Why are London dispersion forces present in all molecules?

London dispersion forces are present in all molecules because they arise from temporary fluctuations in the electron distribution around atoms.

London dispersion forces, also known as van der Waals forces, are a type of intermolecular force that exist between all atoms and molecules. They are named after the German physicist Fritz London who first proposed their existence in 1930. These forces are the weakest of all the intermolecular forces, but they are universal and affect all atoms and molecules to some degree.

The origin of London dispersion forces lies in the quantum mechanical nature of electrons. Electrons in an atom or molecule are not stationary, but constantly move around the nucleus. This movement creates a fluctuating electron cloud, which at any given moment may be unevenly distributed around the atom. This temporary uneven distribution of electrons creates a temporary dipole, with a slightly negative charge on one side of the atom and a slightly positive charge on the other.

This temporary dipole can induce a similar temporary dipole in a neighbouring atom or molecule, causing the two to be attracted to each other. This is the London dispersion force. The strength of the London dispersion force increases with the size and shape of the atoms or molecules involved, as larger and more complex structures have more electrons and thus greater fluctuations in their electron clouds.

Even in molecules that have permanent dipoles, such as water, London dispersion forces still exist. They add to the other intermolecular forces, such as dipole-dipole forces and hydrogen bonding, to determine the overall properties of the substance. For example, they contribute to the boiling and melting points, viscosity, and surface tension of liquids.

In summary, London dispersion forces are present in all molecules because they are a fundamental consequence of the quantum mechanical nature of electrons. They arise from temporary fluctuations in the electron distribution around atoms, and they affect the physical properties of all substances.