COMPOUNDS MIXTURES VALENCE, ETC. 17 



CuS04-5H2O. Likewise, the formula of gypsum, or hy- 

 drated calcium sulphate, may be written CaSO4-2H2O. 

 When this is heated, it loses three-fourths of its water, 

 and the resulting plaster of Paris has the formula 

 (CaSO 4 ) 2 -H 2 0. 



The number of bonds which each component part of 

 a compound possesses determines its formula. Thus, in 

 sodium chloride, sodium and chlorine each have one bond, 

 and therefore the formula is NaCl. In sodium sulphide, 

 the sulphur atom has two bonds, and therefore requires 

 two sodium atoms, each with its one bond, to satisfy it; 

 consequently the formula is Na2S. In arsenious oxide, it 

 requires two arsenic atoms, each with three bonds or six 

 together, and three oxygen atoms, each with two bonds, 

 to make the formula, As20s, in which the total bonds of 

 the arsenic atoms shall equal the total bonds of the oxy- 

 gen atoms. In some conditions, arsenic develops five bonds, 

 and consequently the formula of arsenic oxide is As2Os. 



19. Criss-cross Rule. In compounds which are com- 

 posed of two units, the formula may be determined by 

 taking as many atoms or parts of one component as there 

 are bonds of the other component. This is more apparent, 

 if, first, the bonds of each part are indicated. Thus, in 

 determining the formula of aluminium oxide, write the 

 symbols and indicate the bonds of each, A1'"O", then, 

 applying the " criss-cross " rule, the formula becomes 

 AbOs. In the formula for copper sulphide the bonds 

 would be Cu"S", and the formula, 01282; but it is cus- 

 tomary to reduce such formulas to their lowest terms, which 

 in this case will be CuS. 



In formulas having more than two components, very 

 frequently two or more of the elements group themselves 

 together and act as a unit. Such groups do not generally 

 separate when they take part in a chemical reaction. They 

 are termed radicals. Thus, in the formula of phosphoric 

 acid, HaPCU, the P04 acts as a radical, and in the formula 



