According to the dipole moment's definition, as polarity increases the distance between the centres of positive charge & negative charge increases.In the ionic bond ,as the polarity is maximum, distance between the two charged centres is maximum.Is this concept correct? If not what is the relationship between distance and polarity. I m not getting any intuition.


Actually dipole moment is: $$\huge \vec\mu=q.\vec d$$ where $q$ is the magnitue of charge on any particle and $\vec d$ is the distance between them; This must imply that if charge increases or distance between the charges increases, dipole moment increases; The larger the dipole moment, the greater the polarityenter image description here

Actually when the more electronegative elemets pulls electrons, the partial charge, $q\le e$, increases and in case of 100% polar compund, $q=e$ whereas in 100% covalent compunds, $q=0$; so, you can see dipole moment, and polarity is maximum for ioninc compounds.But if two atoms have greater bond length, surely dipole moment and polarity increases.

  • $\begingroup$ $d$ is the distance between what? When more electronegative atom pulls the electron pair,does the distance increase ie. more polar means more distance?Can u plz elaborately explain? $\endgroup$ – user5764 Jun 22 '14 at 12:56
  • $\begingroup$ I just only want to know what is the role of the distance;does it increase as polarity increases? It was my question i m doing but no one is answering satisfactorily. $\endgroup$ – user5764 Jun 22 '14 at 13:02
  • $\begingroup$ How has $d$ come in the measurement of polarity?PLZ explain! $\endgroup$ – user5764 Jun 22 '14 at 13:04
  • $\begingroup$ yes if you increase distance polarity increases, but vice-versa may not be true as there is no direct way of increasing polarity other than changing q or d. $\endgroup$ – RE60K Jun 22 '14 at 13:10
  • $\begingroup$ yes you are correct that if d is increased $\mu$ increases , also if $\mu$ is maximum then there's no limit to maximization of d, but there is for q. $\endgroup$ – RE60K Jun 22 '14 at 13:19

I'm not sure if the guideline actually works out in empirical observations. There are at least two factors affecting bond length - atomic radii - and polarity of the bond. A more polar bond will have a less even distribution of electrons. The electron density will generally be closer to the more electronegative atom. This isolation of electrons may have the effect of "stretching" the bond.

Consider the following two molecules, both of which are linear and polar:

enter image description here

enter image description here

From polarity considerations alone, we might expect the $\ce{H-F}$ bond to be longer than the $\ce{H-Cl}$ bond because fluorine is more electronegative than chlorine. However, chlorine is a bigger atom than fluorine by nature of having additional sublevels of electrons.

Also, consider the below data: bond length actually increases with decreasing polarity (quantified through dipole moments).

Fluoromethane Bond length 136pm, Bond strength 108 kcal/mol, Dipole moment (D) 1.85

Bromomethane Bond length 178pm, Bond strength 84 kcal/mol, Dipole moment (D) 1.87

Chloromethane Bond length 193pm, Bond strength 70 kcal/mol, Dipole moment (D) 1.81

Iodomethane Bond length 214pm, Bond strength 56 kcal/mol, Dipole moment (D) 1.62

The unfortunate thing about your guideline is that I thought that it would work at least across periods - in which atomic size stays relatively constant. However, the facts do not fit the case. Consider $\ce{H-N}$ and $\ce{H-O}$. Oxygen is more electronegative than nitrogen; we'd expect the $\ce{H-O}$ bond to be more polar, and thus longer than the $\ce{H-N}$, at least according to your guideline. The opposite is actually true; the $\ce{H-O}$ bond is actually shorter than the $\ce{H-N}$ bond.

  • $\begingroup$ So what is the role of distance in measuring the degree of polarity.According to u , polarity is inversely proportional to the distance?Can u plz explain? $\endgroup$ – user5764 Jun 22 '14 at 5:20
  • $\begingroup$ I didn't say that. I think that you should consider bond length on a case by case basis as there are several factors. $\endgroup$ – Dissenter Jun 22 '14 at 5:27
  • $\begingroup$ Can u give me the actual answer:what is the role of $d$ in measuuring the polarity ? $\endgroup$ – user5764 Jun 22 '14 at 10:16
  • $\begingroup$ How does $d$ affect the dipole moment? $\endgroup$ – user5764 Jun 22 '14 at 10:17
  • $\begingroup$ d is a measure of the dipole moment. $\endgroup$ – Dissenter Jun 22 '14 at 14:51

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