MAKING OF FORMULAE AND EQUATIONS 105 



Molecular Formulae of Simple Substances. With oxy- 

 gen (0 2 ) and hydrogen (H 2 ), however, the double formulae are 

 used, instead of the simpler O and H, because the weights 2 X 16 

 and 2 X 1.008 are the ones which fill the cube. The molecules 

 of all elements are not diatomic, however. Thus the cube-full of 

 mercury vapor weighs only 200.6, the same as the atomic weight, 

 and the correct molecular formula of the element is therefore 

 Hg. Similarly, the correct formulae are Na (sodium), K (potas- 

 sium), and Zn (zinc). But the weight of 22.4 liters requires us to 

 write Cl2 for chlorine, N 2 for nitrogen, P 4 for phosphorus vapor 

 and 82 for sulphur vapor (above 1000). 



After all, there is nothing surprising in the fact that the mole- 

 cules even of elementary substances should, in some cases, contain 

 several atoms. All that it means, in the case of an element with 

 diatomic molecules, such as oxygen (0 2 = 32), is that, when 

 oxygen combines with another element, each molecule of oxygen 

 will be divided between two molecules of the product if the latter 

 contain only 16 parts of oxygen each. 



As an illustration, the union of hydrogen and oxygen to form 

 steam (p. 61) may be considered. 



Hydrogen Oxygen Steam 



1000 1000 I + 1000 



If each of the above rectangles represents a small volume con- 

 taining 1000 molecules of gas, then 2000 molecules of hydrogen 

 and 1000 molecules of oxygen give 2000 molecules of water vapor. 

 Since each molecule of water vapor must contain at least one 

 atom (see p. 86) of oxygen, at least 2000 atoms of oxygen were 

 required, and must have been furnished by the 1000 molecules 

 of oxygen. Each of these molecules must therefore have split 

 into at least two atoms. We have no reason, however, for sup- 

 posing that there are more than two atoms in the oxygen molecule. 

 Hence we accept the formula O 2 as correct. 



