Intermolecular forces (IMFs) are one of two kinds of forces that take place in and around a molecule. They come in many forms, giving us insight into how molecules interact with each other as well as what chemical properties a substance may have.
What Are IMFs?
Molecules are operated by two forces of attractions:
- Intramolecular Forces: These are the forces that happen inside a single molecule. They operate between atoms, binding them to hold the molecule together.
- Intermolecular Forces: These are the forces that happen between molecules. They mediate how molecules interact with each other.
Intramolecular forces are more easily recognised as chemical bonds. Ionic bonds, covalent bonds and metallic bonds are all examples of intramolecular forces at work within a molecule.
Intermolecular forces are much weaker than the forces inside a molecule because they aren’t involved in the transfer or sharing of electrons. Intramolecular forces, on the other hand, create their bonds by sharing electrons. As a result, each element in the bond has an octet of electrons in the valence shell, forming an incredibly stable arrangement. Without this configuration, intermolecular forces are much weaker as less energy is required to break them apart.
While the intermolecular forces between molecules may be comparatively weak, they are still very important. They help to determine the properties of a substance, most commonly the melting and boiling points. IMFs can also shed light on how molecules interact with each other.
Types of Intermolecular Forces
There are several different types of intermolecular forces that operate between molecules. The five main ones covered in this article can be categorised into dipole-dipole interactions and Van der Waals forces.
As one of the strongest intermolecular forces of attraction, these occur between molecules that have partially charged ions. When two polar molecules are close together, the positive molecule interacts with its negative neighbour and a dipole-dipole interaction takes place.
Hydrogen chloride (HCl) experiences a dipole-dipole interaction because its polar covalent bonds contain atoms with different electronegativities. The unequal sharing of electrons between these atoms results in the chlorine atom having a partial negative charge and the hydrogen atom having a partial positive charge.
In this dipole-dipole interaction, the positively charged hydrogen atom of one molecule is attracted to the negatively charged chlorine atom in the neighbouring molecule.
Since most molecules are polar, a dipole-dipole interaction is the most common type of intermolecular force. It also appears in compounds like sulphur dioxide (SO2) and chloroform (CHCl3).
This is a strong, electrostatic form of a dipole-dipole interaction. It happens when a hydrogen atom in one molecule binds to one of three highly electronegative atoms in another molecule: nitrogen, oxygen or fluorine.
Hydrogen bonding is just as strong as a dipole-dipole interaction and a considerable amount of energy is required to break the attraction force between molecules. In fact, hydrogen bonds are what hold DNA together.
The strength of these bonds is also why substances that undergo hydrogen bonding, like water (H2O) or hydrogen fluoride (HF), have extremely high melting and boiling points. This tells us how the behaviour of different intermolecular forces impacts the properties of a compound.
When an ion encounters a polar molecule, it is called an ion-dipole interaction. This IMF is a form of dipole-dipole interaction that involves ions instead of polar molecules. This makes it the strongest intermolecular force because the charge of an ion is always much greater than the charge of a dipole.
The part of the molecule that attracts or repels the ion is determined by the charge of the ion. For example, a cation is a positively charged ion and so would be attracted to the negative portion of a molecule while being repelled by the positive portion. The opposite process happens with anions, which are negatively charged ions.
An ion-dipole interaction can be seen when sodium chloride (NaCl) is dissolved in water. When this happens, the compound separates into Na+ cations and Cl- anions. While the Na+ ions are attracted to the negative dipole of the oxygen atom in water, the Cl- ions have an ion-dipole interaction with the positive dipole of the hydrogen atom. Ion-dipole interactions also help to trigger the deionisation process.
Van de Waals Intermolecular Forces
Some of the weakest IMFs are the Van der Waals forces. Named after Johannes Diderik van de Waals, a Dutch scientist, these are distant-dependent intermolecular forces that take place between uncharged atoms or molecules.
The Van der Waals forces are only significant in atoms and molecules that have no other types of attractions, like non-polar molecules or Group 0 elements. This form of IMF is used to explain:
- The universal attraction between bodies
- The physical absorption of gases
- The cohesion of condensed phases
Van der Waals forces include the Keesom interaction, the Debye force and, most notably, the London Dispersion forces.
London Dispersion Forces
London dispersion forces (LDFs) may belong to the Van der Waals category of intermolecular forces, but it was actually Fritz London who first suggested how these types of IMFs may arise. This is where the LDF gets its name from.
LDFs are the only intermolecular forces present in materials that are entirely made up of neutral molecules, i.e. materials without polar dipole molecules. These include the noble gases, like helium, xenon, argon and neon.
While London dispersion forces are the weakest of all IMFs, they are the only forces capable of holding gas molecules together. Therefore, LDFs are responsible for condensing gases into liquids and play an important role in the physical properties of these materials.
London dispersion forces occur when the electrons in two neighbouring molecules form a temporary dipole. This happens when the electrons in a neutral molecule have an uneven distribution and gather on one side. This forms a temporary dipole and the molecule can now induce another temporary dipole in a nearby molecule.
The formation of dipoles between two neutral molecules results in a temporary attractive force, known as the London dispersion force, which continuously forms and disappears.
In helium, the London dispersion forces are particularly weak. This is shown by the fact that it has a very low boiling point (-270°C). This is the temperature at which its molecules will have enough strength to stick together and condense into a liquid.
LDFs get stronger as the size of an atom increases, and so its forces are more significant in materials with large, heavy atoms. Xenon is a good example of this as its heavier atoms allow the London dispersion forces to pull its molecules together at a higher temperature. This is reflected in the high boiling point (-108°C) that xenon has in comparison to helium, and shows how LDFs affect the physical properties of many substances.
Intermolecular forces are crucial to how molecules interact with each other and how chemical properties are determined. Every substance is experiencing IMFs, even the chemicals in our online store!
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