Dipoles & Dipole Moments: Molecule Polarity

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  • 0:03 Dipoles
  • 2:33 Dipole Moments
  • 4:42 Finding the Dipole Moment
  • 6:32 Polar and Nonpolar Molecules
  • 7:46 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn about dipoles and dipole moments in this lesson. Understand the relationship between dipole moments and molecule polarity, and learn how to determine if a molecule is polar or nonpolar.

Dipoles

Let's talk about some of the mysteries of the universe. Carbon dioxide and water are both very common molecules on Earth. Carbon dioxide molecules are almost two-and-a-half times heavier than water molecules, yet water is a liquid at room temperature and carbon dioxide is gas? What could possibly explain this crazy phenomenon?

Let's start solving this problem by looking at the structure of these molecules and where their electrons are. Nonmetal atoms like carbon, oxygen and hydrogen bond by sharing electrons in what is known as a covalent bond. Sometimes electrons are shared evenly, other times one atom will hog the electrons. Uneven sharing of electrons results in the formation of a dipole. A dipole is best defined as a separation of charges between two covalently bonded atoms. The atom that hogs the electrons will have a partial negative charge, often represented with a lowercase delta and a negative sign. The electron deprived atom will have a partial positive charge, represented with a lowercase delta and positive sign. Arrows are also commonly used to indicate a dipole. The arrow lies parallel to the bond and points towards the atom with the partial negative charge.

Dipoles are determined by examining electronegativity values for bonded atoms. Electronegativity is the ability of an atom to draw electrons to itself. In other words, electronegativity values describe just how greedy an atom is for electrons. You can determine the electronegativity of an atom by looking at a chart or using the general electronegativity trend for the periodic table. The more electronegative atom in a bond will always be the one with the partial negative charge. Draw the dipole arrow towards the more electronegative atom. When two elements have the same electronegativity, there is no dipole.

Let's draw the dipoles for all the bonds in carbon dioxide and water using the Lewis dot structures. Which ways will the dipole arrows point?

Dipoles
dipoles for carbon dioxide and water

In the case of carbon dioxide, we have two dipoles. According to an electronegativity chart, oxygen is more electronegative than carbon, so we draw our dipoles pointing from carbon to oxygen. In the case of water, oxygen is more electronegative than hydrogen, so both dipoles point from hydrogen to oxygen.

Dipole Moments

The next step in solving our conundrum is to determine the dipole moment. A molecule has a dipole moment when there is an overall unequal distribution of electrons across the entire molecule. Dipole moments are illustrated with a massive arrow pointing from the partially positive area of the molecule to the partially negative area of the molecule.

Unfortunately, you can't just glance at a Lewis dot structure and know the dipole moment. Generally, we must consider the shape of the molecule and the presence of lone pairs, too.

A molecule like CO2 may be composed of two dipoles, but it has no dipole moment. This is because the charge is equally distributed amongst the entire molecule. When molecules have an even charge distribution and no dipole moment, then they are nonpolar molecules. CO2 is a linear molecule, so our dipoles are symmetrical; the dipoles are equal in magnitude but point in opposite directions. Generally, when dipole distribution is symmetrical, there is no dipole moment. In order to get an accurate picture of this, we must visualize molecule shape. For a review on this topic, please see the video on VSEPR theory.

Some molecules like H2O may appear to be linear according to their Lewis dot structure, when in reality they are bent. Assuming H2O is linear might lead us to think it has no dipole moment because the dipoles appear to be equal in size and pointing in opposite directions.

Dipole Moment
dipole moment for water

However, when we draw the structure of H2O as bent, suddenly the dipoles are no longer perfectly opposite one another. The dipoles point towards oxygen, indicating that the oxygen is partially negatively charged. The dipoles are equal in magnitude, so we can assume that the dipole moment bisects the space between the hydrogens perfectly, pointing towards oxygen. Molecules that have an uneven distribution of charge and a dipole moment are called polar molecules.

Another clue that a molecule is polar is the presence of lone pairs. Water has two sets of lone pairs on oxygen, making that area particularly negative.

Finding the Dipole Moment

Here are some quick steps to determine the dipole moment of the molecule:

Step 1) Obtain the Lewis dot structure for the molecule.

Step 2) Draw dipoles for each bond. Dipole arrows point towards the more electronegative element.

Step 3) Predict the molecule's geometry.

Step 4) Look at the dipoles. Cancel out any of equal magnitude pointing in opposite directions.

Step 5) Determine the dipole moment. If all dipoles cancel out, there is no dipole moment. If dipoles cannot be canceled out, draw a large arrow pointing in the direction of the most negative part of the molecule according to your dipoles. This is your dipole moment.

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