96 TEXTBOOK OF BOTANY 



the atom of oxygen, and the second indicates the electrically charged 

 ions into which the water molecule may be separated. A vessel of water 

 always contains some free H+ and OH~ ions. Likewise, every molecule 

 is made up of atoms held together by electrical forces. 



In stable molecules the positive and negative charges balance each 

 other and the molecule is neutral; hence it is less active chemically than 

 the free charged ions. Moreover, the electrical charges are important 

 factors in determining the structural arrangement of the atoms and ions 

 within a molecule, and also the stability and constancy of the molecules. 

 Ions may separate under certain conditions and recombine under others 

 without losing their identity. 



Oxygen and hydrogen are gases with specific properties. As gases 

 their molecules are widely dispersed. Each molecule of oxygen and of 

 hydrogen is composed of two atoms, expressed as O2 and H2. In fact, 

 every chemical element (hydrogen, oxygen, nitrogen, carbon, etc.) is 

 characterized by a particular kind of atom. 



In turn, the properties of atoms are deteiTnined by still smaller units 

 of which they are composed. An atom is visualized no longer as a minute 

 solid ball, but as a system of electrons, neutrons, and protons in which 

 the negatively charged electrons are arranged about a central body com- 

 posed mostly of neutrons and the positively charged protons. The ar- 

 rangement and movement of these particles within the atom have been 

 likened to the solar system, in which several planetary bodies (the 

 planets) revolve about a central body, the sun. In an atomic system, 

 however, the planetary electrons may not all lie in a single plane and 

 their orbits of motion may not encircle the central body. Since the posi- 

 tive charges on the protons of an atom are balanced by the negative 

 charges on its electrons, an atom is electrically neutral.^ 



Some chemical reactions are dependent upon the movement of an 

 electron from one atomic system to another atomic svstem. Since the 

 electron is negatively charged, the atom from which it departs becomes 

 a positively charged ion and the atom to which it becomes attached be- 

 comes a negatively charged ion, both of which are chemically active and 

 readily unite with other oppositely charged ions. Many of the common 

 salts, such as Na+Cl~, are formed by the union of oppositely charged 



^ For further information about the constitution of atoms see a modern textbook of 

 chemistry such as that by W. McPherson and W. E. Henderson, Course in General Chem- 

 istry, 4th ed., Ginn and Company, 1936, pp. 200-226; H. T. Briscoe, Introduction to 

 College Chemistry, Houghton Mifflin Company; W. H. Hatcher, Introduction to Chemi- 

 cal Science, John Wiley & Sons, Inc. 



