Mass Spectra of the Alkali Metals, 441 



When this table is taken in connexion with that o£ the 

 elements given by the ordinary discharge-tube method (vide 

 Phil. Mag. vol. xlii. p. 143), it will be seen that the mass- 

 spectrum analysis of three groups of the periodic table — 

 the halogens, the alkali metals, and the inactive gapes — is now 

 complete as far as the element caesium ; and with the 

 possible exception of K 39 and the doubtful CI 39 there are no 

 isobares. The intermingling of the atomic weights so deter- 

 mined is very remarkable, and is particularly striking in the 

 case of the ten consecutive integers representing the isotopes 

 of bromine, krypton, and rubidium: — Kr 78, Br 79, Kr 80, 

 Br 81, Kr 82, Kr 83, Kr 84, Rb 85, Kr 86, Rb 87. It is 

 quite clear that the exact order of the chemical atomic 

 weights has little significance and that the anomalies such as 

 argon and potassium are merely due to the unequal relative 

 proportions of their isotopic constituents. 



An examination of the isotopes of these groups of elements 

 shows that of the halogens (odd atomic numbers) all 6 are 

 odd, of the alkali metals (odd atomic numbers) 7 are odd and 

 1 even, while of the inactive gases (even atomic numbers) 

 14 are even and 3 odd. Therefore the interesting relation 

 emerges that for the great majority of the possible configura- 

 tions even atomic weight is associated with even atomic 

 number and odd with odd. This means that in the nuclei of 

 most types of atom the number of the electrons is an even 

 number*. 



Summary. 



An apparatus for the production of positive rays of the 

 alkali metals by means of a hot anode is described. 



The results obtained with the metals Li, Na, K, Rb, and 

 Cs are given showing that Na and Cs appear to be simple 

 and the others mixtures of two isotopes each. 



The relations between these figures and those previously 

 obtained with the halogens and the inactive gases are briefly 

 discussed. 



Cavendish Laboratory, 



Cambridge, May 1921. 



* See also Harkins (' Nature,' April 14, 1921), who deduces a similar 

 result from considering the total abundance of various elements. 



