Final answer:
The strength of London dispersion forces, which determines the melting and boiling points of substances, increases with the size and polarizability of molecules, leading to higher boiling points for larger molecules.
Step-by-step explanation:
The increase in melting and boiling points with increasing atomic/molecular size is due to the corresponding increase in the strength of London dispersion forces. The polarizability of a molecule determines how easily an electrostatic charge can distort a molecule's charge distribution, or electron cloud. Highly polarizable molecules have larger dispersion forces, which contribute to higher melting and boiling points. As molecules increase in size, they tend to have more electrons that are further from the nucleus and more easily polarizable, strengthening the dispersion forces between molecules.
For example, n-pentane, with a larger surface area for intermolecular interactions, has a higher boiling point compared to neopentane, which has a smaller surface area and is less polarizable. Thus, the relationship between polarizability, molecular size, and boiling points is evident in the comparison of these compounds with the same molecular formula (C5H12).