London dispersion forces are the weak intermolecular forces that occur when the molecules are in close proximity to each other. The strength of these forces is determined by the atomic size of the element.
In smaller atoms, the atomic radius is smaller, which means that the positively charged nucleus holds the outermost valence electrons more firmly, leaving less room for their dispersion or polarization.
As the size of an atom increases, the number of shells also increases, and the outermost electron moves farther away from the nucleus. This results in a larger atomic size. The nuclear attraction is low in larger atoms, which causes the electrons to disperse and create significant temporary dipoles in molecules with larger atomic sizes. As a result, the molecules interact strongly, leading to clusters of molecules getting closer. These stronger interactions result in higher melting and boiling points, which means that more energy is required to break the forces of attraction between the molecules.
For instance, I2 molecules have a larger size and significant London forces between them, which makes their grouping strongest among the halogens. As a result, I2 molecules are solid at room temperature and have higher melting points than F2 and Cl2 molecules.
Another example is benzene (C6H6), naphthalene (C10H8), and anthracene (C14H10), with one, two, and three fused aromatic rings. Their melting point increases with the increase in the number of fused rings.
Melting Points
Benzene(C6H6) naphthalene (C10H8) anthracene (C14H10)
5.5°C 80.2°C 215°C.
Related Reading: London Dispersion Force