550 



equal number of electrons, indicated by the difference between 

 the atomic weight and the atomic number. 



The atomic numbers apparently determine all the properties 

 of each element, and are more fundamental than the atomic 

 weights. The latter are secondary properties, in most cases 

 modified by other factors, and in a few cases actually thrown out 

 of order by such factors. 



Atomic Numbers of Radioactive Elements; Isotopes. 



When an atom of a radioactive element loses an atom of helium, 

 it also loses two free positive charges from its nucleus. Its atomic 

 number is consequently reduced by two (for example, Radium 

 = 88, Niton = 86). When, on the other hand, a radioactive 

 change takes place involving the loss of an electron, a positive 

 charge in the nucleus, previously bound, becomes free, and the 

 atomic number is found to be increased by one. 



With these facts in mind, an examination of the uranium 

 disintegration series discloses that several elements (for example, 

 Radium -B, Radium -D, Radium -G and Lead) must ex- 

 ist which possess the same atomic numbers, but different atomic 

 weights. Such elements are known as isotopes. Isotopes are 

 identical in all of their chemical properties, although they differ 

 in atomic mass (see p. 20). This shows conclusively that atomic 

 weight is not a fundamental property, but atomic number. 



Ordinary lead chloride contains the elements lead and chlorine 

 combined in the following proportions by weight: 



Lead (207.2) Chlorine (70.9) Lead chloride (278.1). 

 Richards, however, has found that the lead contained in uranium 

 ores gives a chloride in which as little as 206.1 parts by weight of 

 lead may be combined with 70.9 parts of chlorine, while Soddy 

 has shown that the lead extracted from thorium ores gives a 

 chloride which contains as much as 208.4 parts by weight of lead 

 to 70.9 parts of chlorine. We have, therefore, three lead chlorides, 

 all possessing the same specific properties, and being therefore the 



