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Chapter V Dielectric Study of ……
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non-polar, while the molecules of a typical ionic compound such as KI with a
hetro-polar bond have a very high dielectric moment µ = 23 x 10 -30 Cm [1]. This
moment is directed from the negatively charged ion I - to the positively charged
ion K + , to which is shown in figure (5.3-a).
It is quite obvious that the polarity of a molecule is judged by its
geometrical picture and actual arrangements of charges in space. A molecule
of carbon dioxide exhibits symmetric structure with a centre of symmetry as
shown in figure (5.3-b), and for this reason carbon dioxide is not polar;
whereas a molecule of water has the form of isosceles triangle as shown in
figure (5.3-c), therefore, water displays sharply pronounced polar properties with
µ = 6.1 x 10 -30 Cm [1].
All hydrocarbons are non-polar or weakly polar substances. However in
hydrocarbons, if hydrogen atoms are substituted by the atoms of halogens (F,
Cl, Br or I) or by the groups (-OH, -NH2, -NO2, etc), the asymmetric molecules
are formed in which µ > 0.
Considering simplest hydrocarbon methane CH4 and the products of
substitution of hydrogen by chlorine are methyl chloride CH3Cl, methylene
chloride CH2Cl2, chloroform CHCl3 and carbon tetrachloride CCl4, whose
structural formulae are as shown in figure (5.4).
Figure 5.4: (a) methane, (b) methyl chloride, (c) methylene chloride, (d) Chloroform, (e) Carbon
tetrachloride.
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