The group 17 elements: the halogens

The elements

The Group 17 elements have a particular name: the halogens meaning born of salt . This is due to the formation of salts when they form compounds with a metal. [link] lists the derivation of the names of the halogens.

Discovery

Fluorine

The mineral fluorspar (also known as fluorite ) consists mainly of calcium fluoride and was described in 1530 by Agricola ( [link] ) for its use as a flux. Fluxes are used to promote the fusion of metals or minerals, and it was from this use that fluorine derived its name. In 1670 Heinrich Schwanhard found that when he mixed fluorspar with an acid the fumes produced (hydrogen fluoride) etched the glasses he was wearing. Despite many researchers investigating the chemistry of hydrogen fluoride (HF) the elemental form of fluorine was not isolated until 1886 when Henri Moissan ( [link] ) studied the electrolysis of a solution of potassium hydrogen difluoride (KHF ₂ ) in liquid hydrogen fluoride (HF). The mixture was needed because hydrogen fluoride is a non-conductor. The device was built with platinum/iridium electrodes in a platinum holder and the apparatus was cooled to −50 °C.

The generation of elemental fluorine from hydrofluoric acid proved to be exceptionally dangerous, killing or blinding several scientists who attempted early experiments on this halogen. The victims became known as fluorine martyrs .

Chlorine

Archaeological evidence has shown that sodium chloride (known as table salt ) has been used as early as 3000 BC and brine (the saturated water solution) as early as 6000 BC. It is thought that hydrochloric acid was probably known to alchemist Jābir ibn Hayyān ( [link] ) around 800 AD, while aqua regia (a mixture of nitric acid and hydrochloric acid) began to be used to dissolve gold sometime before 1400 AD. Upon dissolving gold in aqua regia, chlorine gas is released along with other nauseating and irritating gases.

Chlorine was first prepared and studied in 1774 by Carl Wilhelm Scheele ( [link] ), and therefore he is credited for its discovery despite his failing to establish chlorine as an element, mistakenly thinking that it was the oxide obtained from the hydrochloric acid. Regardless of what he believed, Scheele did isolate chlorine by reacting MnO ₂ (as the mineral pyrolusite) with HCl, [link] .