NATURE OF LIFE AND LIVING MATERIAL 9 



Molecules: The molecule that is formed when oxygen and hy- 

 drogen unite is water. One speaks of molecules thus formed of 

 different types of atoms as chemical compounds. For convenience, 

 the molecule is represented by symbols of the atoms of which it is 

 composed, showing how many and what kind of atoms are pres- 

 ent. Thus two atoms of oxygen form a molecule of oxygen, writ- 

 ten O2; the water molecule is H2O; sulphuric acid is written 

 H2SO4, being composed of two hydrogen, one sulphur, and four 

 oxygen atoms. Any given chemical compound always contains the 

 same constituents always in the same proportions; a molecule of 

 water always consists of two hydrogen and one oxygen atoms. If 

 the number of atoms is changed the characters of the compound 

 are likewise changed. If an atom of oxygen is added to the water 

 molecule, H2O, a new compound, H2O2, hydrogen peroxide, is 

 formed, the characteristics of which are different from those of 

 water. 



Organic Compounds: If a bit of plant or animal substance, for 

 example, a leaf or a piece of meat, is heated in a tube not easily 

 reached by the air, there remains a residue of black materials, 

 largely composed of charcoal, or a crude form of carbon. This sim- 

 ple test demonstrates that the element carbon is a conspicuous part 

 of protoplasm. This being true, it becomes necessary to understand 

 at least some of the characteristics of the carbon atom. 



It is known that the carbon atom has four electrons on its outer 

 shell or orbit and will accept four more, that is, it has the valence 4. 



