CHAPTER XXIII 

 THE PERIODIC SYSTEM 



IN an earlier chapter (p. 208) we saw that the elements fluorine, 

 chlorine, bromine and iodine exhibited striking similarities in 

 their chemical properties, and we grouped these four elements 

 together under the name of the halogen family. Now there are 

 two rather rare elements, selenium and tellurium, which resemble 

 sulphur very markedly in their chemical properties. Both give com- 

 pounds with hydrogen, hydrogen selenide H 2 Se and hydrogen 

 telluride H 2 Te, corresponding with hydrogen sulphide H 2 S, but 

 less stable. Both give compounds with oxygen, selenium dioxide 

 Se0 2 and tellurium dioxide TeO 2 , corresponding with sulphur 

 dioxide S0 2 . These dioxides dissolve in water to form weak 

 acids similar to sulphurous acid. These acids, again, can be oxi- 

 dized to yield selenic acid H 2 Se04 and telluric acid H 2 TeO4, analo- 

 gous to sulphuric acid H 2 S0 4 . All these compounds show grad- 

 ations in properties, as we go upwards in atomic weights from 

 S to Te, which are very reminiscent of the gradations encountered 

 in a series of halogen compounds, such as HC1, HBr, HI. 



Furthermore, just as we have in the halogen family a first mem- 

 ber with rather irregular habits, fluorine, so we note in the sulphur 

 family a corresponding light element showing decided peculiari- 

 ties, oxygen. Oxygen forms a compound with hydrogen H 2 O, 

 which is akin to H 2 S in being a weak acid and, as might be pre- 

 dicted, is much more stable. The metallic oxides are very sim- 

 ilar in their properties to the metallic sulphides. Ozone may be 

 regarded as oxygen dioxide 00 2 , analogous to sulphur dioxide S0 2 . 

 The family resemblance in other compounds, however, is more diffi- 

 cult to trace. 



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