The group of halogens is the onlyperiodic table group that contains elements in three of the mainstates of matter atstandard temperature and pressure, though not far above room temperature the same becomes true of groups1 and15, assuming white phosphorus is taken as the standard state.[n 1] All of the halogens form acids when bonded to hydrogen. Most halogens are typically produced fromminerals orsalts. The middle halogens—chlorine, bromine, and iodine—are often used asdisinfectants. Organobromides are the most important class offlame retardants, while elemental halogens are dangerous and can be toxic.
The fluorine mineralfluorospar was known as early as 1529. Early chemists realized that fluorine compounds contain an undiscovered element, but were unable to isolate it. In 1860,George Gore, an English chemist, ran a current of electricity throughhydrofluoric acid and probably produced fluorine, but he was unable to prove his results at the time.[citation needed] In 1886,Henri Moissan, a chemist in Paris, performedelectrolysis onpotassium bifluoride dissolved in anhydroushydrogen fluoride, and successfully isolated fluorine.[7]
Hydrochloric acid was known toalchemists and early chemists. However, elemental chlorine was not produced until 1774, whenCarl Wilhelm Scheele heated hydrochloric acid withmanganese dioxide. Scheele called the element "dephlogisticated muriatic acid", which is how chlorine was known for 33 years. In 1807,Humphry Davy investigated chlorine and discovered that it is an actual element. Chlorine gas was used as apoisonous gas duringWorld War I. It displaced oxygen in contaminated areas and replaced common oxygenated air with the toxic chlorine gas. The gas would burn human tissue externally and internally, especially the lungs, making breathing difficult or impossible depending on the level of contamination.[7]
Bromine was discovered in the 1820s byAntoine Jérôme Balard. Balard discovered bromine by passing chlorine gas through a sample ofbrine. He originally proposed the namemuride for the new element, but theFrench Academy changed the element's name to bromine.[7]
Iodine was discovered byBernard Courtois, who was usingseaweed ash as part of a process forsaltpeter manufacture. Courtois typically boiled the seaweed ash with water to generatepotassium chloride. However, in 1811, Courtois added sulfuric acid to his process and found that his process produced purple fumes that condensed into black crystals. Suspecting that these crystals were a new element, Courtois sent samples to other chemists for investigation. Iodine was proven to be a new element byJoseph Gay-Lussac.[7]
In 1811, the German chemistJohann Schweigger proposed that the name "halogen" – meaning "salt producer", from αλς [hals] "salt" and γενειν [genein] "to beget" – replace the name "chlorine", which had been proposed by the English chemistHumphry Davy.[9] Davy's name for the element prevailed.[10] However, in 1826, theSwedishchemist BaronJöns Jacob Berzelius proposed the term "halogen" for the elements fluorine, chlorine, and iodine, which produce asea-salt-like substance when they form acompound with an alkaline metal.[11][12]
The English names of these elements all have the ending-ine. Fluorine's name comes from theLatin wordfluere, meaning "to flow", because it was derived from the mineralfluorite, which was used as aflux in metalworking. Chlorine's name comes from theGreek wordchloros, meaning "greenish-yellow". Bromine's name comes from the Greek wordbromos, meaning "stench". Iodine's name comes from the Greek wordiodes, meaning "violet". Astatine's name comes from the Greek wordastatos, meaning "unstable".[7] Tennessine is named after the US state ofTennessee, where it was synthesized.
The halogens fluorine, chlorine, bromine, and iodine arenonmetals; the chemical properties of astatine and tennessine, two heaviest group 17 members, have not been conclusively investigated. The halogens show trends in chemical bond energy moving from top to bottom of the periodic table column with fluorine deviating slightly. It follows a trend in having the highest bond energy in compounds with other atoms, but it has very weak bonds within the diatomic F2 molecule. This means that further down group 17 in the periodic table, the reactivity of elements decreases because of the increasing size of the atoms.[13]
Halogens are highlyreactive, and as such can be harmful or lethal tobiological organisms in sufficient quantities. This high reactivity is due to the highelectronegativity of the atoms due to their higheffective nuclear charge. Because the halogens have seven valence electrons in their outermost energy level, they can gain an electron by reacting with atoms of other elements to satisfy theoctet rule.Fluorine is the most reactive of all elements; it is the only element more electronegative than oxygen, it attacks otherwise-inert materials such as glass, and it forms compounds with the usually inertnoble gases. It is acorrosive and highly toxic gas. The reactivity of fluorine is such that, if used or stored in laboratory glassware, it can react with glass in the presence of small amounts of water to formsilicon tetrafluoride (SiF4). Thus, fluorine must be handled with substances such asTeflon (which is itself anorganofluorine compound), extremely dry glass, or metals such as copper or steel, which form a protective layer of fluoride on their surface.
The high reactivity of fluorine allows some of the strongest bonds possible, especially to carbon. For example, Teflon is fluorine bonded with carbon and is extremely resistant to thermal and chemical attacks and has a high melting point.
The stable halogens formhomonucleardiatomicmolecules.Due to relatively weak intermolecular forces, chlorine and fluorine form part of the group known as "elemental gases".
The elements become less reactive and have higher melting points as the atomic number increases. The higher melting points are caused by strongerLondon dispersion forces resulting from more electrons.
All of the halogens have been observed to react with hydrogen to formhydrogen halides. For fluorine, chlorine, and bromine, this reaction is in the form of:
H2 + X2 → 2HX
However, hydrogen iodide and hydrogen astatide can split back into their constituent elements.[15]
The hydrogen-halogen reactions get gradually less reactive toward the heavier halogens. A fluorine-hydrogen reaction is explosive even when it is dark and cold. A chlorine-hydrogen reaction is also explosive, but only in the presence of light and heat. A bromine-hydrogen reaction is even less explosive; it is explosive only when exposed to flames. Iodine and astatine only partially react with hydrogen, formingequilibria.[15]
All of the hydrogen halides areirritants. Hydrogen fluoride and hydrogen chloride are highlyacidic. Hydrogen fluoride is used as anindustrial chemical, and is highly toxic, causingpulmonary edema and damaging cells.[17] Hydrogen chloride is also a dangerous chemical. Breathing in gas with more than fifty parts per million of hydrogen chloride can cause death in humans.[18] Hydrogen bromide is even more toxic and irritating than hydrogen chloride. Breathing in gas with more than thirty parts per million of hydrogen bromide can be lethal to humans.[19] Hydrogen iodide, like other hydrogen halides, is toxic.[20]
All the halogens are known to react with sodium to formsodium fluoride,sodium chloride,sodium bromide,sodium iodide, and sodium astatide. Heated sodium's reaction with halogens produces bright-orange flames. Sodium's reaction with chlorine is in the form of:
However, when iron reacts with iodine, it forms onlyiron(II) iodide.
Fe + I2 → FeI2
Iron wool can react rapidly with fluorine to form the white compoundiron(III) fluoride even in cold temperatures. When chlorine comes into contact with a heated iron, they react to form the blackiron(III) chloride. However, if the reaction conditions are moist, this reaction will instead result in a reddish-brown product. Iron can also react with bromine to formiron(III) bromide. This compound is reddish-brown in dry conditions. Iron's reaction with bromine is less reactive than its reaction with fluorine or chlorine. A hot iron can also react with iodine, but it forms iron(II) iodide. This compound may be gray, but the reaction is always contaminated with excess iodine, so it is not known for sure. Iron's reaction with iodine is less vigorous than its reaction with the lighter halogens.[15]
Interhalogen compounds are in the form of XYn where X and Y are halogens and n is one, three, five, or seven. Interhalogen compounds contain at most two different halogens. Large interhalogens, such asClF3 can be produced by a reaction of a pure halogen with a smaller interhalogen such asClF. All interhalogens exceptIF7 can be produced by directly combining pure halogens in various conditions.[21]
Interhalogens are typically more reactive than all diatomic halogen molecules except F2 because interhalogen bonds are weaker. However, the chemical properties of interhalogens are still roughly the same as those ofdiatomic halogens. Many interhalogens consist of one or more atoms of fluorine bonding to a heavier halogen. Chlorine and bromine can bond with up to five fluorine atoms, and iodine can bond with up to seven fluorine atoms. Most interhalogen compounds arecovalent gases. However, some interhalogens are liquids, such as BrF3, and many iodine-containing interhalogens are solids.[21]
Many syntheticorganic compounds such asplasticpolymers, and a few natural ones, contain halogen atoms; these are known ashalogenated compounds ororganic halides. Chlorine is by far the most abundant of the halogens in seawater, and the only one needed in relatively large amounts (as chloride ions) by humans. For example, chloride ions play a key role inbrain function by mediating the action of the inhibitory transmitterGABA and are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production ofthyroid hormones such asthyroxine. Organohalogens are also synthesized through thenucleophilic abstraction reaction.[22]
Polyhalogenated compounds are industrially created compounds substituted with multiple halogens. Many of them are very toxic and bioaccumulate in humans, and have a very wide application range. They includePCBs,PBDEs, andperfluorinated compounds (PFCs), as well as numerous other compounds.
Chlorine has maximum solubility of ca. 7.1 g Cl2 per kg of water at ambient temperature (21 °C).[24] Dissolved chlorine reacts to formhydrochloric acid (HCl) andhypochlorous acid, a solution that can be used as adisinfectant orbleach:
Iodine, however, is minimally soluble in water (0.03 g/100 g water at 20 °C) and does not react with it.[26] However, iodine will form an aqueous solution in the presence of iodide ion, such as by addition ofpotassium iodide (KI), because thetriiodide ion is formed.
The table below is a summary of the key physical and atomic properties of the halogens. Data marked with question marks are either uncertain or are estimations partially based onperiodic trends rather than observations.
Fluorine has one stable and naturally occurringisotope, fluorine-19. However, there are trace amounts in nature of the radioactive isotope fluorine-23, which occurs viacluster decay ofprotactinium-231. A total of eighteen isotopes of fluorine have been discovered, with atomic masses ranging from 13 to 31.
Chlorine has two stable and naturally occurringisotopes, chlorine-35 and chlorine-37. However, there are trace amounts in nature of the isotopechlorine-36, which occurs viaspallation of argon-36. A total of 24 isotopes of chlorine have been discovered, with atomic masses ranging from 28 to 51.[7]
There are two stable and naturally occurringisotopes of bromine, bromine-79 and bromine-81. A total of 33 isotopes of bromine have been discovered, with atomic masses ranging from 66 to 98.
There is one stable and naturally occurringisotope of iodine,iodine-127. However, there are trace amounts in nature of the radioactive isotopeiodine-129, which occurs via spallation and from the radioactive decay of uranium in ores. Several other radioactive isotopes of iodine have also been created naturally via the decay of uranium. A total of 38 isotopes of iodine have been discovered, with atomic masses ranging from 108 to 145.[7]
There are no stableisotopes of astatine. However, there are four naturally occurring radioactive isotopes of astatine produced via radioactive decay ofuranium,neptunium, andplutonium. These isotopes are astatine-215, astatine-217, astatine-218, and astatine-219. A total of 31 isotopes of astatine have been discovered, with atomic masses ranging from 191 to 227.[7]
From left to right:chlorine,bromine, andiodine at room temperature. Chlorine is a gas, bromine is a liquid, and iodine is a solid.Fluorine could not be included in the image due to its highreactivity, and astatine and tennessine due to their radioactivity.
Approximately six million metric tons of the fluorine mineralfluorite are produced each year. Four hundred-thousand metric tons of hydrofluoric acid are made each year. Fluorine gas is made from hydrofluoric acid produced as a by-product inphosphoric acid manufacture. Approximately 15,000 metric tons of fluorine gas are made per year.[7]
The mineralhalite is the mineral that is most commonly mined for chlorine, but the mineralscarnallite andsylvite are also mined for chlorine. Forty million metric tons of chlorine are produced each year by theelectrolysis ofbrine.[7]
Approximately 450,000 metric tons of bromine are produced each year. Fifty percent of all bromine produced is produced in theUnited States, 35% inIsrael, and most of the remainder inChina. Historically, bromine was produced by addingsulfuric acid and bleaching powder to natural brine. However, in modern times, bromine is produced by electrolysis, a method invented byHerbert Dow. It is also possible to produce bromine by passing chlorine through seawater and then passing air through the seawater.[7]
In 2003, 22,000 metric tons of iodine were produced. Chile produces 40% of all iodine produced,Japan produces 30%, and smaller amounts are produced inRussia and the United States. Until the 1950s, iodine was extracted fromkelp. However, in modern times, iodine is produced in other ways. One way that iodine is produced is by mixingsulfur dioxide withnitrate ores, which contain someiodates. Iodine is also extracted fromnatural gas fields.[7]
Even though astatine is naturally occurring, it is usually produced by bombarding bismuth with alpha particles.[7]
Tennessine is made by using a cyclotron, fusing berkelium-249 and calcium-48 to make tennessine-293 and tennessine-294.
Both chlorine and bromine are used asdisinfectants for drinking water, swimming pools, fresh wounds, spas, dishes, and surfaces. They killbacteria and other potentially harmfulmicroorganisms through a process known assterilization. Their reactivity is also put to use inbleaching.Sodium hypochlorite, which is produced from chlorine, is the active ingredient of mostfabric bleaches, and chlorine-derived bleaches are used in the production of somepaper products.
Halogen lamps are a type ofincandescent lamp using atungsten filament in bulbs that have small amounts of a halogen, such as iodine or bromine added. This enables the production of lamps that are much smaller than non-halogenincandescent lightbulbs at the samewattage. The gas reduces the thinning of the filament and blackening of the inside of the bulb resulting in a bulb that has a much greater life. Halogen lamps glow at a higher temperature (2800 to 3400kelvin) with a whiter colour than other incandescent bulbs. However, this requires bulbs to be manufactured fromfused quartz rather than silica glass to reduce breakage.[38]
Indrug discovery, the incorporation of halogen atoms into a lead drug candidate results in analogues that are usually morelipophilic and less water-soluble.[39] As a consequence, halogen atoms are used to improve penetration throughlipid membranes and tissues. It follows that there is a tendency for some halogenated drugs to accumulate inadipose tissue.
The chemical reactivity of halogen atoms depends on both their point of attachment to the lead and the nature of the halogen.Aromatic halogen groups are far less reactive thanaliphatic halogen groups, which can exhibit considerable chemical reactivity. For aliphatic carbon-halogen bonds, the C-F bond is the strongest and usually less chemically reactive than aliphatic C-H bonds. The other aliphatic-halogen bonds are weaker, their reactivity increasing down the periodic table. They are usually more chemically reactive than aliphatic C-H bonds. As a consequence, the most common halogen substitutions are the less reactive aromatic fluorine and chlorine groups.
Fluoride anions are found in ivory, bones, teeth, blood, eggs, urine, and hair of organisms. Fluoride anions in very small amounts may be essential for humans.[40] There are 0.5 milligrams of fluorine per liter of human blood. Human bones contain 0.2 to 1.2% fluorine. Human tissue contains approximately 50 parts per billion of fluorine. A typical 70-kilogram human contains 3 to 6 grams of fluorine.[7]
Chloride anions are essential to a large number of species, humans included. The concentration of chlorine in thedry weight of cereals is 10 to 20 parts per million, while inpotatoes the concentration of chloride is 0.5%. Plant growth is adversely affected by chloride levels in thesoil falling below 2 parts per million. Human blood contains an average of 0.3% chlorine. Human bone typically contains 900 parts per million of chlorine. Human tissue contains approximately 0.2 to 0.5% chlorine. There is a total of 95 grams of chlorine in a typical 70-kilogram human.[7]
Some bromine in the form of the bromide anion is present in all organisms. A biological role for bromine in humans has not been proven, but some organisms containorganobromine compounds. Humans typically consume 1 to 20 milligrams of bromine per day. There are typically 5 parts per million of bromine in human blood, 7 parts per million of bromine in human bones, and 7 parts per million of bromine in human tissue. A typical 70-kilogram human contains 260 milligrams of bromine.[7]
Humans typically consume less than 100 micrograms of iodine per day. Iodine deficiency can causeintellectual disability.Organoiodine compounds occur in humans in some of theglands, especially thethyroid gland, as well as thestomach,epidermis, andimmune system. Foods containing iodine includecod,oysters,shrimp,herring,lobsters,sunflower seeds,seaweed, andmushrooms. However, iodine is not known to have a biological role in plants. There are typically 0.06 milligrams per liter of iodine in human blood, 300 parts per billion of iodine in human bones, and 50 to 700 parts per billion of iodine in human tissue. There are 10 to 20 milligrams of iodine in a typical 70-kilogram human.[7]
Astatine, although very scarce, has been found in micrograms in the earth.[7] It has no known biological role because of its high radioactivity, extreme rarity, and has a half-life of just about 8 hours for the most stable isotope.
Tennessine is purely man-made and has no other roles in nature.
The halogens tend to decrease in toxicity towards the heavier halogens.[41]
Fluorine gas is extremely toxic; breathing in fluorine at a concentration of 25 parts per million is potentially lethal.Hydrofluoric acid is also toxic, being able to penetrate skin and causehighly painful burns. In addition, fluoride anions are toxic, but not as toxic as pure fluorine. Fluoride can be lethal in amounts of 5 to 10 grams. Prolonged consumption of fluoride above concentrations of 1.5 mg/L is associated with a risk ofdental fluorosis, an aesthetic condition of the teeth.[42] At concentrations above 4 mg/L, there is an increased risk of developingskeletal fluorosis, a condition in which bone fractures become more common due to the hardening of bones. Current recommended levels inwater fluoridation, a way to preventdental caries, range from 0.7 to 1.2 mg/L to avoid the detrimental effects of fluoride while at the same time reaping the benefits.[43] People with levels between normal levels and those required for skeletal fluorosis tend to have symptoms similar toarthritis.[7]
Chlorine gas is highly toxic. Breathing in chlorine at a concentration of 3 parts per million can rapidly cause a toxic reaction. Breathing in chlorine at a concentration of 50 parts per million is highly dangerous. Breathing in chlorine at a concentration of 500 parts per million for a few minutes is lethal. In addition, breathing in chlorine gas is highly painful because of its corrosive properties. Hydrochloric acid is the acid of chlorine, while relatively nontoxic, it is highly corrosive and releases very irritating and toxic hydrogen chloride gas in open air.[41]
Pure bromine is somewhat toxic but less toxic than fluorine and chlorine. One hundred milligrams of bromine is lethal.[7] Bromide anions are also toxic, but less so than bromine. Bromide has a lethal dose of 30 grams.[7]
Iodine is somewhat toxic, being able to irritate the lungs and eyes, with a safety limit of 1 milligram per cubic meter. When taken orally, 3 grams of iodine can be lethal. Iodide anions are mostly nontoxic, but these can also be deadly if ingested in large amounts.[7]
Astatine isradioactive and thus highly dangerous, but it has not been produced in macroscopic quantities and hence it is most unlikely that its toxicity will be of much relevance to the average individual.[7]
Tennessine cannot be chemically investigated due to how short its half-life is, although its radioactivity would make it very dangerous.
Certain aluminium clusters have superatom properties. These aluminium clusters are generated as anions (Al− n withn = 1, 2, 3, ... ) in helium gas and reacted with a gas containing iodine. When analyzed by mass spectrometry one main reaction product turns out to beAl 13I− .[44] These clusters of 13 aluminium atoms with an extra electron added do not appear to react with oxygen when it is introduced in the same gas stream. Assuming each atom liberates its 3 valence electrons, this means 40 electrons are present, which is one of the magic numbers for sodium and implies that these numbers are a reflection of the noble gases.
Calculations show that the additional electron is located in the aluminium cluster at the location directly opposite from the iodine atom. The cluster must therefore have a higher electron affinity for the electron than iodine and therefore the aluminium cluster is called a superhalogen (i.e., the vertical electron detachment energies of the moieties that make up the negative ions are larger than those of any halogen atom).[45] The cluster component in theAl 13I− ion is similar to an iodide ion or a bromide ion. The relatedAl 13I− 2 cluster is expected to behave chemically like thetriiodide ion.[46][47]
^This could also be the case forgroup 12, althoughcopernicium's melting and boiling points are still uncertain.
^The number given inparentheses refers to themeasurement uncertainty. This uncertainty applies to theleast significant figure(s) of the number prior to the parenthesized value (i.e., counting from rightmost digit to left). For instance,1.00794(7) stands for1.00794±0.00007, while1.00794(72) stands for1.00794±0.00072.[27]
^The average atomic weight of this element changes depending on the source of the chlorine, and the values in brackets are the upper and lower bounds.[28]
^abThe element does not have any stablenuclides, and the value in brackets indicates themass number of the longest-livedisotope of the element.[28]
^Jones, Daniel (2017) [1917]. Peter Roach; James Hartmann; Jane Setter (eds.).English Pronouncing Dictionary. Cambridge: Cambridge University Press.ISBN978-3-12-539683-8.
^Union internationale de chimie pure et appliquée, ed. (2005).Nomenclature of inorganic chemistry: IUPAC Recommendations 2005. Cambridge: Royal Society of Chemistry. p. 51.ISBN978-0-85404-438-2.
^Schweigger, J.S.C. (1811)."Nachschreiben des Herausgebers, die neue Nomenclatur betreffend" [Postscript of the editor concerning the new nomenclature].Journal für Chemie und Physik (in German).3 (2):249–255. On p. 251, Schweigger proposed the word "halogen":"Man sage dafür lieber mit richter WortbildungHalogen (da schon in der Mineralogie durchWerner's Halit-Geschlecht dieses Wort nicht fremd ist) von αλςSalz und dem alten γενειν (dorisch γενεν)zeugen." (One should say instead, with proper morphology, "halogen" (this word is not strange since [it's] already in mineralogy via Werner's "halite" species) from αλς [als] "salt" and the old γενειν [genein] (Doric γενεν) "to beget".)
^Snelders, H. A. M. (1971). "J. S. C. Schweigger: His Romanticism and His Crystal Electrical Theory of Matter".Isis.62 (3):328–338.doi:10.1086/350763.JSTOR229946.S2CID170337569.
^In 1826, Berzelius coined the termsSaltbildare (salt-formers) andCorpora Halogenia (salt-making substances) for the elements chlorine, iodine, and fluorine. See:Berzelius, Jacob (1826)."Årsberättelser om Framstegen i Physik och Chemie" [Annual Report on Progress in Physics and Chemistry].Arsb. Vetensk. Framsteg (in Swedish).6. Stockholm, Sweden: P.A. Norstedt & Söner: 187. From p. 187:"De förre af dessa, d. ä.de electronegativa, dela sig i tre klasser: 1) den första innehåller kroppar, som förenade med de electropositiva, omedelbart frambringa salter, hvilka jag derför kallarSaltbildare (Corpora Halogenia). Desse utgöras af chlor, iod och fluor *)." (The first of them [i.e., elements], the electronegative [ones], are divided into three classes: 1) The first includes substances which, [when] united with electropositive [elements], immediately produce salts, and which I therefore name "salt-formers" (salt-producing substances). These are chlorine, iodine, and fluorine *).)
^Bonchev, Danail; Kamenska, Verginia (1981). "Predicting the properties of the 113–120 transactinide elements".The Journal of Physical Chemistry.85 (9):1177–86.doi:10.1021/j150609a021.
^Fawell, J.; Bailey, K.; Chilton, J.; Dahi, E.; Fewtrell, L.; Magara, Y. (2006)."Guidelines and standards"(PDF).Fluoride in Drinking-water. World Health Organization. pp. 37–9.ISBN978-92-4-156319-2.
^Giri, Santanab; Behera, Swayamprabha; Jena, Puru (2014). "Superhalogens as Building Blocks of Halogen-Free Electrolytes in Lithium-Ion Batteries†".Angewandte Chemie.126 (50): 14136.Bibcode:2014AngCh.12614136G.doi:10.1002/ange.201408648.
^Ball, Philip (16 April 2005). "A New Kind of Alchemy".New Scientist.
