224 



PHENOMENA, ATOMS, AND MOLECULES 



where a is the polarizability. A liquid made up of non-polar molecules of 

 this kind will have a dielectric constant 8 which is greater than unity, and 

 the molar polarisation of the liquid is calculated from 8 by means of the 

 equation 



/ 8- i \M 



V 8 -4- 2 / P 



>84-2 



where M is the molecular weight and p is the density of the liquid. The 

 relation between P and a is given by the equation 



4 jr, 

 — Na 



2.54 X 10^* a 



where A'' is the Avogadro number, 6.06 X 10-^. The quantity P has the 

 dimensions of a volume and is thus measured in cubic centimeters. For 

 ordinary organic liquids the value of P is roughly about 0.3 of the volume 

 of a gram-molecule. A few values of P and a are given in table i. From 

 the value a = 10"-^, which is of the order of magnitude found in most 

 organic liquids, we can calculate that the electric field needed to give to a 

 non-polar molecule a dipole moment of 10"^^ is F = 3.0 X 10'^ volts per 

 centimeter. This is the field that would exist at a distance r = yX lO'^/^/e 

 cm. from an electron. A field of this magnitude would be found at the 

 distance r = 2.7 X io~^/(e)^''^ in the direction along the axis of the dipole. 

 From the foregoing discussion, we see that the effects of the electric 

 fields of molecules are of three kinds which we may classify as (i) segre- 

 gation, (2) orientation, and (3) deformation. As an example of segrega- 

 tion, we have the segregation of positive ions around negative ions in 

 electrolytes. Dipole molecules become orientated in the field produced by 



any other molecule and thus are attracted toward the molecule producing 

 the field. Non-polar molecules, under the influence of fields of other mole- 

 cules, become deformed or polarized so that they acquire a dipole moment 

 in the direction of the field and are thus attracted by the molecule produc- 

 ing the field. 



