Dipole-Dipole Interactions

Molecules with permanent dipoles (polar molecules), attract each other, this pushes up the boiling points.

In a cloud of molecules which aren't polar, as they move around due to diffusion, there will be very small changes in the electric fields as the molecules experience slight attractions and repulsions. This results in temporary attractions between the particles.

 The greater the number of electrons in a molecules the stronger these forces become, this explains why the boiling point increases with atomic mass in the alkanes and Group VII.