CHAPTER XIX 



CHEMICAL NOMENCLATURE 



Atoms and Molecules. We are now in a position to under- 

 stand the nomenclature employed by chemists to represent the 

 various chemical changes which we have been studying. All 

 matter is supposed to be built up of minute particles termed 

 atoms. These usually exist combined with other atoms to 

 form molecules, and these molecules are the smallest particles 

 of matter which can have a separate existence. In a compound 

 the molecules are composed of a number of different kinds of 

 atoms united together. In an element the atoms are all of the 

 same kind. 



Chemists represent the atoms of the various elements by 

 letters. These are in general the first letter or, sometimes, the 

 characteristic letters of the name (or Latin name) of the element. 

 Thus the letter H represents the atom of hydrogen, and O the 

 atom of oxygen. To represent a compound, a formula consist- 

 ing of a combination of symbols is employed. Thus water must 

 evidently be represented by some combination of the letters H 

 and O, but we do not yet know the number of H's and O's which 

 we must employ. 



Avogadro's Law. In order to deduce the formulae applicable 

 to the different compounds, we must make use of a law first enun- 

 ciated by the Italian chemist Avoyadro, and which is now 

 almost universally accepted by chemists. This law states that 

 Equal volumes of all gases under similar conditions of tem- 

 perature and pressure contain equal numbers of molecules. 



Formula for Hydrochloric Acid Gas. From what we have 

 learnt concerning the formation of hydrochloric acid gas, we l<n< >w 



