1 6 SCIENCE AND SOIL 



two elements represented in the molecule. In the name of such a 

 compound both elements are expressed, and sometimes the name 

 also includes the number of atoms of each of the elements in the 

 molecule, as phosphorus pentachlorid (PC1 6 ). As a rule, the name 

 of one element is modified slightly so as to end in -id, a termination 

 that means that the compound is binary, containing but two 

 elements. (Any exception to this rule will be self-explanatory.) 

 Thus sodium chlorid must contain only two elements, sodium 

 and chlorin, because of the names and the ending -id] and, since 

 sodium must be monovalent and chlorin may be, the formula 

 for the molecule is probably Na Cl, which is correct for com- 

 mon salt. 



As a matter of fact, chlorin is always monovalent in binary 

 compounds with metals. Calcium oxid (quicklime) must be a 

 binary compound of the two strictly bivalent elements, calcium 

 and oxygen, and the molecular formula may be Ca=O, which is 

 also correct. Calcium chlorid should be Ca=Cl 2 ; and magnesium 

 chlorid, Mg=Cl 2 ; and potassium oxid, K 2 =O; hydrogen sulfid, 

 H 2 = S; sulfur dioxid, SO 2 (O = S=O); and sulfur trioxid, SO 3 

 (in which the sulfur atom must use six bonds), all of which are 

 correct. More complex molecules, which, however, are easily 

 understood, are aluminum oxid (A1 2 O 3 ) and phosphorus pentoxid 

 (P 2 O 6 ) . The aluminum atom has three bonds (or hands) , and the 

 phosphorus uses five bonds in this oxid, while the oxygen atom is 

 always bivalent, having but two hands. There are six bonds of 

 union in aluminum oxid and ten bonds in phosphorus pentoxid, 



p=o 



thus: A1 ;>O, /O. Thus, if one knows the name, the atomic 

 A \0 P=0 

 % 



weight, and the valence of the fifteen most important elements, 

 he has the key to the formula and percentage composition of their 

 binary compounds. 



Third, matter may exist in ternary (triple) compounds, with 

 three elements represented in the molecule. Most ternary com- 

 pounds contain oxygen, and, in naming such compounds, the most 

 common rule is to express the names of only two of the elements 



