I want to know if I can calculate the dipole moment of a molecule given the electronegativity values for the atoms comprising it.

I think that dipole moments determine the polarity of molecules, so if one molecule has a larger dipole moment than another, the one with the larger dipole moment is more polar than the other. Is this correct?

Can the magnitude of how polar a molecule is be calculated from the difference in electronegativities among the elements comprising the molecule?


Yes you can. By looking at the different electronegativities of each element in a molecule, you are able to determine the dipole moment for each bond. Then by adding the dipole moments for each bond, you are able to determine whether a molecule is non-polar or polar and if it is polar, how polar it is.

For example, in $\ce{CH4}$, the carbon atom is bonded to four hydrogen atoms. The electronegativity of carbon is 2.55, while the electronegativity of hydrogen is 2.2 This means that for each $\ce{C-H}$ bond, there will be a dipole moment from the hydrogen to carbon atom. However, since methane has a perfect tetrahedral structure, the four dipole moments cancel each other out so that there is no net dipole moment. Hence, methane despite having polar bonds, is actually not a polar molecule, shown below. dipole moment for methane

Another example that we can consider is ammonia, $$\ce{NH3}$$. Ammonia consists of 3 $\ce{N-H}$ bonds. The electronegativity of nitrogen is 3.04. That means that ammonia consists of 3 polar $\ce{N-H}$ bonds. The molecular structure of ammonia is trigonal pyrimadal and hence unlike methane, when you add up the 3 dipole moments, they don't cancel each other out. Instead, they result in a net upward dipole moment. Hence, the molecule itself is also polar, shown below

dipole moment of ammonia

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  • $\begingroup$ Is there a mathematical equation between the dipole moment and electronegativity difference? $\endgroup$ – Swaroop Chandra Jul 16 '17 at 15:02
  • $\begingroup$ What about ozone ($\ce{O3}$)? Does your explanation account for why it is polar even though the atoms (and thus electronegativities) on both sides of each bond are the same? $\endgroup$ – user102008 Aug 12 '17 at 16:57
  • $\begingroup$ @user102008 ozone is slightly more complicated. In short, since O3actually exists in resonance, the two end O atoms have a slight delta negative charge. Since O3 is bent, these dipoles dont cancel each other out, hence O3 is polar. $\endgroup$ – Nanoputian Aug 14 '17 at 9:27
  • $\begingroup$ @Nanoputian: I know, but it doesn't seem to be explained by the method you described in your answer, i.e. determining the dipole moment of each bond by looking at the electronegativities of the atoms. $\endgroup$ – user102008 Aug 14 '17 at 16:51

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