The halogens are five non-metallic elements. Found in Group 17 (also known as Group VIIA in the older system) of the periodic table, these elements are among the most useful to modern life. The name "halogen" means "salt-former," derived from the halogens' tendency to bond with other elements to create many of the most common salts.
There are five halogen elements: fluorine (F, atomic number 9), chlorine (Cl, atomic number 17), bromine (Br, atomic number 35), iodine (I, atomic number 53) and astatine (At, atomic number 85). The currently-undiscovered element that would have an atomic number of 117 is also a potential halogen.
Atoms of the halogens share many characteristics, but they differ in size. Fluorine has the smallest atom, with a mass of only 18.998 atomic masses. Going down the group, the atoms of each element get more massive. Chlorine atoms are 35.5 atomic masses, bromine is 79.9, iodine is 126.9 and astatine is about 210 atomic masses. Astatine is so massive, in fact, that it has an unstable and radioactive atom.
The principal common feature of the halogens is that each element has an outer electron shell with seven electrons. Since a full electron shell needs eight electrons, each of these elements needs only one additional electron to fill the shell. Such a need means that all halogens are extremely reactive. Halogens can react with metallic ions to form ionic salts (like NaCl, table salt), with hydrogen to form strong acids (including HF, hydrofluoric acid) or with other atoms of the same element to form diatomic molecules (such as Cl2, chlorine gas).
Halogens are notable as being the only group on the periodic table in which the elements exist in all three states of matter at room temperature. Fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids.
Halogens have a wide variety of uses in modern life. Teflon is made by bonding fluorine to carbon, creating a solid surface that does not react with other materials. Teflon coatings are found on cooking surfaces and in electronics. Chlorine, bromine and iodine are all used as disinfectants, while chlorine is also particularly effective as a bleach. Halogen lamps are incandescent lamps with a small amount of a halogen included. The addition of the halogen allows the filament to last much longer and to burn more efficiently.
Due to their high reactivity, all halogens are potentially dangerous, especially if they have been isolated by chemical processes. Fluorine is especially problematic, as the element will react with most other materials. Even storage materials like glass can react with fluorine and create dangerous results. While the other halogens are less reactive, they are still extremely dangerous. Chlorine gas is particularly toxic in high concentrations.