196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



viz, its nuclear charge. It is known that the variation of the atomic 

 weights of the elements with atomic number, while showing certain 

 well-marked relationships, shows no definite regularity. From the 

 point of view of the nucleus theory, the atomic weight of an element, 

 while in some cases approximately proportional to its atomic num- 

 ber, is in reality a complicated function of the actual structure of 

 the nucleus. The question Avhy the atomic mass should not neces- 

 sarily be proportional to the atomic number has already been dis- 

 cussed on page 193. While the main properties of an atom are con- 

 trolled by its nuclear charge, the property of gravitation and also 

 of radioactivity are to be ascribed mainly, if not entirely, to the 

 nucleus. 



RADIOACTIVE ELEMENTS AND THE PERIODIC SERIES. 



Since the nucleu.s charge of an atom determines the main physical 

 and chemical properties of an atom, it is possible that elements may 

 exist of equal nuclear charges but different atomic weights. For 

 example, if it were possible to add a helium nucleus to the nucleus 

 of another atom, it would increase the nuclear charge by two and 

 the mass by about four ; if instead of the helium nucleus two hydro- 

 gen nuclei were added, the charge would be the same, but the mass of 

 the resulting atom two units less than with helium. In such a case 

 two atoms would be possible of identical nuclear charge but different 

 atomic weights. In a similar v/ay it may be possible for elements to 

 exist of the same atomic mass but different nuclear charges. This 

 would be brought about by the loss or gain of one or more negative 

 electrons in the nucleus. 



The study of radioactive elements has in the last year thrown a 

 flood of light not only on this problem but on the underlying mean- 

 ing of the periodic law of the elements. Russell, Fajans, and Soddy 

 independently put forward a remarkable and important generaliza- 

 tion 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 shifts its place two groups lower in the direction of diminish- 

 ing mass. On the other hand, the element resulting from a beta ray 

 transformation shifts one place in the opposite direction. For ex- 

 ample, radium, which is in Group II, changes after loss of an alpha 

 particle into the emanation into group O, 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 



