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LXXXXL Some Dimensions of the Atom. 

 By L. St. C. Broughall * 



IN the majority of the theories so far advanced on the 

 subject o£ the structure of the atom, little has been said 

 about the actual dimensions. 



The diameter of the molecule varies approximately between 

 1 and 10 X 10 ~ 8 cm., the value for hydrogen being somewhere 

 in the region o£ 2 x 10~ 8 cm. Now in the case of the inert 

 gases we conclude, on the evidence afforded by the ratio o£ 

 the specific heat at constant pressure to the specific heat 

 at constant volume, that the molecules are monatomic. 

 Therefore, any investigation on the size of the molecule is 

 also, for these gases, an investigation on atomic dimensions. 



There are several methods which have been applied to the 

 measurement of a' the diameter of the molecule of a gas, 

 and the experimental values for the inert gas have been 

 found particularly in two ways, namely — (i.) the value 

 deduced from " b " in Van der Waals' equation; (ii.) the 

 value obtained from the viscosity of the gas. , 2 



Now in the first case we have the equation lp-\ ) (v — b) 



= R#, where " b " is a numerical constant required to 

 ■correct the simple gas law for the volume of the molecules 

 present. In order to obtain the diameter we use the equation 



*=(tT^J where N = 2-75x10 19 . 



In the second case a can be calculated from the equation 



(T=l'0912~-\ where p = density of gas in gms./c.c. at 



S.T.P., G- = velocity of molecule in cms./sec. at 0° C, 

 yj = viscosity, N = number of molecules per c.c. at 0° C. and 

 76 cm. pressure = 2"75 x 10 19 . 



Using the above formulae we find that the values for the 

 inert gases are as follows : — 



Molecular diameter deduced from 

 Gas. t], b. 



Helium 2'02 x 10" 8 cm. 2'30 x 10" 8 cm. 



Neon 2-37 „ „ 



Argon 3-41 ., „ 2S6 ., 



Krypton ... 3'89 „ „ 314 „ 



Xenon 4*58 ,, „ 3*42 „ ,, 



* Communicated bv the Author. 



