Structure of the Atom. 



497 



brought in support of such a contention. Soddy * has 

 pointed out that the recent generalisation of the relation 

 between the chemical properties of the elements and the 

 radiations can be interpreted by supposing that the atom 

 loses two positive charges by the expulsion of an a particle, 

 and one negative by the expulsion of a high speed electron. 

 From a consideration of the series of products of the three 

 main radioactive branches of uranium, thorium, and actinium, 

 it follows that some of the radioactive elements may be 

 arranged so that the nucleus charge decreases by one unit as 

 we pass from one element to another. It would thus appear 

 that van den Broek's suggestion probably holds for some if 

 not all of the heavy radioactive elements. Recently Moseley f 

 has supplied very valuable evidence that this rule also holds 

 for a number of the lighter elements. By examination of 

 the wave-iength of the characteristic X rays emitted by 

 twelve elements varying in atomic weight between calcium 

 {40) and zinc (65'4), he has shown that the variation of 

 wave-length can be simply explained by supposing that the 

 •charge on the nucleus increases from element to element by 

 exactly one unit. This holds true for cobalt and nickel, 

 •although it has long been known that they occupy an 

 anomalous relative position in the periodic classification of 

 the elements according to atomic weights. 



There appears to be no reason why this new and powerful 

 method of analysis, depending on an examination of the fre- 

 quency of the characteristic X ray spectra of the elements, 

 should not be extended to a large number of elements, so 

 that further definite data on the point may be expected in 

 the near future. 



It is clear on the nucleus theory that the physical and 

 chemical properties of the ordinary elements are for the most 

 part dependent entirely on the charge of the nucleus, for the 

 latter determines the number and distribution of the external 

 electrons on which the chemical and physical properties must 

 mainly depend. As Bohr has pointed out, the properties of 

 gravitation and radioactivity, which are entirely uninfluenced 

 by chemical or physical agencies, must be ascribed mainly if 

 not entirely to the nucleus, while the ordinary physical and 

 chemical properties are determined by the number and distri- 

 bution of the external electrons. On this view, the nucleus 

 charge is a fundamental constant of the atom, while the 

 atomic mass of an atom may be a complicated function of 

 the arrangement ot the units which make up the nucleus. 



* Soddy, Jahr. d. Bad. x. p. 188 (1913). 

 t Moseley, Phil. Mag. xxvi. p. 1024 (1913). 



