Dipole Moment: Definition, Equation & Examples

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  • 0:00 What Are Polarity And…
  • 2:10 Dipole Moment Equation
  • 2:45 Example Problem
  • 3:20 Lesson Summary
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Lesson Transcript
Instructor: LaRita Williams

LaRita holds a master's degree and is currently an adjunct professor of Chemistry.

In this lesson, we define the term dipole moment and discuss some background on molecular polarity. We also introduce the dipole moment equation and practice using it in an example problem.

Polarity and Dipole Moment Defined

Have you ever shared a bed with someone who likes to hog all of the blankets? You may both go to sleep with the intention of sharing the covers equally, but somehow during the night, the comforter and sheets are naturally drawn towards their side of the bed, leaving you cold and uncomfortable. In chemistry, this type of unequal sharing is the basis of bond polarity and dipole moments.

When atoms share electrons, they form a covalent bond. If one of the atoms in that bond has a stronger attraction for electrons than the other, then the electrons are unequally shared and the bond is classified as a polar covalent bond. Polar covalent bonds result in an uneven charge distribution between atoms. Since electrons innately carry a negative charge, the atom with the stronger attraction for electrons becomes surrounded by a partial negative charge, and the atom with a weaker attraction for electrons is left with a partial positive charge.

A dipole moment is simply the measure of net polarity in a molecule. If a molecule contains polar bonds that are unevenly distributed about the center, there will be an uneven charge distribution across the entire molecule, making it a polar molecule. Polar molecules exhibit a large difference in electrical charge (a positive end and a negative end), otherwise known as a dipole moment.

Dipole Moment Example, NHsub3

For example, ammonia (NHsub3) is a polar molecule. As you can see, ammonia consists of one nitrogen atom covalently bonded to three hydrogen atoms. Since nitrogen has a stronger attraction for electrons than hydrogen, the electrons that they share are drawn towards nitrogen and away from hydrogen. This gives nitrogen a partial negative charge (which is denoted by by a minus sign on N) and hydrogen a partial positive charge (which is denoted by a plus sign on each H). The presence of both a positive and negative charge on the molecule means that ammonia is polar and, therefore, exhibits a net dipole moment.

Dipole Moment Equation

A molecule's dipole moment can be quantified using Equation 1. Here, the Greek symbol mu represents the calculated dipole moment in units of debyes (D) or Coulomb meters (C*m); capital Q represents the value of the charge at either end of the molecule, which is measured in Coulombs (C); and lower case r is the distance between charges in meters (m).

Dipole Moment Equation

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