London dispersion forces (also known as van der Waals forces) are temporary, weak attractive forces that occur between all molecules, including nonpolar molecules like pentane. They arise from the instantaneous fluctuations in the electron distribution of molecules. As the electrons move about the molecule, they create temporary dipoles, which can induce dipoles in neighboring molecules. These temporary dipoles can then attract each other, creating a net attractive force between the molecules.
In pentane, the London dispersion forces are the only significant intermolecular forces present. This is because the carbon-carbon and carbon-hydrogen bonds in pentane are nonpolar, meaning that they do not have a permanent positive or negative charge. As a result, pentane molecules are relatively weakly attracted to each other, and they therefore have a low boiling point (-36.1 °C) and a low melting point (-129.8 °C).
