Intermolecular Forces

• Intermolecular forces are the forces of attraction that exists between molecules. These are weaker than intramolecular forces, which are the forces within molecules.

• There are three main types of intermolecular forces: London dispersion forces, dipole-dipole forces and hydrogen bonding.

• London dispersion forces, also known as Van der Waals’ forces, are the weakest type of intermolecular force. They exist between all molecules, irrespective of their polarity. This force results from temporary dipoles caused by the movement of electrons.

• Dipole-dipole forces are stronger than dispersion forces and occur between polar molecules. In a polar molecule, one end of the molecule has a slightly positive charge while the other end has a slightly negative charge. This induces a force of attraction between opposite charges.

• Hydrogen bonding is the strongest form of intermolecular force. Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine and is in close proximity to another electronegative atom.

• The strength of these intermolecular forces determines many physical properties of substances including their boiling and melting points, viscosity, surface tension and volatility.

• Melting and boiling points increase with the strength of the intermolecular forces. Substances with strong intermolecular forces will have higher boiling and melting points than substances with weaker forces.

• It’s important to note that while hydrogen bonding is typically strongest, other factors such as molecule size and shape can influence the net strength of intermolecular forces.

• Phase changes (solid to liquid, liquid to gas, etc.) are caused by a change in intermolecular forces. When heat is added to a substance, it can break these forces and cause a phase change.