Mastering intermolecular forces at 14–16

Molecules are one of the key particles in chemistry. An understanding of molecules helps us explain much of the material and natural world around us. 

At 14–16, students need to understand three main points about molecules.

• Covalently bonded substances can form small molecules.
• Weak intermolecular forces exist between molecules.
• The bigger the molecule, the stronger the intermolecular forces.

These points may seem simple enough, but students often struggle to visualise what molecules are and how they are different from giant structures. We also introduce the idea of intermolecular forces often without any explanation on how they form.

Clearing up misconceptions

Temporary dipoles

Intermolecular forces is a catchall term for several different types of attraction between particles. We discuss London forces at 14–16, which can be described as transient, induced dipole–dipole interactions. They form because small fluctuations in the electron charge cloud of a molecule cause small temporary areas of negative or positive charge to form. These are temporary dipoles, also called transient or instantaneous dipoles. A temporary dipole in one molecule can induce a dipole in a neighbouring molecule and the two dipoles can attract each other.

The bigger the molecule, the more dipoles will form, and so the stronger the attractive force. This explains why halogens have increasing melting points down the group. The forces can be large enough to make a substance a solid at room temperature, such as iodine. It also explains the idea that the bigger the alkane, the higher the boiling points and viscosity.

When discussing intermolecular forces with 14–16 students, it’s important to reinforce the idea that all bonding is electrostatic in nature and that London forces are just the weakest example. All molecules have these forces but, at 16–18, students are introduced to other types of intermolecular forces. This can help explain properties like anomalously high melting points of some molecules and differences in solubility in water.

Permanent dipoles and polar molecules

In a covalent bond the electrons are not always shared equally. All atoms of elements attract the bonding electrons more or less strongly, in a sort of electron tug of war. We call this property electronegativity – the stronger the pull, the higher the electronegativity. Elements such as fluorine, oxygen, nitrogen and chlorine have high electronegativities. Atoms of these elements in molecules therefore tend to bear a small negative charge, with other bonded atoms bearing a small positive charge. This charge separation is permanent, so we call this a permanent dipole.

Permanent dipoles in one molecule can attract the dipole in a neighbouring molecule. These permanent dipole–dipole interactions are stronger than London forces, and occur in addition to London forces. Substances with permanent dipoles tend to have higher boiling points than substances with only London forces between the molecules.

Molecules with permanent dipoles are often called polar molecules, whereas ones that only form temporary dipoles are called nonpolar. The polar nature of substances can affect both the physical and chemical properties of a substance. The existence of polar molecules is a useful insight for students into the continuum of bonding between ionic and covalent.

Hydrogen bonding

When atoms of the three most electronegative elements (nitrogen, oxygen and fluorine) bond to hydrogen atoms, a particularly strong dipole forms. The lone pair of electrons on these electronegative atoms are attracted to the positive hydrogen atoms in a neighbouring molecule. The small size of hydrogen atoms allows a close approach of the two atoms, which makes the force of attraction stronger. This is hydrogen bonding of the intermolecular forces. Compounds that exhibit hydrogen bonding, like water and ammonia, have anomalously high boiling points compared to those that do not exhibit this type of bonding.

Molecules with significant hydrogen bonding, such as ammonia and ethanol, can dissolve in water because they can form hydrogen bonds with the water molecules. Nonpolar molecules such as hexane will not be able to form hydrogen bonds with water and so will be immiscible in water.