6 SCIENCE AND SOIL 



one, as in monotone) ; bivalent atoms have two bonds (bi or di 

 means two); trivalent atoms have three bonds; tetravalent, four 

 bonds; pentavalent, five bonds; hexavalent, six bonds; hepta- 

 valent atoms have seven bonds; and octovalent atoms have eight 

 bonds, with power to hold four of the bivalent atoms of oxygen. 



There are a few cases in which the atom does not make common 

 use of all the bonds it possesses. Thus the nitrogen atom has five 

 bonds, or hands, but in some compounds only three bonds are used 

 to hold other atoms. It might be conceived in this case that the 

 other two hands are clasped together, and this conception might 

 even be extended to cover a molecule composed of a single biva- 

 lent atom (such as mercury and, possibly, argon). One atom of 

 nitrogen and three atoms of hydrogen form the compound called 

 ammonia (NHg). This compound is frequently sold in fertilizers, 

 but the hydrogen has no money value because water (I^O) con- 

 tains hydrogen. The molecular weight of ammonia is 17, of which 

 the nitrogen atom is 14 and the hydrogen atoms are 3. If a fer- 

 tilizer is guaranteed to contain 17 per cent of ammonia, it should 

 contain 14 per cent of the element nitrogen; while 8| per cent of 

 ammonia is equivalent to only 7 per cent of nitrogen. Ammonia 

 itself contains ^, or 82 per cent, of the element nitrogen. 



In the compound called ammonium chlorid (NH 4 C1), the atom 

 of nitrogen is pentavalent; that is, it has and uses five bonds: 

 K H 



N-C1. The molecular weight of this compound is 53.5 



(14 + 4 + 35-5), and it contains -^-, or 26 per cent, of nitrogen. 



53-5 

 Phosphorus is another element which sometimes uses only three 



/H 



bonds, as in hydrogen phosphid, P^-H, and in phosphorus trichlorid, 



X H 



/Cl 

 P^-C1, and sometimes five bonds, as in phosphorus pentachlorid, 



X C1 

 CL JC\ 



^P^-C1. Thus, the hydrogen phosphid contains f^, or 91 per 

 CK X C1 



