The halogens are the elements in group 7. Their atoms join in pairs to form molecules, such as Cl2, Br2 and I2. At room temperature they show a clear pattern in their appearance, and all of them are toxic.
Chlorine is a pale green gas. Bromine is a red-brown liquid that gives off an orange-brown vapour. Iodine is a grey solid, and when it is heated it gives off a purple vapour. So going down the group from chlorine to iodine the state changes from gas to liquid to solid, and the colour gets darker.