The halogens are the elements astatine, fluorine, chlorine, bromine, and iodine. They are non-metals, and make up part of the seventh group in the periodic table.

The halogens all have a strong unpleasant odor and will burn flesh. They do not dissolve well in water.

The five elements are strongly electronegative. They are oxidizing agents, with fluorine being the strongest and astatine being the weakest. They react with most metals and many non-metals.

As the atomic structure of the halogens becomes more complex with increasing atomic weight there is a gradiation in physical properties. For example: Fluorine is a pale green gas of low density. Chlorine is a greenish-yellow gas 1.892 times as dense as fluorine. Bromine is a deep reddish-brown liquid which is three times as dense as water. Iodine is a grayish-black crystalline solid with a metallic appearance. And astatine is a solid with properties which indicate that it is somewhat metallic in character.