40 PRINCIPLES OF CHEMISTRY. 



atom of mercury and one atom of oxygen, we are justified in 

 saying that a mercury atom is 12.5 times heavier than an oxygen 

 atom. 



In a manner similar to this, the weights of the atoms of all 

 different elements have been compared with each other, and the 

 element having the lightest atom has been selected as the unit 

 of comparison. The element having the lightest atom is hydro- 

 gen, and we say the atomic weight of hydrogen is 1, and 

 compare with this weight the weights of all other elements. 

 In doing this, we find that the atom of oxygen weighs sixteen 

 times as much as the atom of hydrogen, and we consequently 

 say the atomic weight of oxygen is 16. 



We have learned before, from the decomposition of the red 

 oxide of mercury, that the mercury atom is 12.5 times as heavy 

 as that of oxygen. As the atomic weight of this element is 16, 

 the atomic weight of mercury must be 12.5 times 16, or 200. 



Whilst atomic weight is the weight of the atom of any ele- 

 ment as compared to the weight of an atom of hydrogen, 

 molecular weight is the combined weight of the atoms forming 

 the molecule. Thus the molecular weight of oxide of mercury 

 is 200+16=216. 



Chemical symbols. For reasons to be better understood here- 

 after, chemists designate each element by a symbol, and the 

 first or the first two letters of the Latin name of the element 

 have generally been selected. Thus, the symbol of hydrogen 

 is H, of oxygen 0, of mercury Hg (from hydrargyrum), of 

 sulphur S, etc. These symbols designate, moreover, not only 

 the elements, but one atom of these elements. For instance : 

 O not only signifies oxygen, but one atom or 16 parts by weight 

 of oxygen ; and Hg, one atom or 200 parts by weight of mercury. 



Chemical formulas. In a similar manner as atoms of elements 

 are represented by symbols, the molecules of a compound sub- 

 stance are designated, and such a representation of a compound 

 substance by symbols is called its formula. Thus, HgO is the 

 formula of the red oxide of mercury, and it tells at once that it 

 is a substance composed of one atom or 200 parts by weight of 

 mercury, and one atom or 16 parts by weight of oxygen. 



In the molecule of a compound body there must be at least 



