Page:Popular Science Monthly Volume 87.djvu/141

Rh The study of radioactive elements has in the last year thrown a flood of light not only on this problem but on the underlying meaning of the periodic law of the elements. Russell, Fajans and Soddy independently put forward a remarkable and important generalization in regard to the change of chemical properties of the successive products of transformation of the primary radioactive elements. This generalization can be very simply expressed in terms of the usual arrangement of the elements in groups according to the periodic law. It is found that after a transformation in which alpha particles are expelled, the resulting element has chemical properties which shift its place two groups lower in the direction of diminishing mass. On the other hand, the element resulting from a beta ray transformation shifts one place in the opposite direction. For example, radium, which is in group II., changes after loss of an alpha particle into the emanation into group 0, which included all the inert gases of the helium-argon type. The emanation after loss of another particle becomes radium A, which belongs to group VI., and this in turn becomes radium B belonging to group IV. Since radium B is transformed by the loss of a beta particle, the resulting element radium C takes up a position in group V. By this simple rule, it has been found possible to define the essential chemical properties of all known radioactive elements. It was found that on this theory one element was missing in the general scheme. This element was discovered a few weeks later by Fajans and Göhring, and found to have the general chemical properties predicted for it.

This generalization is capable of a very simple explanation on the nucleus theory. The loss of an alpha particle of charge 2 lowers the nuclear charge of the resulting elements two units; the loss of a beta particle, which carries a unit negative charge, raises the nuclear charge by one unit. In other words, the atomic number of an element shifts two units lower after loss of an alpha particle and shifts one unit higher after loss of a beta particle.

The atomic numbers of the elements in the uranium-radium series can be simply deduced from this rule if the atomic number of one element is known. We shall see later that the atomic number of radium B is 82 and identical with that of lead. The actual atomic numbers of the various elements are given in the circles representing the atoms in Fig. 12. It is seen that uranium, the heaviest known element, has an atomic number 92, while the elements radium B, radium D and the end product, which is believed to be lead, have the same atomic number, viz., 82. The evidence of the correctness of this striking conclusion will now be discussed.

As a result of a careful examination of the radioactive substances, it has been found that in a number of cases elements, which are of different atomic weight and exhibit different radioactive properties, yet