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London dispersion forces, also known as van der Waals forces or instantaneous dipole-induced dipole forces, are weak intermolecular forces present between all molecules, regardless of their polarity. These forces arise due to temporary fluctuations in electron distribution within the electron cloud of a molecule, which can create transient dipoles. These transient dipoles can then induce dipoles in neighboring molecules, resulting in attractive forces between them.

The strength of London dispersion forces is proportional to the number of electrons a molecule has since a larger electron cloud is more susceptible to temporary fluctuations in electron distribution. This means that larger molecules with more electrons will have more significant fluctuations in their electron clouds, which can create stronger temporary dipoles, leading to stronger dispersion forces.

As molecular size increases, the number of electrons within the molecule usually increases as well. This means that molecules with larger molecular sizes possess larger electron clouds and are more likely to experience fluctuations in their electron distribution. These fluctuations ultimately lead to stronger temporary dipoles and, in turn, stronger London dispersion forces between these larger molecules. In conclusion, the strength of London dispersion forces increases with increasing molecular size because larger molecules have more electrons and larger electron clouds, which are more susceptible to temporary fluctuations in electron distribution. These fluctuations produce stronger temporary dipoles and, as a result, stronger London dispersion forces between molecules.