The halogens are the elements in group 7. Their physical properties change in a regular way down the group. Chlorine is a pale green gas at room temperature, bromine is a red-brown liquid and iodine is a dark grey solid. So the colour of the halogens gets darker and the state changes from gas to liquid to solid as you go down the group.
The melting points and boiling points increase down the group. The molecules get bigger, so there are stronger forces between them and more energy is needed to separate them. That is why chlorine boils at a very low temperature and iodine is a solid.
This pattern lets us predict the properties of the other halogens. Fluorine is above chlorine, so it should be even paler (it is a pale yellow gas) with lower melting and boiling points than chlorine. Astatine is below iodine, so it should be a black solid with higher melting and boiling points than iodine.