14 THE FOUNDATION Part I 



Boston Common and the dead bench on which he sits both abound in speed- 

 ing molecules (Figs. 2.4 and 2.5). 



Characteristics of Atoms. Nobody has seen the atoms. Their existence was 

 assumed by John Dalton (1766-1844) and it has been proved by patient, 

 skillful experimentation with radioactivity and other means. 



Fig. 2.4. Molecules are continuously repelled 

 and attracted in a random jumpy dance. Those in 

 a thin gas move in free curves. Those in a fluid 

 or a solid are packed together as if in a crowded 

 hall. (Courtesy, Gerard: Unresting Cells. New 

 York, Harper & Bros., 1940.) 



The relatively small center body or nucleus contains practically all of the 

 atom's mass. Electrically negative particles rotate around it. In comparison 

 with their size, they swing through space relatively as great as that in which 

 planets rotate about the sun (Fig. 2.6). The nucleus is composed of protons 

 carrying positive charges of electricity and neutrons that carry no charge. The 

 sum of their masses is the weight of the atom. The electrical charge of the 

 nucleus indirectly controls the nature and behavior of the atom. Atomic nuclei 

 are bound together by a force that was unimagined until experimental splitting 

 demonstrated its reality. As interdependence permeates living organisms, so 

 interdependence of parts is the keystone of the atoms that are the foundation 

 of living matter. 



Within the space around the nucleus are particles called electrons, so light 

 that they are ignored in the computation of atomic weight. Each carries a 

 negative charge of electricity and spins like a coin that is spun upon a table 

 top. It is generally believed that electrons revolve around the nucleus, but 

 their spinning is independent of it. The number of electrons in an atom governs 

 its chemical properties. Electrons, for example, determine that one atom of 

 oxygen will unite with two atoms of hydrogen to form water (H^O). 



Isotopes. Isotopes are different forms of atoms existing in the same element 

 (Fig. 2.6). They have nearly the same chemical properties but differ in the 

 number of neutrons in their nuclei. Since the weight of an atom is the sum of 

 the numbers of its protons and neutrons, the isotopes of an atom have differ- 

 ent atomic weights. For example, hydrogen has three known isotopes: hydro- 

 gen, atomic weight 1; deuterium (heavy hydrogen), atomic weight 2; tritium, 

 atomic weight 3. Isotopes that have few neutrons in their nuclei are called 

 light isotopes and those with the most neutrons heavy isotopes. In general the 

 heavy isotopes are less stable, since an excess of neutrons weakens the co- 



