136 THE POPULAR SCIENCE MONTHLY 



have all the nucleus charges from 1 for hydrogen to 92 for lead. There 

 is, however, another very important consequence that follows from this 

 conception of the atom. Disregarding for a moment the atomic weight 

 which depends mainly on the structure of the nucleus, the main phys- 

 ical and chemical properties of the atom are determined by the nucleus 

 charge and not by the atomic mass. This must obviously be the case, for 

 the number and distribution of electrons round the nucleus is determined 

 by the electric forces between the electrons and the nucleus, and this is 

 dependent on the magnitude of the nucleus charge which may be re- 

 garded as a point charge. Without entering into the difficult question 

 of the actual distribution of the exterior electrons in any atom, it is 

 obvious that the number and position of the outlying electrons of the 

 atomic structure, which probably mainly influence the chemical and 

 physical properties of the atom, are determined by the charge on the 

 nucleus. No doubt if the electrons are in motion, their positions rela- 

 tive to the nucleus and possibly also their rates of vibration will be 

 slightly influenced by the mass of the nucleus as well as its charge, but 

 the general evidence indicates that this effect must be very small. 



We thus see that there is in the structure of every atom a quantity 

 which is more fundamental and important than its atomic weight, 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 number, is in reality a 

 complicated function of the actual structure of the nucleus. The ques- 

 tion why the atomic mass should not necessarily be proportional to the 

 atomic number has already been discussed. While the main proper- 

 ties of an atom are controlled by its nuclear charge, the property of 

 gravitation and also that of radioactivity are to be ascribed mainly, if 

 not entirely, to the nucleus. 



Radioactive Elements axd the Pekiodic Series 

 Since the nucleus 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 ex- 

 ample, if it were possible to add a helium nucleus to the nucleus of an- 

 other atom, it would increase the nuclear charge by two and the mass 

 by about four; if instead of the helium nucleus two hydrogen 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 way, 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. 



