5.9: Shapes and Properties- Polar and Nonpolar Molecules

Learning Objective

• Determine if a molecule is polar or nonpolar.

Molecular Polarity

Many of the physical properties of compounds are the result of the polarity of the molecule as a whole. To determine the polaroti of a molecule, we will have to consider the polar bonds and their location.

Polarity in Diatomic Molecules

If there is only one bond in the molecule, the bond polarity determines the molecular polarity. Any diatomic molecule in which the two atoms are the same element must be a nonpolar molecule. Examples are I₂, O₂, and H₂.

A diatomic molecule that consists of a polar covalent bond, such as Hydrogen fluoride (HF), is a polar molecule.

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 like Hydrogen fluoride is pictured below (see figure below).

Figure \(\PageIndex{1}\): A molecular dipole results from the unequal distribution of electron density throughout a molecule.

Polarity on Molecules with a Central Atom

For molecules with more than two atoms, the molecular geometry must also be taken into account when determining if the molecule is polar or nonpolar. Figure (\PageIndex{2}\) shows a comparison between carbon dioxide and water. Carbon dioxide (CO₂) 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. However, 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. CO₂ is a nonpolar molecule. In H₂O, the orientation of the two O–H bonds is bent: one end of the molecule has a partial positive charge, and the other end has a partial negative charge. In short, the molecule itself is polar. 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.

Figure \(\PageIndex{2}\): Physical Properties and Polarity. The physical properties of water and carbon dioxide are affected by their polarities.

Similarly, in BF₃ (planar), the effect of a B-F bond is cancelled by the sum of the other two B-F bonds. Hence, a planar molecule (BF₃) is nonpolar because the bond polarities cancel each other, but a pyramidal molecule(NH₃) is polar.

Figure \(\PageIndex{3}\): The molecular geometry of a molecule (planar vs. pyramid) 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.

Figure \(\PageIndex{4}\):The same molecular geometry but peripheral bonds are of different electronegativity. CCl₄ is nonpolar but CH₃Cl is polar.

Perspective on Symmetry

Nonpolar compounds will be symmetric, meaning all of the sides around the central atom are identical - bonded to the same element with no unshared pairs of electrons. Notice that a tetrahedral molecule such as \(\ce{CCl_4}\) is nonpolar Figure (\(\PageIndex{1}\). Another non polar molecule shown below is boron trifluoride, BF₃. BF₃ is a trigonal planar molecule, and all three peripheral atoms are the same.

Figure \(\PageIndex{5}\) Some examples of nonpolar molecules based on molecular geometry (BF₃ and CCl₄).

Polar molecules are asymmetric, either containing lone pairs of electrons on a central atom or having atoms with different electronegativities bonded. This works pretty well - as long as you can visualize the molecular geometry. That's the hard part.

To know how the bonds are oriented in space, you have to have a strong grasp of Lewis structures and VSEPR theory. Assuming you do, you can look at the structure of each one and decide if it is polar or not - whether or not you know the individual atom electronegativity. This is because you know that all bonds between dissimilar elements are polar, and in these particular examples, it doesn't matter which direction the dipole moment vectors are pointing (out or in).

Three other polar molecules are shown below with the arrows pointing to the more electron-dense atoms. Just like the water molecule, none of the bond moments cancel out.

Figure \(\PageIndex{6}\) Some examples of polar molecules based on molecular geometry (HCl, NH₃ and CH₃Cl).

To summarize,

To be polar, a molecule must:

1. Contain at least one polar covalent bond.
2. Have a molecular structure such that the sum of the vectors of each bond dipole moment does not cancel - Assymetric molecule.

Steps to Identify Polar Molecules

1. Draw the Lewis structure
2. Figure out the geometry (using VSEPR theory)
3. Visualize or draw the geometry
4. Find the net dipole moment (you don't have to actually do calculations if you can visualize it)
5. If the net dipole moment is zero, it is non-polar. Otherwise, it is polar.

Example \(\PageIndex{1}\):

Label each of the following as polar or nonpolar.

1. Water, H₂O:
2. Methanol, CH₃OH:
3. Hydrogen Cyanide, HCN:
4. Oxygen, O₂:
5. Propane, C₃H₈:

Solution

1. Water is polar. Any molecule with lone pairs of electrons around the central atom is polar.
2. Methanol is polar. This is not a symmetric molecule. The \(\ce{-OH}\) side is different from the other 3 \(\ce{-H}\) sides.
3. Hydrogen cyanide is polar. The molecule is not symmetric. The nitrogen and hydrogen have different electronegativities, creating an uneven pull on the electrons.
4. Oxygen is nonpolar. The molecule is symmetric. The two oxygen atoms pull on the electrons by exactly the same amount.
5. Propane is nonpolar, because it is symmetric, with \(\ce{H}\) atoms bonded to every side around the central atoms and no unshared pairs of electrons.

Exercise \(\PageIndex{1}\)

Label the following as polar or nonpolar.

NH₃

Answer

polar

Summary

• Nonpolar molecules are symmetric.
• Polar molecules are asymmetric, either containing lone pairs of electrons on a central atom or having atoms with different electronegativities bonded.