Intermolecular forces

Van der walls and hydrogen bond

Intermolecular forces keep molecules together. Not to be confused with intramolecular bonding, e.g. covalent or ionic bonds, which are responsible for keeping atoms together to form molecules.

Intermolecular forces affect properties like boiling point and melting point of substances. The stronger the intermolecular forces, the higher will be boiling and melting points. Substances with very weak intermolecular forces are likely to be gases, because that is the state of matter where molecules are the furthest away ( in liquids and solids the molecules are closer to each other).

Van der Walls forces

There are 3 different intermolecular forces which, together, are named Wan der Walls forces (this sometimes causes confusion as some textbooks sometimes refers to only one or two of those as Wan der Walls forces).

1) Dipole-dipole interactions.

This is the interaction between molecules that have a permanent dipole moment, caused by the presence of atoms with different electronegativities. The electrons will tend to be located near the more electronegative atom and that constitutes a dipole. Dipoles attract each other (electrostatic force). A special type of dipole-dipole interaction is the hydrogen bond, described below.

2) Dispersion or London forces.

This is the interaction due to instantaneous dipoles that are produced in the molecules. Instantaneous dipoles arise as a result of the movement of electrons. All molecules present this kind of interaction.

Polar molecules, which have the dipole-dipole interactions described above, also interact through dispersion forces.

3) Induction (or polarization).

A polar molecule can distort the electron cloud of a neighbouring molecule and induce a dipole on it as a result. The resulting interaction between these 2 molecules is called induction or polarization.

Hydrogen bond

This is the strongest type of intermolecular force.It is a special case of the Van der Walls dipole-dipole interaction described above.

It occurs when there are any of the 3 most electronegative elements, namely F, O, N, bonded to H.

An ubiquitous substance that presents hydrogen bonds is water, where the large dipoles observed in the O-H bonds are responsible for a strong attraction between pairs of molecules. That is why water has a relatively high boiling point of 100 degrees celsius.

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