26 Introduction 



(Fig. 2). This table is based upon the number of protons in the 

 nucleus and the electrons in the rings. The atoms are arranged in the 

 order in which the number of electrons and protons increase in an 

 orderly fashion. Each element bears an atomic number that indicates 

 its position. One electron rates position one in the table. As the first 

 ring can have but two electrons, only one other element is in the 

 series with hydrogen; this element is helium which has two electrons 

 in this orbit. Beginning the next series of elements is lithium with 

 three electrons : two in the first ring and one in the second. Copper, 

 number 29, has 2 electrons in the inner ring, 8 in the second, 18 in the 

 third, and 1 in the fourth. In general, no ring starts filling with elec- 

 trons until the ring below it is saturated. Each atom has a certain 

 weight known as its atomic weight. These atomic weights of the various 

 elements are given in relationship to that of oxygen which is assigned 

 a weight of 16. Sulfur, for example, has an atomic weight of 32.06, 

 hydrogen, 1.01, and carbon, 12.01. 



I 2 



H HE 



Fig. 2. — A portion of the periodic table of the elements. The atomic number 

 of each element is indicated below each diagram, together with the abbreviation for 

 the element. 



Isotopes. — Carbon from any natural source, however, is actually 

 a mixture of several atomic weights. These various carbons are known 

 as carbon 11, 12, 13, or 14, according to their varying atomic weights. 

 It would be very simple to demonstrate that the chemical activity of 

 these various materials is identical despite the varying weights. Thus 

 it must be assumed that the electron orbits are all identical. The dif- 

 ferences must lie in the nuclei. The nucleus, as mentioned previously, 

 is made up principally of protons and neutrons, with the protons exactly 

 balancing the electrons of the orbits. The differences in atomic weights 

 then must be due to different numbers of neutrons. 



