CHAPTER V 

 COMPOSITION OF MATTER 



29. Chemical Elements. In studying the structure of 

 matter, we have discovered that it is made up of exceedingly 

 small particles called molecules, but we have yet to learn what 

 molecules are made of. Chemists and physicists who have 

 investigated this problem are of the opinion that molecules 

 are composed of still smaller particles called atoms. There are, 

 of course, as many different kinds of molecules as there are 

 different kinds of matter, but atoms are less common ; in fact, 

 there are only about eighty different kinds. Since the mole- 

 cules are built up of these atoms, the differences that they 

 exhibit must be due to differences in the number and kinds of 

 atoms of which they are composed. When a substance has 

 only one kind of atom in its molecules, it is called a chemical 

 element. Gold, copper, iron, and other metals, mercury, sul- 

 phur, carbon, and phosphorus are examples of chemical ele- 

 ments. According to our definition, there can be, of course, 

 only as many different kinds of chemical elements as there are 

 different kinds of atoms. Most substances, therefore, are 

 composed of more than one kind of atom, and are called 

 chemical compounds. Often a very slight difference in the 

 structure of the molecule may make a great difference in the 

 substance. For instance, such substances as sugar, vinegar, 

 starch, wood, alcohol, fat, and oil are merely different combina- 

 tions of the three chemical elements, carbon, hydrogen, and 

 oxygen. About a hundred thousand different combinations of 

 carbon with other elements are known. 



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