THE CHEMISTRY OF LIFE 39 



complex — it has six neutrons and six protons in its nucleus and six 

 electrons in a double orbit surrounding it. The atom has often been 

 called a miniature solar system. There is always the central nucleus 

 and the electrons revolving around it much like the planets revolve 

 around the sun in our solar system. The number of electrons is al- 

 ways equal to the number of protons in the nucleus of an atom. Thus, 

 a state of electrical neutrality exists within the atom. 



Most of the volume of an atom is empty space as is true of our 

 solar system, if we wish to carry the analogy a little further. The 

 central nucleus makes up by far the greater portion of the mass of the 

 atom. A proton has weight which is 1,800 times as great as an elec- 

 tron, and neutrons have essentially the same mass as protons. Thus, 

 we have a relatively large central core of the atom around which revolve 

 the infinitesimal electrons, with the greater portion of the atom being 

 empty space. If we could enlarge an atom to a size 100 feet in diam- 

 eter, then the nucleus would appear at the center about the size of a 

 grape. The electrons would be tiny specks, too small to be seen with 

 the naked eye, but they would be whirling around the nucleus at such 

 terrific speeds that they would give a dim blur which would outline 

 the outer limits of the atom. 



Isotopes of Atoms 



We hear a great deal about isotopes these days, especially the radio- 

 active isotopes developed in connection with atomic research. Since 

 such isotopes have great value in biological research, we shall learn 

 something about them at this point in our study. An atom of some 

 specific element behaves the way it does because of the number of 

 protons and neutrons in its nucleus and the number and arrangement of 

 electrons in its orbit or orbits. The number of neutrons may vary in 

 some cases, but this will not change the chemical properties of the 

 atom. It will change the weight of the atom, however. For instance, 

 when an extra neutron is added to the single proton of hydrogen, we 

 have heavy hydrogen (deuterium) which has twice the atomic weight 

 of ordinary hydrogen. It has exactly the same chemical properties as 

 ordinary hydrogen, although its speed of chemical reaction is slower 

 because of its greater weight. Hence, rather than calling deuterium 

 a separate element it is known as an isotope of hydrogen. A still 

 heavier kind of hydrogen was discovered which has two neutrons in 

 the nucleus. It was called tritium. When necessary, we distinguish 

 between these three with superscripts, H\ H 2 , and H 3 , and for most 



