xvin THE CLASSIFICATION OF THE ELEMENTS 465 



have been found nor have the gaps between silver and 

 gold, cadmium and mercury, tin and lead, or antimony and 

 bismuth been filled by the discovery of additional elements 

 belonging to any of these families. It is therefore probable 

 that these gaps do not in fact exist, and that Lothar Meyer 

 was right in his suggestion that the differences in a family 

 of elements may be 16 or 46 or 90, as in the two series 



Li = 7 Na=23 ' K - 39 Rb = 85j 3=133 

 Differences 16 16 46% 4J\ 



N=i4 P = 3i As = 75 Sb-i2o Bi=2o8 

 Differences 17 44 45 88 



There is, however, a gap of about 40 units at the point 

 where the differences undergo this sudden increase. This 

 gap, which extends from barium (Ba -Sr=i37~88 = 49) to 

 tantalum (Ta-Nb= 181 - 93= 88) is filled by a series of 

 RARE EARTH ELEMENTS, derived mainly from monazite sand. 

 In addition to scandium and yttrium, two elements of much 

 smaller atomic weight, fourteen elements of known atomic 

 weight have been separated (see Table F, p. 492) as follows : 



La Ce Pr Nd Sa Eu Gd Tb Ds Ho Er Tm Yb Lu 



i39'o 140-25 140-6 144-3 i5<>'4 I52'c 157-3 159-2 162*5 163-5; l6 7'7 ^S'S 172*0 174-0 



These elements exhibit a remarkable resemblance to 

 one another; their compounds are so similar that their 

 separation has occupied more than a century, and even now 

 may not be quite complete. They differ far less than 

 MendeleefFs transition-elements and cannot possibly be 

 scattered over the eight families of the short periods, or the 

 eighteen families of the long periods of the conventional 

 classification. With unmistakable clearness these elements 

 all claim a place in the same family as scandium and yttrium 

 and must be placed with these elements in Group III of 

 Tables D and E. 1 The regular periodic change in the 



1 Armstrong, Encycl. Brit., Ninth Edition, 1902, XXVI, 712; 

 compare Biltz, Ber., 1902, 35, 562-568. 



H H 



