How can I tell which elements have larger London Dispersion forces?

Question

When given two atoms (ex. H2O VS. H2S) how can I tell which has the larger London dispersion forces?

Answer 1

The bigger the size of the atom the stronger the dispersion forces. The size increases as you go down the periodic tale.

However, in the specific case of H2O and H2S you have other type of forces which are dominant. H2O exhibits very strong H-bonds while H2S very weak because S is not as electronegative as O and thus H-S bonds are less polarized than H-O. Also both molecules have a net dipole moment so you also have dipole-dipole interactions. These 2 types of forces (H-bonds and dipole-dipole) will determine the properties of these molecules and not the difference in the dispersion forces because of the different size of O and S.

Foe molecules, dispersion forces also depend on the shape of the molecule. The bigger the surface available for interaction, the stronger the forces will be. Thus if the molecule has an extended shape it will have stronger dispersion forces than if it is packed.

E.g. CH3CH2CH2CH3 > CH3CH(CH3)CH3 > C(CH3)4