Definition of Intermolecular Forces (Dipole-Dipole, Ion-Dipole, London, and P. Hydrogen)

Candela Rocío Barbisan | Nov. 2021
Chemical engineer

Basically, there are three intermolecular forces that are the most typical and the ones that we will address in this section. Now, why is the study of these types of forces interesting? Well, because it allows predicting some chemical properties such as Boiling Points and Melting Points.

Suppose we have the following compounds MgO, NO₂, HF and F₂ and we must sort them by increasing Boiling Point. We know that as the strength from attraction between them, we must deliver more Energy to break the links. Therefore, we must understand what are the forces that interact.

In the case of MgO, it is an ionic compound, so the forces that hold it together are electrostatic, the most intense of all, therefore, it will have the highest Boiling Point. Then, if we analyze NO versus HF and F

Based on this analysis, it is known that the highest Boiling Point will be MgO, followed by HF, then NO ₂ and finally F₂.

London forces

Also known as dispersion forces, they exist in all molecular compounds. However, in polar molecules they lose importance due to the existence of dipoles that will cause other more relevant forces to exist. Therefore, in apolar molecules they are the only forces present.

The greater the molar mass, the greater the London Forces. In turn, nonpolar molecules form transient or temporary dipoles, that is, the electronic cloud is deformed by the continuum movement of its electrons. The larger that electronic cloud and the more polarizable it is, the greater the interacting London Forces.

Typical example are diatomic compounds such as Cl₂ where there is symmetry in the structure added to the fact that the two atoms that form it have the same electronegativity, therefore, the bond is apolar and the molecule is also apolar. In the case of CO₂, the predominant forces are also Scattering Forces; however, we observe polar bonds that, given the symmetric structure of the molecule, cancel their dipoles, forming an apolar molecule.

Dipole-dipole forces

When the molecules do not show symmetry and permanent dipoles are generated, it is said that the molecule is polar or that its dipole moment is not zero. This implies the presence of dipole-dipole forces that generate attractions between the charged ends of the molecules, the end with positive electron density of one molecule and the end with negative electron density of another molecule. Of course, when working with electron densities these forces are more intense than London's forces, which, as we said, are present in all molecules.

Typical examples are H molecules₂S and HBr where, due to their geometry, areas with negative charge densities strongly interact with positively charged densities of another molecule.

Hydrogen Bridging Forces

This type of force refers to a specific case of dipole-dipole forces that are the bonds between Hydrogen with Fluorine, Nitrogen or Oxygen. They are forces product of dipoles between the mentioned atoms that bind strongly and, therefore, are designates with a particular name, since they are of greater intensity than any other force dipole-dipole. Such is the case of water molecules (H₂O) or ammonia (NH₃).

Ion - dipole forces

It is the last type of intermolecular force that we will see and it occurs in cases where an ion participates in a compound. This interaction will then occur between the ion and the dipoles of a polar molecule, for example, in the dissolution from you go out in water, as MgCl₂ in water. The permanent dipoles of the polar molecules of the water interaction with the ionic species dissolved Mg⁺² and Cl^-.

It should be noted that these types of forces seen are weaker than covalent bonds and ionic bonds, present in covalent solids and ionic compounds respectively.