76 ADVENTURES IN RADIOISOTOPE RESEARCH 



The question of the identity of different elements was given increased 

 attention since this was the basis of arranging the radioelements in the 

 periodic system^. K. Fajans^ has indicated that this idea can be carried 

 throughout the periodic system and that the ordinary elements also 

 are probably mixtures. Fajans has given the tame "pleiade" to such 

 a group of elements which occupy the same position in the periodic 

 system ; the separate members were called "isotopic'' elements by 

 SoDDY. The lack of an ionium spectrum in ionium-thorium samples^ 

 could scarcely be explained on any assumption other lhan that the 

 isotopic elements show no differences in then- spectra. 



The theory of isotopic elements was not readily acceptable to chemists 

 and physicists ; to the former, because ever since the formulation of 

 the periodic system thej^ had been accustomed to regard the atomic 

 weight as a fundamental property of an element ; to the latter, because 

 there was no known instance in which two different elements exhibited 

 the same spectrum and such a hypothesis seemed difficult to unify with 

 the prevailing ideas on the origin of spectrum lines^. These doubts were 

 removed and the whole concept of the nature of isotopic elements was 

 simultaneously given considerably more weight by the ideas, developed 

 by E. Rutherford^ and N. Bohr^, on the constitution of the atom, 

 and by the experiments of Moseley'^ on the X-ray spectra of the ele- 

 ments. According to the Rutherford model of the atom, the mass of the 

 atom is associated with an extremely small volume at the positively 

 charged centre and the number of positive charges, and not the atomic 

 weight, is primarily responsible for the properties of the corresponding 

 element. Since the number of electrons Avhich occupy the volume bet- 

 ween the nucleus and surface of the atom is given by the size of charge 

 on the positive nucleus and all chemical and physical properties of the 

 element depend on the number and arrangement of these electrons ; 

 gravitation and radioactivity are excepted. Instability of the nucleus 

 results in radioactive phenomena and the fact that the nuclei of two 

 atoms have the same charge and the same physical and chemical pro- 

 perties but different mass and stability (e. g. radium -D and lead) agrees 

 very well witli Ihe Rutherford-Bohr theory. 



1 A. S. Russell, Chem. News 107, 49 (1913) : K. Fa-t^^ns, Phijs. Z. 14. 13r, 

 (1913); F. SoDDY, Chcm. Netcs 107, 97 (1913). 

 2K. Fajans, Chem. Ber. 46, 422 (1913). 



3 F. EXNER and E. Haschek, Sitz. Ber. Akad. Wiss. Wienl2\, 175 (1912); A. S. 

 Russell and R. Rossi, Proc. Roy. Soc. 87, 478 (1912). 



4 A. Schuster, Nature 91, 30 (1913). 



5E. Rutherford, FMl. Mag. 21, 669 (1911). 



«N. Bohr, Fhil. Mag. 26, 1 (1913). 



7 IT. Moseley, Phil. Mag. 26, 1024 (1913). 



