CHEMICAL SYMBOLS. 49 



weight is taken as 1. On that basis the atomic 

 weights (or numbers representing the comparative 

 weights of atoms— the atomic numbers) are as fol- 

 lows, viz.: 



Hydrogen (H) - - - . _ i 



Carbon (C) - - - . - 12 



Nitrogen (N) - - - - . 14 



Oxygen (O) ----- 16 



Sodium (Na) ----- 23 



Magnesium (Mg) - - - . - 24 



Silicon (Si) - - - - - 28 



Phosphorus (P) - - - - - 31 



Sulphur (S) 32 



Chlorine (CI) - - - . - 35.5 



Potassium (K) - - . - - 39.1 



Calcium (Ca) - - - - - 40 



Iron (Fe) - - ... 56 



Copper (Cu) - - - . - 63.5 



Arsenic (As) - . - - . 75 



A group of atoms (or molecule) forming a chemi- 

 €al compound, is represented by writing together 

 the symbols or characters of the atoms. The mole- 

 cule of hydrochloric (or muriatic) acid, for instance, 

 consists of one atom each of hydrogen and chlorine, 

 consequently it is represented in this way: H CI. 

 Few compounds, however, are formed in this sim- 

 ple way. Sometimes two, sometimes three or more 

 atoms of the same element are required in the for- 

 mation of a chemical compound with atoms of other 

 elements. The molecule of water consists of two 

 atoms of hydrogen, and one of oxygen. Its symbol 

 is: H2O. Small arable figures, following after the 

 letter which represents an elementary atom, always 

 indicate the number of such atoms. O2 means two 

 atoms of oxygen ; P4 four atoms of phosphorus, etc. 

 To represent several groups of atoms (molecules) a 

 large figure is placed before the symbol of the com- 

 pound. H2 O represents a single molecule of water; 



