68 ELEMENTS OF GENEKAL SCIENCE 



up of two or more atoms and that these atoms may be of 

 the same kind or of different kinds. 



Each molecule of water is believed to contain both hydro- 

 gen and oxygen atoms. When we decomposed water by the 

 electric current, we secured twice as much hydrogen as oxy- 

 gen. That would suggest that in the molecule there is twice 

 as much hydrogen as oxygen. Further study by chemists 

 has shown that in a molecule of water there are two hydrogen 

 atoms and one oxygen atom. 



Chemists let a certain letter stand for a certain kind of 

 atom. Thus, II stands for a hydrogen atom and O for an 

 atom of oxygen. If there is more than one atom, a small 

 figure is written below and to the right of the letter to tell 

 how many atoms are meant, Written in this way the symbol 

 for a molecule of water is H 2 O. 



71. Explanation of some chemical terms. When a substance 

 is made up of molecules having two or more kinds of atoms 

 in them, as in water, it is clear that it is possible to separate 

 such a substance into two or more different substances, as we 

 did with water. Such a substance is called a compound sub- 

 stance. Water is a compound. A substance whose molecules 

 have in them only one kind of atom, as oxygen, cannot 

 be separated into simpler substances. It is called a simple 

 substance (element). Hydrogen, oxygen, iron, silver, lead, 

 copper, and gold are examples of simple substances. 



When a compound is separated into its elements or into 

 simpler compounds, the process is called chemical decomposition. 

 Have you seen an example of chemical decomposition ? It is 

 sometimes said in that case that the compound has " broken 

 down." Any change in a substance in which the molecule 

 is changed by atoms being taken away from it, added to it, 

 or exchanged for others is called a chemical change. So long 

 as the molecules remain the same, there is no chemical change. 

 The following table contains the names of the more im- 

 portant simple substances, their symbols, and a few words 



