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BELL SYSTEM TECHNICAL JOURNAL 



occur only in those made up of molecules which are electrically 

 asymmetrical. 



Polar molecules (that is molecules with permanent electric moments) 

 are, by definition, those in which the centroid of the negative charges 

 does not coincide with the centroid of the positive charges, but falls 

 at some distance from it. All materials must be classed either as 

 polar or non-polar, the latter class including those which are elec- 

 trically symmetrical. Some simple examples of non-polar molecules 



METHYL CHLORIDE 

 (CH3 Cl) 



Fig. 2 — Methane and carbon tetrachloride are non-polar molecules each having 

 four equal vector moments whose sum is zero. Methyl chloride is polar because the 

 sum of the vector moments is not zero. 



are H2, N2, O2, CH4, CCI4 and CeHe. In these molecules each C — H, 

 C — Cl or other bond may be regarded as having a vector dipole mo- 

 ment of characteristic magnitude located in the bond. Where the 

 sum of these vector moments is zero the molecule will be non-polar. 

 Both CH4 and CCI4 meet this requirement but CH3CI is polar because 

 the C — Cl vector moment is considerably greater than the resultant of 

 the three C — H vectors. (See Fig. 2.) Polar molecules are the rule 

 and non-polar the exception. 



