RADIOACTIVITY— ARTIFICIAL AND NATURAL 301 



known with certainty and the element itself must still be regarded as 

 missing. (Furthermore there should be at least three more rows 

 numbered 93, 94 and 95, and containing stars but no circles.) This is 

 what is meant by saying that every known element, hydrogen alone 

 excepted, has at least one radioactive form. 



Figure 2 shows that at the beginning of the Table of the Elements, 

 the stable types of nuclei outnumber the unstable ones. The pre- 

 ponderance is gradually shifted as Z increases, and Fig. 4 exhibits to 

 us how greatly the radioactive nuclei outnumber the stable ones among 

 the elements of which the atomic numbers range from 81 to 84. In- 

 deed the circle which is lowest and most to the right in Fig. 4 represents 

 the most massive and most highly charged of all the stable nuclei 

 which are known (it is the solitary isotope of bismuth, atomic num- 

 ber 83 and mass-number 209). All the rows after 83 are occupied 

 entirely by stars. ^ 



203 TO 218 



81 O O O ^ ^ -)f 



62 

 63 



Fig. 4 — Isotopes of the elements numbered 81 to 84. 



As the title of this talk has already suggested, the radioactive nuclei 

 are of two classes: the "natural" and the "artificial," the types already 

 existing in the rocks of the earth and the types made in the laboratory 

 by physicists employing the art of transmutation. Nearly all of the 

 natural types lie beyond 80 in atomic number, and most of them were 

 discovered in the first fifteen years after Becquerel found the first. 

 Two of them are identical with two of the man-made types. Apart 

 from these two, every one of the artificial types is a creation of the 

 years since 1933. One guesses that while the natural radioactive 

 bodies may be many, the artificial ones must surely as yet be few; 

 how surprising then to learn that while there are some forty of the 

 former, the latter after four brief years already number two hundred 

 and thirty! Unlike the natural ones, these artificial isotopes are 

 sprinkled liberally throughout the whole of the Table of the Elements, 

 from the second onward to the end. Not only in number but in di- 



* It must though be admitted that some of the heaviest nuclei, though demon- 

 strably radioactive, may exist for hundreds of millions of years before they dis- 

 integrate; "instability" is indeed a very relative concept! 



