COMPOUNDS MIXTURES VALENCE, ETC. 15 



fore called a monad, or univalent element. The chlorine 

 atom usually acts as a monad and we may picture that, 

 when hydrogen chloride is produced, the bond of a chlorine 

 atom ties itself to the bond of a hydrogen atom, thereby 

 making a stable molecule. One atom of chlorine will 

 combine with one atom of potassium to form a molecule 

 of potassium chloride. The potassium atom is therefore 

 univalent. 



The atoms of some elements have the capacity to hold 

 in combination two atoms of a univalent element. We 

 may think of such elements as having two bonds attached 

 to each atom; two atoms of hydrogen are required to unite 

 to one atom of oxygen so as to form water, therefore, the 

 valence of oxygen is two. Oxygen is called a diad or 

 bivalent element. A number of important elements are 

 bivalent, such as calcium, magnesium, and zinc. One 

 atom of nitrogen unites with three atoms of hydrogen to 

 form ammonia. Therefore nitrogen in this case is a triad 

 or trivalent element. Other elements usually trivalent are 

 phosphorus and aluminium. Carbon as indicated by the 

 four atoms of hydrogen, which unite with one atom of 

 carbon to form marsh gas, is tetravalent. 



Sometimes an element may develop one valence under 

 one set of conditions and a different valence under other 

 conditions. Hydrogen never changes valence, oxygen prac- 

 tically never, and the same is true of some of the metallic 

 elements, such as calcium, sodium, and potassium. When 

 oxygen combines with an element it is liable to develop 

 in that element an increase in its valence. For example, 

 when sulphur unites with hydrogen to produce the ill- 

 smelling gas, hydrogen sulphide, the valence of the sul- 

 phur atom is two, as indicated by the two atoms of hydrogen 

 which it requires. When sulphur is burned in air, most 

 of the resulting compound is sulphur dioxide, the two oxygen 

 atoms of the compound indicating that sulphur has four 

 bonds; in this action a small amount of sulphur trioxide 



