66 BELL SYSTEM TECHNICAL JOURNAL 



properties of other non-radioactive elements which share particular 

 atomic numbers and are mixed together in varying proportions; and 

 they establish that such "elements" are indistinguishable except in 

 such properties as are influenced to a measurable extent by the mass of 

 the nucleus. 



These facts make it necessary to redefine the word element, which in 

 its long journey through the centuries from Lucretius has modified its 

 meaning time and time again to keep pace with the gradual refinement 

 of scientific thought, though all the while it kept its spelling intact. 

 These are the alternatives: either to confer the status of a separate 

 "element" upon each substance (apart, of course, from the com- 

 pounds!) possessing a distinctive mass and radioactivity of its own, so 

 that there may be several distinct elements sharing a given set of chem- 

 ical properties — or to link the term "element" with a characteristic set 

 of chemical and physical properties, with a specific atomic number and 

 position in the Periodic Table, so that a given element may be an en- 

 semble of several different kinds of matter differing in radioactivity or 

 mass or both. Reasons of science require that one or the other of these 

 alternatives be chosen, but the actual choice is determined by reasons of 

 language and expediency. These reasons — I will not pause to develop 

 them — favor the second alternative. Inconvenient though it may be 

 to refer to RaB and RaD and RaG and ThB and several other radioac- 

 tive substances as the same element, the inconveniences entailed by the 

 other policy would in the end be immensely greater. One element to 

 each atomic number, one place in the Periodic Table to each element — 

 this is the choice which the prior usage and the associations of the word 

 element recommend ; and some other name must be selected to dis- 

 tinguish the substances which share a common atomic number but 

 differ in mass or radioactivity or both. 



Such a name is Soddy's word isotope, constructed out of Greek 

 words to signify "in the same place." Radium B and RaD and all the 

 other substances which appear in the column labelled "82" in Fig. 1 

 are isotopes of the element 82; radon and thoron and actinon are 

 isotopes of the element 86. In these eleven places of the Periodic 

 Table extending from 81 to 92, the individual isotopes enjoy names of 

 their own, the elements are best known by their numbers. The names 

 thallium, lead, bismuth and uranium are, it is true, generally attached 

 to the elements 81, 82, 83 and 92 ; but the first three of these names are 

 used by some people to mean the elements in question and by others to 

 designate only those of their isotopes which are not radioactive, and 

 there is danger of confusion. ^^ Elsewhere in the Periodic Table, where 



12 The names polonium, radium, actinium, thorium and protactinium signify par- 



