9.7: Molecular Polarity

Learning Objectives

• To predict whether a molecule is polar or non-polar

Introduction to Molecular Polarity

You have already seen that covalent bonds are polar when they link two different atoms. In a polar bond, one atom is positively charged and the other is negatively charged. A molecule (or polyatomic ion) is polar when one side of the molecule is more positive (or more negative) than the other.  This occurs when the polarities of the bonds do not cancel out.

For example in CO₂, each carbon-oxygen bond is polar, but CO₂ is a nonpolar molecule. The molecule is linear, so the two bonds point in opposite directions. They are equally polar, so their effects cancel out. The two sides of the molecule are identical; neither one is more negative than the other.

On the other hand, OCS is a polar molecule. This molecule is also linear, but the C=O and C=S bonds are not equally polar, because O is more electronegative than S.  In this molecule, the oxygen side is more negative than the sulfur side.

Water is also polar, but for a different reason.  In this case, the two bonds are equally polar, but they do not cancel each other out, because they do not point in opposite directions.  Water is a bent molecule, so the O­–H bonds are roughly 109.5˚ apart (actually about 105˚).  In the drawing below, the left side of the molecule is positively charged and the right side is negatively charged.

As a general rule, molecules that are distorted are also polar. The only exceptions are linear molecules in which the outer atoms are dissimilar; these molecules are not distorted, but they are polar. OCS (above) is an example.

Polar Molecules Have a Net Dipole Moment

Molecular polarity can be described more formerly in terms of the summation of bond dipole moments. In complex molecules with polar covalent bonds, the three-dimensional geometry and the compound’s symmetry determine whether there is a net dipole moment. Mathematically, dipole moments are vectors; they possess both a magnitude and a direction. The dipole moment of a molecule is therefore the vector sum of the dipole moments of the individual bonds in the molecule. If the individual bond dipole moments cancel one another, there is no net dipole moment. Such is the case for CO₂, a linear molecule (Figure \(\PageIndex{1a}\)). Each C–O bond in CO₂ is polar, yet experiments show that the CO₂ molecule has no dipole moment. Because the two C–O bond dipoles in CO₂ are equal in magnitude and oriented at 180° to each other, they cancel. As a result, the CO₂ molecule has no net dipole moment even though it has a substantial separation of charge. In contrast, the H₂O molecule is not linear (Figure \(\PageIndex{1b}\)); it is bent in three-dimensional space, so the dipole moments do not cancel each other. Thus a molecule such as H₂O has a net dipole moment. We expect the concentration of negative charge to be on the oxygen, the more electronegative atom, and positive charge on the two hydrogens. This charge polarization allows H₂O to hydrogen-bond to other polarized or charged species, including other water molecules.

Figure \(\PageIndex{1}\): How Individual Bond Dipole Moments Are Added Together to Give an Overall Molecular Dipole Moment for Two Triatomic Molecules with Different Structures. (a) In CO₂, the C–O bond dipoles are equal in magnitude but oriented in opposite directions (at 180°). Their vector sum is zero, so CO₂ therefore has no net dipole. (b) In H₂O, the O–H bond dipoles are also equal in magnitude, but they are oriented at 104.5° to each other. Hence the vector sum is not zero, and H₂O has a net dipole moment.

Other examples of molecules with polar bonds are shown in Figure \(\PageIndex{2}\). In molecular geometries that are highly symmetrical (most notably tetrahedral and square planar, trigonal bipyramidal, and octahedral), individual bond dipole moments completely cancel, and there is no net dipole moment. Although a molecule like CHCl₃ is best described as tetrahedral, the atoms bonded to carbon are not identical. Consequently, the bond dipole moments cannot cancel one another, and the molecule has a dipole moment. Due to the arrangement of the bonds in molecules that have V-shaped, trigonal pyramidal, seesaw, T-shaped, and square pyramidal geometries, the bond dipole moments cannot cancel one another. Consequently, molecules with these geometries always have a nonzero dipole moment.

Figure \(\PageIndex{2}\): Molecules with Polar Bonds. Individual bond dipole moments are indicated in red. Due to their different three-dimensional structures, some molecules with polar bonds have a net dipole moment (HCl, CH₂O, NH₃, and CHCl₃), indicated in blue, whereas others do not because the bond dipole moments cancel (BCl₃, CCl₄, PF₅, and SF₆).

Molecules with asymmetrical charge distributions have a net dipole moment.

Example \(\PageIndex{1}\)

Which molecule(s) has a net dipole moment?

1. \(\ce{H2S}\)
2. \(\ce{NHF2}\)
3. \(\ce{BF3}\)

Given: three chemical compounds

Asked for: net dipole moment

Strategy:

For each three-dimensional molecular geometry, predict whether the bond dipoles cancel. If they do not, then the molecule has a net dipole moment.

Solution:

1. The total number of electrons around the central atom, S, is eight, which gives four electron pairs. Two of these electron pairs are bonding pairs and two are lone pairs, so the molecular geometry of \(\ce{H2S}\) is bent (Figure \(\PageIndex{6}\)). The bond dipoles cannot cancel one another, so the molecule has a net dipole moment.
   
2. Difluoroamine has a trigonal pyramidal molecular geometry. Because there is one hydrogen and two fluorines, and because of the lone pair of electrons on nitrogen, the molecule is not symmetrical, and the bond dipoles of NHF₂ cannot cancel one another. This means that NHF₂ has a net dipole moment. We expect polarization from the two fluorine atoms, the most electronegative atoms in the periodic table, to have a greater affect on the net dipole moment than polarization from the lone pair of electrons on nitrogen.
   
3. The molecular geometry of BF₃ is trigonal planar. Because all the B–F bonds are equal and the molecule is highly symmetrical, the dipoles cancel one another in three-dimensional space. Thus BF₃ has a net dipole moment of zero:

Exercise \(\PageIndex{1}\)

Which molecule(s) has a net dipole moment?

• \(\ce{CH3Cl}\)
• \(\ce{SO3}\)
• \(\ce{XeO3}\)

Answer

\(\ce{CH3Cl}\) and \(\ce{XeO3}\)

Summary

Molecules with polar covalent bonds can have a dipole moment, an asymmetrical distribution of charge that results in a tendency for molecules to align themselves in an applied electric field. Any diatomic molecule with a polar covalent bond has a dipole moment, but in polyatomic molecules, the presence or absence of a net dipole moment depends on the structure. For some highly symmetrical structures, the individual bond dipole moments cancel one another, giving a dipole moment of zero.