number, i.e., a multiple of unity. This served as the basis for the revival of Prout's hypothesis regarding the unity of matter: the atoms of all elements consist of a whole number of protons, i.e., hydrogen nuclei with an atomic weight of 1.