300 BELL SYSTEM TECHNICAL JOURNAL 



same whether the atom is part of a soUd, a liquid or a gas; it is immune 

 also to the chemical state of the atom, i.e. it is the same whether the 

 radioactive element is isolated or is part of a chemical compound. It 

 is also immune to heat and to all of the many other agencies which 

 physicists and chemists have at their command. 



Radioactivity being a feature of the nucleus, every chemical symbol 

 which I use from now on will refer to the nucleus of an atom and not to 

 the atom as a whole. "Be" will stand for beryllium nuclei, "F" for 

 fluorine nuclei, "Al" for aluminium nuclei. For most elements, 

 though, there are two or more different sorts of nuclei distinguished 

 from each other by their masses, and the symbol must tell us which is 

 meant. The custom is to write the mass-number of the isotope in 

 question as if it were an exponent: H^ and H- and H' for the three kinds 

 of hydrogen nuclei, He^ and He^ for the two kinds of helium nuclei, 

 Li^ and Li^ to distinguish between the isotopes of lithium, and so on. 

 If in addition one wants to remind the reader of the atomic number, 

 one writes it as a subscript before the chemical symbol : iH\ iH^, 2He'*, 

 oF^^ and the like. Purists object that either the chemical symbol or the 

 value of Z is superfluous when both are given, but others often like to 

 see them both. And now for some names: there are three nuclei which 

 have names of their own. The Greek words for "first" and "second" 

 are applied to iH^ and iH-; they are the proton and the deuteron. The 

 name for 2He* is alpha-particle; this nucleus is indeed the particle which, 

 as Rutherford discovered long ago, makes up one of the three kinds 

 of rays which radioactive bodies emits, and there never was a greater 

 piece of good fortune in language than that whereby this all-important 

 particle received the name of the first letter of the Greek alphabet, for 

 indeed it is the alpha of modern nuclear physics. And now another 

 reference to masses: the mass of the electron (when not moving ex- 

 tremely fast) is only about .0005 of the mass-unit which is being used 

 throughout this talk, and therefore the mass-numbers at the heads of 

 the columns in Figs. 1 and 2 and others are about as good approxima- 

 tions to nuclear masses as they are to atomic masses, and I shall use 

 them as such. 



Now let us notice not only the circles of Figs. 2 and 3, but the stars 

 as well. The stars also stand for nuclei, but these are radioactive — 

 or unstable, two words which have practically the same meaning when 

 applied to a nucleus. At least one star appears in every row, the first 

 in Fig. 2 excepted. If the figure had room for ninety-two rows, one 

 for each element from hydrogen to uranium, there would appear at 

 least one star in every row below the first, excepting three (atomic 

 numbers 61, 85, 87) for which no isotope either stable or unstable 



