THE HISTORY OF AN IDEA · 1774–1940
A family that resists being isolated
Chlorine, iodine, bromine and fluorine were all recognized in compounds long before they could be separated from them, and fluorine took more than a century.
Explore the supporting references at the foot of this article.
01
A green gas that was not an oxide
Scheele obtained a choking green-yellow gas in 1774 by treating a manganese with acid, and described it accurately as a gas that bleached, dissolved in water and attacked metals. He did not call it an element: the prevailing framework treated it as a compound of an acid with oxygen, and it kept that reading for over thirty years until Davy’s repeated failures to remove oxygen from it made the simpler interpretation the better one. The substance never changed; the category did.
02
Discoveries out of manufacture
Two of the family came out of industrial processes rather than mineral analysis. Courtois, making saltpetre for gunpowder from seaweed ash, added too much acid and watched a violet vapor rise that crystallized on the cold surfaces above — iodine, named for the color. Balard found bromine in the mother liquors left after salt crystallized from Mediterranean brine. Both were working with the concentrated residues of an industry, which is where a minor constituent of seawater becomes findable.
03
The element that destroyed its own apparatus
Fluorine defeated everyone for a century. The problem was circular: it is the most reactive element, so anything used to free it — reagent, , vessel — tends to be consumed by the product, and several chemists were poisoned or killed trying. Moissan succeeded in 1886 by electrolysing hydrogen fluoride containing potassium fluoride at low temperature in a platinum–iridium apparatus with fluorspar windows, a solution in which the choice of materials was as much the discovery as the chemistry. Astatine, the last member, has no stable and was made in 1940 by bombardment rather than found.
What followed
Every member of the family is now made at scale or, in astatine’s case, made atom by atom for research; the chemistry that made them hard to isolate is the same chemistry that makes them useful in water treatment, refrigeration, and pharmaceuticals.
RSC Periodic Table — ChlorineLANL Periodic Table — ChlorineRSC Periodic Table — ChlorineLANL Periodic Table — ChlorineUSGS — Tin
Sources & further reading
- RSC Periodic Table — Chlorine ↗Royal Society of ChemistryReference explanation · Source checked
- LANL Periodic Table — Chlorine ↗Los Alamos National LaboratoryReference explanation · Source checked
- RSC Periodic Table — Chlorine ↗Royal Society of ChemistryReference explanation · Source checked
- LANL Periodic Table — Chlorine ↗Los Alamos National LaboratoryReference explanation · Source checked
- USGS — Tin ↗United States Geological SurveyReference explanation · Source checked
Sources and further reading. What the review labels mean