ARTICLES 



THE DIMENSIONS OF ATOMS 

 AND MOLECULES 



By WILLIAM LAWRENCE BRAGG, M.A., F.R.S. 

 Langworthy Professor of Physics, University, Manchester 



It has long been known that the dimensions of the simpler 

 molecules are of the order io~*cm. When we try to deter- 

 mine these dimensions accurately, however, we are faced with 

 the difficulty of defining with precision that which we are 

 attempting to measure — a difficulty which is due to our ignorance 

 of atomic and molecular structure. Even so, various lines of 

 reasoning have agreed in assigning diameters to the molecule, 

 regarded as a spherical body, which lie between one and four 

 Angstrom units (i A.u. = io~* cm.). It is the purpose of this 

 article to review the evidence on which these estimates are 

 based, and to give examples which will illustrate the general 

 concurrence between the results obtained by different methods. 

 The dielectric constant of a gas is greater than unity, and 

 we must therefore suppose that the molecules become polarised 

 in an electric field. A rearrangement of their structure takes 

 place which has the same effect as that of opposite electrical 

 charges induced at either end of the molecule. According to 

 Mossotti's hypothesis, the molecules of the gas may be regarded 

 as small conducting spheres, and with this assumption the 

 diameter of the spheres can be calculated from the dielectric 

 constant. The dielectric constant K and the radius o- of the 

 molecules are connected by the equation 



K - I =^ 4'rrN<r' (l) 



where N is the number of molecules in unit volume. Values 

 of the atomic diameter for the inert gases calculated in this 

 way are given below : 



2<r (Diameter of atom). 



io~* cm. 



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