468 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



perhaps identical spectra), but differing in atomic weight 

 by a very small number of units. The story of the rare- 

 earth elements is, in fact, assumed to be repeated at almost 

 every stage of the progress from thallium to uranium. The 

 40 radio-elements are distributed as follows : 



Groups. o. I. II. III. IV. V. VI. VII. 



Period VI. Tl(3) Pb( 9 ) Bi( 4 ) -(7) 1 -(o) 



Period VII. Nt(3) - (o) Ra( 4 ) -(2) 2 Th( 5 ) -(i) Ur( 2 ) 



The ultimate products of disintegration appear to be 

 helium and lead, but the existence is indicated of 5 iso- 

 topic forms of lead, devoid of radioactivity, in addition to 

 4 radioactive forms. The theory of isotopic elements has 

 received confirmation from the fact that samples of " lead " 

 derived from different sources have been found to show 

 marked differences of atomic weight (p. 303) 



C. ILLUSTRATIONS AND APPLICATIONS OF THE 

 PERIODIC LAW. 



Periodicity of valency. The ebb and flow of valency, 

 which was described by Meyer in 1869 (p. 459), was 

 discussed more fully in 1871 by Mendeleeff. He suggested 

 (foe. cit. p. 141) that whilst the valency in the hydrides 

 varied in the order indicated by Meyer (p. 459), the 

 valency in the oxides was identical with the group-number, 

 and varied in the order 12345678. The typical 

 hydrides and oxides may therefore be tabulated as follows : 



Group I. II. III. IV. V. VI. VI I. VIII. 



f i 2 34321 



Hydride^ LiH BH 3 CH 4 NH 3 OH 2 FH 



[NaH [CaH 2 4 ] SiH 4 PH 3 SH 2 C1H 



1 Including polonium. 2 Including actinium. 



3 The hydrides of the earlier groups were not known to Mendeleeff. 



4 MgH 2 is not known. 



