halogen

Any of a group of five non-metallic elements with similar chemical bonding properties: fluorine, chlorine, bromine, iodine, and astatine. They form a linked group (Group 7) in the periodic table, descending from fluorine, the most reactive, to astatine, the least reactive. They all have coloured vapours and are poisonous. Melting points and boiling points increase going down the group. They combine directly with most metals to form salts, such as common salt (NaCl). Each halogen has seven electrons in its valence shell, which accounts for the chemical similarities displayed by the group.

Fluorine and chlorine are gases, bromine is a liquid, and astatine is a solid. The colour of the element darkens going down the group from fluorine, which is pale yellow, to astatine, which is black.