176 OUTLINE OF CHEMICAL STUDY 



SECTION II 

 CHEMICAL ACTION 



201. The Atomic Theory. The name molecule has been 

 given to " the smallest particle of any substance that 

 can exist alone " ( 10). Now we have learned that ele- 

 ments combine to form compounds, and that each mole- 

 cule of a compound is just like all the others. This 

 would seem to show that each molecule must contain 

 in itself a small portion of each of the elements in the 

 compound. But this at once raises the question, How 

 can a molecule (the smallest particle that can exist alone) 

 be made up of smaller parts ? 



Scientists have answered the question by formulating 

 an atomic theory. They say that there may be parti- 

 cles smaller than molecules, but that these can never 

 exist alone, that is, they must always be united with at 

 least one other. These smaller particles are called atoms. 

 An atom may unite with others of its kind or of dif- 

 ferent kinds, but it must always be in a union. 



A molecule, then, is said to be composed of atoms. 

 Therefore we see that each molecule of a substance may 

 be just like the others, and yet every one may be made 

 up of atoms of different elements. In other words, when 

 two or more elements unite to form a compound, the mole- 

 cules of each element break up and the atoms of the dif- 

 ferent kinds unite with each other, forming molecules 

 that will be all alike. 



202. Chemical Affinity. The atomic theory allows 

 us still to say that the molecule is the smallest particle 



