Some Dimensions of the Atom. 873 



Now, owing to the differences between the values obtained 

 by the two methods, it is apparent that the diameter can 

 only be fonnd to one significant figure, and in some cases 

 even this accuracy cannot be obtained. On the other hand, 

 we would expect that the values deduced from one source 

 would show any relation existing between the respective 

 molecules. 



Now both sets of values show an increase in a with increase 

 in the atomic number of the element, presuming it is a 

 monatomic gas ; but the values obtained from " 6 " of Van 

 der Waals' equation show a much more remarkable fact, 

 namely, that the increase in moving from one inert gas to 

 the succeeding one is a constant. Thus : 



o- Xe -o- Kr =:*28x 10~ 8 cm.; o" Kr -o- A = -28 X 10~ 8 cm.; 



o- A -<7 He = -56 = -28 x 2 x 10- 8 cm. 



Unluckily I have not got the value for Neon, but it is almost 

 certain that its diameter is 2*58 X 10 ~ 8 cm. 



These figures become much more interesting when compared 

 with LangmmVs "Theory of Atomic Structure," published 

 in the Journal of the American Chemical Society for 1919. 



Now, it was found by H. J. Moseley in 1913 that the 

 number of electrons in the atom of an element is equal to 

 the atomic number of that element. Thus the atomic 

 number (N) for Helium = 2 ; Neon = 10 ; Argon = 18 ; 

 Krypton = 36 ; Xenon = 51 ; Niton = 86. 



These figures were shown in Langmuir's paper to obey a 

 very simple law, namely, that N = 2(1 2 + 2 2 + 2 2 f 3 2 + 3 2 + 4 2 ) 

 for the inert gases. Thus N for Helium = 2(1 2 ). N for 

 Neon = 2(l 2 + 2 2 ), etc. 



On this, and in order to explain the valency of the elements 

 and their magnetic properties, Langmuir advanced the 

 explanation that each of the terms in the above equation 

 represents a complete shell of electrons, but the Neon-Argon 

 shells are formed into one shell, as also the Krypton— Xenon- 

 shells. 



The respective shells are referred to as I, II a, II 6, HI a, 

 III 6, IV. Now he further states' that the distance between 

 the first shell and shell 116 is equal to the distance between 

 shells 116 and III b. Now the above figures prove that this 

 is the case, and it follows that each electron has the same 

 free space at its disposal irrespective of the shell it belongs 

 to, as the number of electrons increases in the same pro- 

 portion as the space available ; but owing to the existence of 

 the shells 116 and III 6, one has to presume that two 

 electrons can be squeezed into the space for one without 



