What are the Van der Waals forces?

The Van der Waals force is a set of attractive forces short range exist between all atoms and molecules. They are named after the Dutch scientist Johannes Diderik Van der Waals.

These forces are relatively weak, but they appear on the entire contact surface between two atoms, between two molecules or between two surfaces, so if this contact surface is large, the Van der Waals forces can be considerable.

Understanding Van der Waals forces enables qualitative prediction of many physical properties of chemicals, such as their melting and boiling points and solubility in different types of solvents. These are the forces that allow us to explain how it is possible that a monatomic gas such as helium, which does not form any type of chemical bond , can condense and pass to the liquid state .

These forces also explain why a substance like water, which is made up of molecules with a molar mass of just 18 g / mol, is liquid, while chlorine, which weighs almost 4 times more, is a gas at room temperature.

In addition to representing a type of attractive force between atoms and molecules, Van der Waals forces do not share many characteristics with chemical bonds . The most outstanding characteristics of this type of force are presented below:

  • They are distance dependent forces. In the simplest case, they decrease with the square of the distance. Furthermore, they strongly depend on molecular geometry.
  • Compared to the attractive forces that occur in ionic, covalent, and metallic bonds, the Van der Waals forces are relatively weak.
  • Except for dipole-dipole interactions, they are temperature independent interactions.
  • They are short-range forces, which means they decay rapidly as atoms and molecules move away.
  • They are general forces that occur between any atom and any molecule.
  • Unlike covalent bonds, they have no directionality. This means that, regardless of the orientation with which two molecules approach, if they get close enough, the Van der Waals forces will always appear.
  • When two molecules or atoms get too close, the force becomes repulsive, preventing the molecules from collapsing with each other.
  • They are additive, which means that, despite being considerably weak, the existence of many simultaneous interactions can make the total force of attraction considerable.

Types of Van der Waals forces

In addition to the repulsive component that ensures that atoms and molecules do not collapse, there are three types of Van der Waals forces, which may or may not act between two molecules or atoms at the same time. These are presented below:

1. Dipole-dipole interactions or Keesom forces

The Keesom force is one of the contributors to the Van der Waals force. It is about the electrostatic interaction forces that appear between the permanent dipoles of polar molecules such as water, acetone and others.

Keesom forces are similar to electrostatic forces between ions, but differ from them in that they are much weaker and decay with the sixth power of the distance (that is, with r 6 ). The reason they are weaker is that the attraction occurs between the partial charges of a dipole, rather than between entire electric charges as in the case of ions.

These interactions only occur between molecules and not between atoms, since atoms do not have permanent dipole moments. Furthermore, the Keesom force is inversely proportional to temperature, which means that when temperature increases, the Keesom force decreases.

Examples of molecules exhibiting dipole-dipole interactions

All polar molecules have dipole-dipole interactions with each other. For instance:

  • Two molecules of acetone.
  • Two chloroform molecules.
  • One molecule of cyclohexanone with one molecule of acetone.

2. Induced dipole-dipole interactions or Debye forces

When a polar molecule approaches an atom or an apolar molecule, the dipole of the first attracts or repels electrons on the surface of the second. This induces a dipole in the apolar molecule, which is called an induced dipole. The newly formed dipole then interacts with the original dipole creating a small attractive force.

These interactions between a permanent and an induced dipole are often known as Debye forces, and they depend not only on the distance between the two molecules or between the polar molecule and an individual atom, but they also depend on the polarizability of the apolar molecule. and the magnitude of the dipole moment of the polar molecule.

Examples of molecules exhibiting induced dipole-dipole interactions

This kind of interaction only occurs between different molecules, since one needs to be polar and the other not. Some examples are:

  • An atom of a noble gas such as argon and a polar compound such as hydrogen chloride. In this case, the positive partial charge on the hydrogen of the HCl attracts the valence electrons of the argon, generating a small induced dipole in the latter. If an argon approaches the chlorine atom, the negative partial charge will repel the valence electrons and an induced dipole is also formed.
  • Molecular bromine (Br 2 ) is considerably soluble in water, despite being an apolar compound and water a strongly polar solvent. The explanation is due, in part, to the high polarizability of bromine that allows it to form considerably strong Debye interactions.

3. Induced dipole-induced dipole interactions or London scattering forces

Of all the Van der Waals forces, the weakest are the London dispersal forces. These interactions occur between all atoms and molecules due to the appearance of instantaneous dipoles in any part of their structure.

An instantaneous dipole is a small dipole that is formed by an asymmetric distribution of valence electrons at a given moment.

How are instantaneous dipoles generated?

As the electrons are rotating around the nucleus, it can happen that at one time there are more electrons on one side than on the other, which gives rise to a dipole. This small dipole quickly disappears as the electrons continue to move around the nucleus, which is why they are called instantaneous.

Instantaneous dipoles can arise anywhere in the structure of a molecule, and at any given time there can be many of them. London dispersion forces consist of the interaction between these instantaneous moments in one molecule and those of another.

Examples of compounds exhibiting London dispersion forces

As mentioned, these types of interactions occur between all atoms and molecules. However, it is in large apolar molecules where its effect can be best observed, since in these cases it is the only type of intermolecular interaction that occurs.

A typical example where these forces are very strong is in the case of long chain aliphatic hydrocarbons, especially polymers such as polyethylene.

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