The halogens contain some of the most reactive chemical elements, including fluorine, which is at the top of the list. Halogens are nonmetals and include gases, solids, and one of the only two liquid elements, bromine.
Because halogens are so reactive, they form very stable compounds with tight bonds. Fire extinguishers with halogen compounds are thus very effective, and teflon does not react with food.
The element fluorine is so dangerous that its pursuit may have caused the most misery and death among chemists until the study of the radioactives. Fluorine is found in the mineral called fluorspar or fluorite (calcium fluoride). Glass cutter Heinrich Schwanhard of Nuremberg in 1670 wrote that when fluorspar is treated with acid, his glasses would become etched. Chemists over the years confirmed this, and in 1771, C.W. Scheele found that the acid of fluorspar (hydrofluoric acid) was so reactive that it could hardly be purified. It tended to contain traces of glass, lead, or whatever composed its container.
Hydrofluoric acid was used routinely in the 18th Century to etch glass. Experienced chemists such as Sir Humphry Davy and Joseph Gay-Lusac harmed themselves by inhaling it. Belgian P. Louyet was poisoned and died, as was Frenchman Jérôme Nicklès. Others suffered but survived. George Gore managed a laboratory explosion. Davy, it should be noted, had isolated many other elements with electrolysis, but this acid corroded his electrodes.
Late in the 19th Century (1886), French chemist Henri Moissan had success. He used platinum-iridium containers for their inertness and screw caps carved from fluorspar. Electrolysis on a mixture of potassium biflouride and hydrofluoric acid produced the actual element.
Fluorine is a pale yellow gas on the rare occasion that it exists as a free element. It reacts vigorously, even explosively, with almost any element and has even been forced to combine with some of the noble gases.
Chlorofluorocarbons are a class of refrigerant gas with the tradename Freon. Freon was used for decades in refrigerators until it was banned in 1996 for attacking earth's ozone layer. Teflon is polytetrafluoroethylene, an inert plastic. Uranium hexafluoride is the gas used in enriching uranium for use in nuclear bombs.
Fluorine is dangerous and poisonous, but the fluoride ion settles in teeth and bones beneficially (when in low concentrations). Fluoridated water inhibits tooth decay.
In the first age of true chemistry, up until the early 19th Century, one subject of experimentation was muriatic acid, also known as hydrochloric acid (HCl). Removing the hydrogen from it left a gaseous remnant that reacted with metals, bleached green leaves, and dissolved slightly in water.
Celebrated chemists such as Antoine Lavoisier and William Henry explored various chemical dead ends with this new gas. Lavoisier's theory was that HCl and all acids must contain oxygen. Joseph Gay-Lussac and Louis Thenard failed to decompose the gas, understandable since it is an element.
Unlike the aforesaid chemists, Humphry Davy believed this gas from HCl to be a new element. His experiments ruled out the presence of oxygen. Iodine and bromine established the likelihood of such an element existing. It was eventually called chlorine.
Chlorine is useful in far too many chemical applications to be listed. It is a component of many plastics and of Freon. Being very reactive, it is used as a chemical tool to create complex molecules. In dilute form, it bleaches and sterilizes.
Bromine is discussed here.
Iodine is discussed here.
In the 20th Century, the periodic table had an obvious place for another halogen below iodine. This element proved to be radioactive and unstable. In fact, it was produced synthetically in 1940 by a team at Berkeley by bombarding bismuth with alpha particles. The name "astatine" means "unstable", and, in fact, its most stable isotope has a half-life of only 8 hours.
Astatine occurs in nature, but only in the tiniest trace amounts as fleeting products of the decay of heavier radioactive elements. Only one gram exists on all of earth at any given moment. It has never been isolated as an element in any visible amount, and so most of its physical properties are only conjectured. It may be a halogen or it may be a metalloid.