Polarity of Molecules

Just as individual bonds can be polar or nonpolar, entire molecules can be polar or nonpolar as well.

A polar molecule is a molecule in which one end of the molecule is slightly positive, while the other end is slightly negative. The two electrically charged regions on either end of the molecule are called poles, similar to a magnet having a north and a south pole. Hence, a molecule with two poles is called a dipole. A simplified way to depict polar molecules is pictured below (see figure below).

 

An oval molecule is shown with a partial positive charge on the left and a partial negative charge on the right.
A molecular dipole results from the unequal distribution of electron density throughout a molecule.

Nonpolar molecules do not have an overall dipole. A molecule that contains only nonpolar bonds must be a nonpolar molecule. 

For molecules with polar bonds, the molecular geometry must also be taken into account when determining if the molecule is polar or nonpolar. The figure below shows a comparison between water (H2O) and carbon dioxide (CO2).

In H2O, the orientation of the two O–H bonds is bent. Each O-H bond is polar, with the dipole pointing towards the oxygen atom. Thus, the oxygen has a partial negative charge while the hydrogens have a partial positive charge. Because of the orientation of the polar bonds, one end of the molecule has a partial positive charge, and the other end has a partial negative charge. In short, the H2O molecule itself is polar.

The molecule, CO2, also contains polar bonds, but it is a linear molecule. The oxygen atoms are more electronegative than the carbon atom, so there are two individual dipoles pointing outward from the C atom to each O atom. Since the dipoles are of equal strength and are oriented directly opposite each other, they cancel each other out, and the overall molecular polarity of CO2 is zero. Thus, CO2 is a nonpolar molecule.

Physical Properties and Polarity. The physical properties of water and carbon dioxide are affected by their polarities.

The polarity of water has an enormous impact on its physical and chemical properties. For example, the boiling point of water (100°C) is high for such a small molecule due to the fact that polar molecules attract each other strongly. On the other hand, the nonpolar carbon dioxide becomes a gas at −77°C, almost 200° lower than the temperature at which water boils.

Similarly, in BF3 (planar triangle), the effect of a B-F bond is cancelled by the sum of the other two B-F bonds (see figure below). Hence, a planar triangle molecule (BF3) can be nonpolar if the bond polarities cancel each other. In contrast, a pyramidal molecule (NH3) is polar because the bond polarities do not cancel each other out. In the NH3 molecule, the dipole arrows point from the hydrogen atoms to the nitrogen atom. The nitrogen side of the molecule has a partial negative charge and the hydrogen side has a partial positive charge.

A ball and stick diagram of BF3 is shown. The shape of the BF3 molecule is planar triangle. The dipole arrows point from the central boron atom to each of the outer fluorine atoms. Because the dipole arrows all point outward and are at 120 degree angles from each other. This cancels out the effects of the dipole arrows and results in an overall nonpolar molecule. Also, a ball and stick model of NH3 is shown. The shape of NH3 is pyramidal. The dipole arrows point from the hydrogen atoms to the central nitrogen atom. The dipole arrows are arranged at 109.5 degree angles from each other. Since all dipole arrows point in the upwards direction towards nitrogen, they result in the nitrogen side of the molecule having a partial negative charge, while the hydrogens on the opposite side have a partial positive charge. This make the molecule polar overall.
The molecular shape of a molecule (planar triangle vs. pyramidal) affects its polarity.

Some other molecules are shown in the figure below. Notice that a tetrahedral molecule such as CCl4 is nonpolar. However, if the peripheral atoms are not of the same electronegativity, the bond polarities don’t cancel and the molecule becomes polar, as in CH3Cl.

A ball and stick model of CCl4 is shown. The shape of the CCl4 molecule is tetrahedral. All dipole arrows point outward from the central carbon atom to the outer chlorine atoms. The dipole arrows are arranged at 109.5 degree angles form each other and cancel each other out. The CCl4 is nonpolar overall. A ball and stick model of CH3Cl is also shown. The shape of CH3Cl is tetrahedral. The three C-H bonds are nonpolar and do not affect the polarity of the molecule. The C-Cl bond is polar, with the dipole arrow pointing from the central carbon to the outer Cl. This dipole arrow results in the overall molecule being polar.
The same molecular shape but peripheral bonds are of different electronegativity. CCl4 is nonpolar but CH3Cl is polar.
HCl is a polar molecule because it consists of one polar bond. The dipole arrow points from H to Cl. BCl3 is a nonpolar molecule because it’s three dipole arrows (each pointing from the central B to the outer Cl atoms) are arranged in a symmetrical planar triangle shape and cancel each other out. CH2O is polar because the oxygen is the most electronegative element in the molecule and all dipole arrows in the molecule point towards the oxygen. NH3 is a polar molecule because all the dipole arrows point in a similar direction toward the nitrogen. Since the dipole arrows in NH3 are in an asymmetrical pyramidal shape, they do not cancel out. CHCl3 is a polar molecule because the Cl atoms pull the electrons away from the H atom, leaving the Cl side of the molecule with a partial negative charge and leaving the H atom with a partial positive charge. CCl4 is a nonpolar molecule because all of the dipole arrows (which each point from C to an outer Cl) are arranged in a symmetrical tetrahedral shape and cancel each other out.
Molecules with Polar Bonds. Individual bond dipoles are indicated in red. Due to their different three-dimensional geometry, some molecules with polar bonds have a net dipole (HCl, CH2O, NH3, and CHCl3), indicated in blue, and the molecule is polar. Other molecules are nonpolar because the bond dipoles cancel due to symmetry (BCl3 and CCl4).

Concept Review Exercises

  1. How do you determine whether a molecule is polar or nonpolar?

Solutions

  1. If all the bonds in a molecule are nonpolar, the molecule is nonpolar. If it contains identical polar bonds that are oriented symmetrically opposite each other (linear, trigonal planar or tetrahedral) then the molecule is nonpolar. If it contains polar bonds that don’t cancel each other’s effects, the molecule is polar.

Attributions

This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html

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