284 SMITH'S INTERMEDIATE CHEMISTRY 



Between cerium (140) and tantalum (181.5) in the fifth series, 

 there occur fourteen rare elements, called the elements of the rare 

 earths, which have been omitted entirely from the Mendelejeff 

 system. What is to be done with these elements is a point on 

 which agreement has not yet been reached. 



Finally, reference to the table will show that in three cases a 

 slight displacement of the order of the elements according to 

 atomic weights is necessary. Argon, an inert gas, is placed before 

 potassium, an alkali metal, although its atomic weight is 0.8 

 higher. Cobalt is put before nickel because it resembles iron 

 more closely. Tellurium and iodine are placed in that order to 

 bring them into the sulphur and halogen groups, respectively. 

 Their valence and other chemical relations both require this. 

 These three cases constitute undoubted exceptions to the Mendele- 

 jeff system of classification. The general agreement, however, is 

 obviously far too remarkable to be due entirely to chance. 



In a later chapter it will be shown that recent work on atomic 

 structure throws considerable light on the several abnormalities 

 discussed above, supplying us with a more logical basis for the 

 periodicity exhibited by the elements in respect to valence and 

 other properties than is furnished by the use of Mendele Jeff's 

 system alone. Nevertheless the latter will be found to be 

 of valuable service to us throughout the remainder of the 

 book. 



Exercises. 1. There is a blank at the end of the fifth long 

 series, where we should expect to find another halogen (see p. 

 278). If the element that should fill this blank were to be dis- 

 covered, what would be its physical and chemical properties? 

 What would be the properties of its compound with hydrogen? 



2. How should you attempt to obtain H 2 Te, and what physical 

 and chemical properties should you expect it to possess? 



3. Make a list of bivalent elements and criticize this method of 

 grouping as a means of chemical classification. 



