TEE CONSTITUTION OF MATTER 137 



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. Eussell, Fajans and Soddy inde- 

 pendently 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 general- 

 ization 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 be- 

 longs 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 par- 

 ticle, 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 Gohring, 

 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 ele- 

 ment 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 dif- 

 ferent atomic weight and exhibit different radioactive properties, yet 



VOL. LXXXVII. — 10. 



