Intermolecular forces

Intermolecular forces

Molecular covalent compounds do not contain covalent bonds. Instead, the molecules are attracted to each other through intermolecular forces. There are three main types:

  • Van der Waal’s forces
  • dipole-dipole bonding
  • hydrogen bonding


Van der Waal’s forces

In a molecule the electrons are constantly in motion. Therefore, the distribution of electrons is never likely to be exactly symmetrical. Instead, there will probably be a slight surplus on one of the atoms. This is called temporary dipole. This state comes and goes due to the constant motion of the electrons.

Adjacent molecules are repelled by the negative section of the dipole and so move accordingly. This is called induced dipole.

The attraction between these molecules is known as a Van der Waal’s force. All molecules are subject to these forces, even if only weak. In fact, these are why it is possible to liquefy and solidify all compounds.

Van der Wall’s forces are usually between 1 kJmol-1 and 50 kJmol-1. The strength of these forces depends on:

  • The number of electrons within the molecule: the higher the number of electrons the higher the frequency and strength of the temporary dipoles. Such substances have higher melting and boiling points. For example, noble gases and alkanes.
  • The surface area of the molecules: the greater a molecule’s surface area the more contact it will have with adjacent molecules and so the greater its ability to induce a dipole. Such substances have higher melting and boiling points. For example, methylpropane and butane.


Dipole-dipole bonding

In some molecules permanent dipole exists. A polar bond and a dipole can result from the difference in electronegativity between atoms which are covalently bonded. However, a lot of the time a permanent dipole does not result from the present of dipole (polar bonds). This is because other dipoles within the molecule cancel the effect of each other out. This is apparent in tetrahedral, trigonal planar, and linear substances.

Molecules with no overall dipole are known as non-polar. They either contain no polar bonds or the dipoles which result from the polar bonds are cancelled out by each other. Only Van der Waal’s forces or temporary-induced dipole attractions occur in such molecules.

A molecule in which the dipoles resulting from polar bonds are not cancelled out has a permanent dipole. They are known as polar molecules. They are attracted to each other by both Van der Waal’s forces and dipole-dipole bonding (an attraction between two permanent dipoles on different molecules).

Dipole-dipole bonding strengthens intermolecular bonding slightly thereby increasing the melting and boiling points of a compound.


Hydrogen bonding

Hydrogen bonding is where a hydrogen atom forms a very strong intermolecular dipole-dipole bond with electronegative atoms on adjacent molecules. In other words, it is the attraction between an electropositive hydrogen atom and an electronegative atom located on an adjacent molecule. It is possible for a hydrogen atom to do this because around its nucleus it has basically no electron density making it very small. However, this bonding is only possible if the hydrogen atom has a bond with an element which is very electronegative. Examples include alcohols, acids and amines.

Hydrogen bonding affects the physical properties of substances in a number of ways:

  • Melting and boiling points: unlike normal dipole-dipole bonds, hydrogen bonding has a very large effects on a substance’s melting and boiling points. For example, H2O, NH3 and HF have extremely high boiling points because of this type of bond even though the Van der Waal’s forces are weaker in these hydrides than others.
  • Low density of ice: hydrogen bonding between molecules creates a hexagonal structure which is very open and consists of large spaces. This is why ice is so dense and floats on water.
  • DNA’s helical structure: the N-H and C=O bonds in DNA molecules allows for hydrogen bonding. This is what causes the molecule’s spiralling structure as both bonds move in towards each other. This type of hydrogen bond is intramolecular as opposed to intermolecular because the attraction is occurring between hydrogen and electronegative atoms in the same molecule and not adjacent molecules.