THK I'KOIJI.KM OF IIIK ISOTOPIC ELEMENTS 77 



Determination of Ihe charge on ihe nucleus can be made approxima- 

 lel}^ as a result of Ihe experiments on scattering of a-particles by M. 

 (Ieiger and E. Marsdex, and more accurately by the recently perfor- 

 med study by H. Moseley on X-ray spectra. A knowledge of the wave- 

 length of the characteristic X-radiation of an elemenl permits calcu- 

 lation of th(> nuclear charge when certain assumptions are made ; it 

 was thus found that this charge always increases by unity on moving 

 from one position in the periodic system to the next higher^. Generally 

 1 his means a climb to the element with the next higher atomic weight 

 l)Ul. in a few exceptional cases, where the chemical properties force 

 t he element with the lower atomic weight to be arranged higher in the 

 system (e. g. cobalt and nickel), the rule stated above still applies and 

 t hus demonstrates that the number of charges, and not the atomic weight, 

 determines the position of an element in the periodic system. According- 

 ly, the separate positions can be numbered by stating the nuclear charge ; 

 aluminium, for example, thus acquires the atomic numlier 13, gold 79, 

 etc.. and between these all the available numbers except three are already 

 representative of known elements. E. Rutherford and C. Andrade- 

 have proved directly, by determining the X-ray spectrum of radium-B. 

 which was found to be the same as that of lead, that there are ele- 

 ments having different atomic weight but the same nuclear charge. 



Isotopic elements differ, according to this observation, only in the 

 structure and mass of the nucleus. The structure does not enter into 

 the ordinary physics and chemistry but is only of importance to the 

 radioactivity. The ladioactive properties, how'ever, were the chief 

 means of differentiating the isotopic element and, with a few exceptions 

 (metaneon, the different kinds of lead), even now we are only aware 

 of the existence of such isotopic elements in those examples in which 

 at least one of them is radioactive. Separation by utilizing the radio- 

 active differences does not seem to be conceivable ; it is otherwise with 

 the second fundamental property of the nucleus, gravitation, which 

 should permit both distinguishing and separating. 



It is useful in these discussions to distinguish between the gravita- 

 tional and electronic properties ; in all applications of weighing (prima- 

 rily determinations of atomic weight and of solubility^, etc.) differences 

 in weight of the atoms are directly of use, and diffusion in the vapour- 

 state also depends noticeably on the mass and even permits separation ; 

 Aston* has thus succeeded in fractionating metaneon and neon. Centri- 

 fuging also is a process in which mass plays a part and can be applied in 



1 H. Mo-SELEY, Phil. Mag. 26, 1024 (19i:}) : Ihid. 27, 705 (1914). Sec also A. 

 VAN DEN Broek, Fluj-'i. Z. 14, 32 (1913). 



2 E. Rutherford and C. Andrade, riiil. Mag. .Ma\ (1914). 

 ^ K. Fajans, Naturwissenschaften 2, 544 (1914). 



* Aston, B)-iti-sh Association Report, Birmingham (1913). 



