518 On the Dimensions of Atoms. 



contemplated is complicated by the fact that in the gaseous 

 state the atoms combine into pairs forming molecules, so that 

 we are no longer able to regard the molecules as spheres, and 

 apply the kinetic theory with the confidence permissible 

 in the consideration of monatomic gases. According to 

 Langmuir's theory we should picture a chlorine atom as 

 identical with the argon arrangement except that the nuclear 

 charge is 17 instead of 18 and that there is one less electron 

 in the outer shell. In other words, we should expect the 

 diameters' of the outer shells of chlorine and argon atoms to 

 be practically the same. In the gaseous state, however, the 

 unstable arrangement of the chlorine atom is avoided by the 

 junction of the atoms in pairs, so that each pair shares two 

 electrons, and thus forms a stable molecule. The fact that 

 two electrons are shared implies intimate contact between 

 the outer electron shells, and, as far as dimensions are con- 

 cerned, therefore, a chlorine molecule is almost identical 

 with two argon atoms in contact. Similarly, a bromine 

 molecule has the size and shape of two krypton atoms in 

 contact, and an iodine molecule is similarly related to tw T o 

 xenon atoms. According to this view, the molecules of 

 chlorine, bromine, and iodine have, respectively, twice the 

 volumes of the atoms of argon, krypton, and xenon. This 

 is in complete agreement with the conclusion arrived at by 

 the author* in 1^15, from deductions from the viscosities of 

 the gases in question. The procedure by which this result 

 was obtained was, of course, only approximate, for it was 

 based on the assumption of spherical molecules, according to 

 the usual statement of kinetic theory. To extend this theory 

 so as to include diatomic molecules of the shape which 

 LangmmVs views lead us to infer, would be a very com- 

 plicated matter. But in so far as it is justifiable to treat 

 such molecules as colliding with the same frequency as they 

 would, were they spheres of the same volume and mass, the 

 diameter of each atom can be calculated for comparison with 

 the values obtained by W. L. Bragg. The results are shown 

 in Table II., the arrangement of which is the same as for 

 Table I. 



It will be seen that here again the " viscosity " estimate 

 of the diameter is in excess of the "crystal "estimate, and 

 that the increments are general accordance. A comparison 

 between the 4th columns' of the two tables will show how 

 nearly the diameters of the atoms of argon, krypton, and 

 xenon are equal respectively to those of chlorine, bromine, 

 and iodine. 



* A. 0. Ranldne, Phil. Mag. vol. xxix. p. 554. 



