1922] Isotopes 119 



which belongs to the group of the monatomic gases — found lodgment 

 in some of these vacant places; but the great majority fall in places 

 already occupied by well-known elements. Thus seven fall in the place 

 occupied by lead. This necessitates the use of the same atomic number 

 for eight elements. These radioactive elements possess identical chem- 

 ical characteristics with lead and can not be separated by chemical 

 means. The minute amount of the radioactive elements available ren- 

 ders a comparison of the spectra difficult. In a number of cases they 

 are known to be identical and according to Soddy's theory should be, 

 but certain generalizations (as that of Hicks) are opposed to this 

 view. 



The name isotope was adopted by Soddy to designate these ele- 

 ments. The word means same place. Thus RaG is an isotope of lead ; 

 RaC is an isotope of bismuth. No strictly chemical means for separat- 

 ing these elements is known. In the same way ionium and thorium 

 are isotopes and there are three other isotopes in that place in the 

 Periodic System. 



As these isotopes have different phj'sical properties, they can be 

 separated sometimes by means of these. The question then arises as 

 to whether in case the atoms of an element are not homogeneous they 

 can be separated or detected by physical methods. J. J. Thompson in 

 1913 devised a method involving the use of the deflecting influence 

 upon the trajectory of an ion by the simultaneous application of elec- 

 tric and magnetic fields and thus obtained evidence of the existence 

 of heterogeneous particles. In some cases further examinations show 

 these to be different molecular aggregations rather than new elements. 

 Thus in hydrogen besides Ho the presence of Ho was proved and 

 there was a possibility of other hydrogen molecules being there. The 

 atomic weight of hydrogen is 1.008. The fractional portion can now 

 be safely regarded as due to an admixture of these heavier molecules. 

 To call such molecular aggregations isotopes would be a serious de- 

 parture from the original definition of the term. They are not dis- 

 tinct and different elements. The term is sometimes carelessly used 

 in the case of molecular aggregations of hydrogen, and other elements 

 and this must lead to needless confusion of ideas. 



An examination of the monatomic gas neon, whose atomic weight 

 is given as 20.2 reveals the presence of a companion gas having an 

 atomic weight of 22. This corresponds with no known element and 

 the new element is called meta-neon. A physical investigation of at- 



