298 BELL SYSTEM TECHNICAL JOURNAL 



pigeonhole for hydrogen should contain three mass-values ; that for 

 helium, two; that for tin, no fewer than ten. It would make the table 

 impossibly crowded to print them all in this way, and consequently I 

 have broken it up into sections, of which Fig. 2 represents the first six 

 elements. 



1 H O O O 



2 He O O 



3 Ll 



4 Be 



6 C ^ O 



Fig. 2 — Isotopes of the first six elements. 



In this figure each element has a row to itself, and each value of 

 mass has a column to itself, and each circle represents a stable kind of 

 atom. I now introduce the technical term "isotope" to distinguish 

 the different kinds of atoms common to a single element. Hydrogen, 

 you see, has three stable isotopes (there is some doubt about the 

 stability of the third, though none about its existence); helium two 

 (again there is doubt about the stability of one) ; lithium two, beryllium 

 only one, boron two, and carbon two of which the second will appear 

 in the next figure. The unit of mass is a very small amount, about 

 1.67 -10"-* of one gram. I do not pause to give it as accurately as I 

 might, for we are not going to be concerned with very exact mass- 

 values in this talk. The masses of the isotopes are not exactly integer 

 multiples of this unit; for instance, those of the three kinds of hydrogen 

 atoms are 1.008, 2.016 and 3.017. The departures from integer 

 multiples are, however, small, as you see in these three cases. Small 

 as they are, they are mightily important; but it is permissible to ignore 

 them for the purposes of this lecture, and I am going to ignore them 

 from now on. I will, however, speak of the integers at the heads of 

 the columns as "mass-numbers" rather than "masses." 



Since the isotopes of an element differ in mass, what is it then that 

 they have in common? I answer this question by describing Ruther- 

 ford's second great achievement, the "nuclear atom-model." Ruther- 

 ford was the first to prove that the atom consists of a positively- 

 charged nucleus surrounded by a swarm of negative electrons. The 



