The halogens and their state of matter
The halogens - fluorine, chlorine, bromine, and iodine - are in group 17 of the periodic table and are all diatomic molecules, X2, at room temperature. Since they are diatomic, they make a good model system of van der Waals dispersion forces.
As we go down in the periodic table, the number of electron shells in an atom increases, and so the atomic size also increases. Larger atoms have larger and wobblier electron clouds which produce stronger temporary dipoles.
As a result, the largest of these halogens - iodine - produces the strongest temporary dipole-dipole force. The strength of the force causes the atoms to pack very closely together, which is why iodine is solid at room temperature.
Figure 1: The properties of the halogens shown. From left to right: fluorine is a yellow gas, chlorine is a green gas, bromine is a red-brown liquid and iodine is a purple solid.