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LXII. On the Dimensions of Atoms. By A. 0. Rankine, 

 D.Sc, Professor of Pliysics in the Imperial College of 

 Science and Technology*. 



1. TN the August number of this Magazine W. L. Bragg f 

 JL has placed on record estimates of the sizes of 

 the domains occupied in crystals by the -atoms of various 

 elements. He has also examined these data, which cover 

 about half the known elements, in terms of Langmuir's } 

 theory of atomic structure, and has shown in a convincing 

 manner that definite values may be assigned to the diameters 

 of the atoms in a limited number of cases. By the diameter 

 of an atom in this connexion is meant the diameter of the 

 outer shell of electrons as defined by Langmuir. It is inter- 

 esting to compare these values with those derived from the 

 well-known and quite different method based on the kinetic 

 theory of gases. It will be seen that the data obtained by 

 calculations from viscosity afford general support to the 

 conclusions which W. L. Bragff has arrived at in the 

 paper referred to. The number of elements to which this 

 procedure can be applied is small, being limited to cases 

 where the element is in the gaseous state at temperatures 

 sufficiently low for the convenient measurement of its 

 viscosity. But it happens, fortunately, that the elements in 

 question are just those for which Bragg does interpret the 

 atomic domain as being nearly coincident with the outer 

 electron shell. It is thus possible to make comparisons for 

 nine elements, including four inert gases, three halogen 

 gases, and oxygen and nitrogen. 



2. The inert gases. — According to Langmuir's theory these 

 elements constitute the most stable arrangements of electrons 

 round positively charged nuclei. They are monatomic and 

 do not enter into chemical combination. A definite physical 

 significance can thus readily be attributed to what is often 

 vaguely called the atomic diameter by regarding it as the 

 diameter of the outer shell of electrons. It is true that no 

 direct calculation of these diameters can be made from 

 crystal measurements, since the inert gases form no com- 

 pounds, and have not yet been obtained themselves in 

 crystalline form. For reasons which appear to be adequate§, 

 however, Bragg has been able to assign probable values, and 

 these are reproduced in column 2 of Table I. The numbers 

 in column 4 of the same table have been calculated from 



* Communicated by the Author. 



f Phil. Mag., Aug. 1920, vol. xl. p. 169. 



j I. Langmuir, Journ. Amer. Chem. Soc. lxi. p. 868. 



§ Loc. cit. 



