THE MORE COMMON ELEMENTS AND COMPOUNDS 17 



and then to employ a short ending to designate the oxygen. 

 Thus, the ending -ate commonly means oxygen. Ternary com- 

 pounds may best be studied in groups, in which the relation of 

 oxygen to one of the other elements is constant. Thus, the car- 

 bonates constitute a large class of compounds in which the group, 

 or radicle (=CO^), is always present. The structural formula for 



~\ 



this radicle is as follows: yC = O. From this it will be seen 



-0 / 



that the carbonate radicle has two free hands, or bonds, capable 

 of holding two monovalent atome or one bivalent atom. Now, 

 the fact is, that almost any metallic element can join hands with 

 this radicle. Thus we have calcium carbonate (CaCO 3 ), magnesium 

 carbonate (MgCO 3 ) , ferrous carbonate (FeCO 3 ) , sodium carbonate 

 etc. 



The nitrate radicle is NO 3 , as in sodium nitrate ( 

 The chlorate radicle is C1O 3 , as in potassium chlorate (KC1O 3 ). 

 The silicate radicle is =SiO 3 (like the carbonate radicle =CO 3 ). 

 The sulfate radicle is =SO 4 , as in calcium sulfate (CaSO 4 ). 

 The phosphate radicle is =PO 4 , as in ferric phosphate (FePO 4 ). 



If we can remember these six radicles, we have the key to the 

 constitution and composition of a large number of ternary com- 

 pounds, some of which are of the greatest importance in soil fer- 

 tility; as, for example, limestone, which is calcium carbonate 

 (CaCO 3 ) ; land plaster, which is calcium sulfate (CaSO 4 ) ; and the 

 important compound in phosphate rock and in bones, called " bone 

 phosphate," which is calcium phosphate, Ca 3 (PO 4 ) 2 , also properly 

 called tricalcium phosphate. 



When the element chlorin, or the element sulfur, or any of these 

 radicles join hands with metallic elements, the resulting compound 

 is called a salt; as NaCl (common salt),Na2SO 4 (Glauber's salt), 

 MgSO 4 (Epsom salt) ; and even limestone (CaCO 3 ) may properly 

 be called a salt of calcium, and ferrous sulfate (copperas, which, 

 however, contains no copper) is a salt of iron (FeSO^). 



Oxids and hydroxids. As already explained, matter may exist 

 in the form of free elements, as nitrogen (N 2 ), sulfur (S 2 ), or phos- 

 phorus (P 4 ). 



