Constitution and Stability of Atom JSiiclei, 315 



number (n), written as a subscript, is that number which, 

 added to twice the atomic number, gives the atomic weight, 

 written as a superscript, or it is equal to the term 2/ in the 

 atomic weight equation of Harkins and Wilson, 



W = P = 2(M+/)=2M + 2/. .... (1) 



It may be considered as the number of neutrons present in 

 any isotope in excess of the composition represented by the 

 same element by the isotopic number zero. Here the term 

 neutron represents one proton plus one electron (pe). The 

 isotopic number may be defined experimentally by the 

 following equations : 



?i = P-?M, (2) 



n="N-P, (3) 



n=N-M, (3) 



so it indicates the excess of twice the number of negative 

 electrons over the number of positive electrons, or the excess 

 of the number of negative electrons over the positive nuclear 

 charge. 



The plot in the lower right-hand corner of fig. 1, PL XII. 

 represents the radioactive disintegration series on an en- 

 larged scale. Here the curvature of the hyperbolas hns 

 become so slight as to be almost indistinguishable. 



It is evident that during a series of alpha disintegrations 

 the isotopic number remains constant, while a single beta 

 disintegration decreases the isotopic number by 2. Of these 

 two units one is due to the decrease of N by one, the other to 

 the increase of M by one. All of the isotopes of one element 

 (atomic number constant) may be said to lie on isotopic 

 lines, which in the present plot are straight lines which 

 slope upward toward the right, the steepness of the slope 

 decreasing with increasing atomic number. The lines of 

 constant isotopic number may be designated as neulronal 

 lines, since each line represents a constant number of 

 neutrons in excess of the composition represented by isotopic 1 

 number zero. 



The addition of a negative electron to any nucleus would 

 increase its isotopic number by 2, but decrease its atomic 

 number by 1, while the addition of a positive electron 

 would decrease its isotopic number by 1, and increase the 

 atomic number by 1. Thus the addition of a positive 

 electron to the nucleus of Mg 12 2 2G would give 1<V 7 , which 

 is aluminium. 



