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In chemistry, an interhalogen compound is a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
which contains two or more different halogen atoms ( fluorine,
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
,
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
, iodine, or
astatine Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-li ...
) and no atoms of elements from any other group. Most interhalogen compounds known are binary (composed of only two distinct elements). Their formulae are generally , where ''n'' = 1, 3, 5 or 7, and X is the less
electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
of the two halogens. The value of ''n'' in interhalogens is always odd, because of the odd valence of halogens. They are all prone to
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...
, and ionize to give rise to polyhalogen ions. Those formed with astatine have a very short half-life due to astatine being intensely radioactive. No interhalogen compounds containing three or more different halogens are definitely known, although a few books claim that and have been obtained, and theoretical studies seem to indicate that some compounds in the series are barely stable. Some interhalogens, such as , , and , are good halogenating agents. is too reactive to generate fluorine. Beyond that,
iodine monochloride Iodine monochloride is an interhalogen compound with the formula . It is a red-brown chemical compound that melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine, this molecule is highly polar an ...
has several applications, including helping to measure the saturation of fats and oils, and as a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
for some reactions. A number of interhalogens, including , are used to form polyhalides. Similar compounds exist with various
pseudohalogen Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic ...
s, such as the halogen azides (, , , and ) and
cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molecu ...
halides (, , , and ).


Types of interhalogens


Diatomic interhalogens

The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
bond between the two atoms has some ionic character, the less
electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
halogen, X, being oxidised and having a partial positive charge. All combinations of fluorine, chlorine, bromine, and iodine that have the above-mentioned general formula are known, but not all are stable. Some combinations of astatine with other halogens are not even known, and those that are known are highly unstable. *
Chlorine monofluoride Chlorine monofluoride is a volatile interhalogen compound with the chemical formula ClF. It is a colourless gas at room temperature and is stable even at high temperatures. When cooled to −100 Â°C, ClF condenses as a pale yellow liquid. M ...
(ClF) is the lightest interhalogen compound. ClF is a colorless gas with a
normal boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
of −100 Â°C. *
Bromine monofluoride Bromine monofluoride is a quite unstable interhalogen compound with the chemical formula BrF. It can be produced through the reaction of bromine trifluoride (or bromine pentafluoride) and bromine. Due to its lability, the compound can be detect ...
(BrF) has not been obtained as a pure compound — it dissociates into the trifluoride and free
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
. It is created according to the following equation: ::Br2(l) + F2(g) → 2 BrF(g) Bromine monofluoride dissociates like this: ::3 BrF → Br2 + BrF3 * Iodine monofluoride (IF) is unstable and decomposes at 0 Â°C, disproportionating into elemental iodine and
iodine pentafluoride Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is one of the fluorides of iodine. It is a colorless liquid, although impure samples appear yellow. It is used as a fluorination reagent and even a solvent in special ...
. *
Bromine monochloride Bromine monochloride, also called bromine(I) chloride, bromochloride, and bromine chloride, is an interhalogen inorganic compound with chemical formula BrCl. It is a very reactive golden yellow gas with boiling point 5 Â°C and melting point â ...
(BrCl) is a yellow-brown gas with a boiling point of 5 Â°C. *
Iodine monochloride Iodine monochloride is an interhalogen compound with the formula . It is a red-brown chemical compound that melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine, this molecule is highly polar an ...
(ICl) exists as red transparent crystals that melt at 27.2 Â°C to form a choking brownish liquid (similar in appearance and weight to
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
). It reacts with HCl to form the strong acid HICl2. The
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains. *
Astatine monochloride In chemistry, an interhalogen compound is a molecule which contains two or more different halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) and no atoms of elements from any other group. Most interhalogen compounds known are binar ...
(AtCl) is made either by the direct combination of gas-phase
astatine Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-li ...
with chlorine or by the sequential addition of astatine and dichromate ion to an acidic chloride solution. * Iodine monobromide (IBr) is made by the direct combination of the elements to form a dark red crystalline solid. It melts at 42 Â°C and boils at 116 Â°C to form a partially dissociated vapour. *
Astatine monobromide Astatine bromide is an interhalogen compound with the chemical formula At Br. Production Astatine bromide is produced by the reaction of astatine with an aqueous solution of iodine monobromide: :At2 + 2 IBr → 2 AtBr + I2 Astatine bromide als ...
(AtBr) is made by the direct combination of astatine with either bromine vapour or an aqueous solution of iodine monobromide. *
Astatine monoiodide Astatine iodide is an interhalogen compound with the chemical formula At I. It is the second heaviest known interhalogen compound, after iodine tribromide. Production Astatine iodide is produced by the direct combination of astatine and iodin ...
(AtI) is made by direct combination of astatine and iodine. No astatine fluorides have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product. Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluorides. In addition, there exist analogous molecules involving
pseudohalogen Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic ...
s, such as the
cyanogen halide A cyanogen halide is a molecule consisting of cyanide and a halogen. Cyanogen halides are chemically classified as pseudohalogens. The cyanogen halides are a group of chemically reactive compounds which contain a cyano group (-CN) attached to a ...
s.


Tetratomic interhalogens

*
Chlorine trifluoride Chlorine trifluoride is an interhalogen compound with the formula ClF3. This colorless, poisonous, corrosive, and extremely reactive gas condenses to a pale-greenish yellow liquid, the form in which it is most often sold (pressurized at room temp ...
(ClF3) is a colourless gas that condenses to a green liquid, and freezes to a white solid. It is made by reacting chlorine with an excess of fluorine at 250 Â°C in a
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
tube. It reacts more violently than fluorine, often explosively. The molecule is planar and
T-shaped Many shapes have metaphorical names, i.e., their names are metaphors: these shapes are named after a most common object that has it. For example, "U-shape" is a shape that resembles the letter U, a bell-shaped curve has the shape of the vertical ...
. It is used in the manufacture of
uranium hexafluoride Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
. *
Bromine trifluoride Bromine trifluoride is an interhalogen compound with the formula BrF3. At room temperature, it is a straw-coloured liquid with a pungent odor which decomposes violently on contact with water and organic compounds. It is a powerful fluorinating a ...
(BrF3) is a yellow-green liquid that conducts electricity — it self-ionises to form rF2sup>+ and rF4sup>−. It reacts with many metals and metal oxides to form similar ionised entities; with other metals, it forms the metal fluoride plus free bromine and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
; and with water, it forms hydrofluoric acid and hydrobromic acid. It is used in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
as a fluorinating agent. It has the same molecular shape as chlorine trifluoride. * Iodine trifluoride (IF3) is a yellow solid that decomposes above −28 Â°C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5. F2 attacks I2 to yield IF3 at −45 Â°C in CCl3F. Alternatively, at low temperatures, the fluorination reaction ::I2 + 3  XeF2 → 2 IF3 + 3  Xe :can be used. Not much is known about iodine trifluoride as it is so unstable. *
Iodine trichloride Iodine trichloride is an interhalogen compound of iodine and chlorine. It is bright yellow but upon time and exposure to light it turns red due to the presence of elemental iodine. In the solid state is present as a planar dimer I2Cl6, with two ...
(ICl3) forms lemon yellow crystals that melt under pressure to a brown liquid. It can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with many metal chlorides to form tetrachloroiodides (), and hydrolyses in water. The molecule is a planar
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...
(ICl3)2, with each iodine atom surrounded by four chlorine atoms. *
Iodine tribromide Iodine tribromide is an interhalogen with chemical formula IBr3. Properties Iodine tribromide is a dark brown liquid that is miscible with ethanol and ethers.Dr. Bernd Dill (Hrsg.), Prof. Dr. Fred Robert Heiker (Hrsg.), Prof. Dr. Andreas Kirschn ...
(IBr3) is a dark brown liquid.


Hexatomic interhalogens

All stable hexatomic and octatomic interhalogens involve a heavier halogen combined with five or seven fluorine atoms. Unlike the other halogens, fluorine atoms have high electronegativity and small size which is able to stabilize them. *
Chlorine pentafluoride Chlorine pentafluoride is an interhalogen compound with formula ClF5. This colourless gas is a strong oxidant that was once a candidate oxidizer for rockets. The molecule adopts a square pyramidal structure with C4v symmetry, as confirmed by ...
(ClF5) is a colourless gas, made by reacting chlorine trifluoride with fluorine at high temperatures and high pressures. It reacts violently with water and most
metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
and nonmetals. *
Bromine pentafluoride Bromine pentafluoride, Br F5, is an interhalogen compound and a fluoride of bromine. It is a strong fluorinating agent. BrF5 finds use in oxygen isotope analysis. Laser ablation of solid silicates in the presence of BrF5 releases O2 for subseq ...
(BrF5) is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine at 200 Â°C. It is physically stable, but decomposes violently on contact with water, organic substances, and most metals and nonmetals. *
Iodine pentafluoride Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is one of the fluorides of iodine. It is a colorless liquid, although impure samples appear yellow. It is used as a fluorination reagent and even a solvent in special ...
(IF5) is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with silver(II) fluoride. It is highly reactive, even slowly with glass. It reacts with water to form hydrofluoric acid and with fluorine gas to form
iodine heptafluoride Iodine heptafluoride, also known as iodine(VII) fluoride or iodine fluoride, is an interhalogen compound with the chemical formula I F7. It has an unusual pentagonal bipyramidal structure, as predicted by VSEPR theory. The molecule can undergo ...
. The molecule has the form of a tetragonal pyramid.


Octatomic interhalogens

*
Iodine heptafluoride Iodine heptafluoride, also known as iodine(VII) fluoride or iodine fluoride, is an interhalogen compound with the chemical formula I F7. It has an unusual pentagonal bipyramidal structure, as predicted by VSEPR theory. The molecule can undergo ...
(IF7) is a colourless gas and a strong fluorinating agent. It is made by reacting iodine pentafluoride with fluorine gas. The molecule is a
pentagonal bipyramid In geometry, the pentagonal bipyramid (or dipyramid) is third of the infinite set of face-transitive bipyramids, and the 13th Johnson solid (). Each bipyramid is the dual of a uniform prism. Although it is face-transitive, it is not a Plat ...
. This compound is the only known interhalogen compound where the larger atom is carrying seven of the smaller atoms. *All attempts to synthesize bromine or chlorine heptafluoride have met with failure; instead,
bromine pentafluoride Bromine pentafluoride, Br F5, is an interhalogen compound and a fluoride of bromine. It is a strong fluorinating agent. BrF5 finds use in oxygen isotope analysis. Laser ablation of solid silicates in the presence of BrF5 releases O2 for subseq ...
or
chlorine pentafluoride Chlorine pentafluoride is an interhalogen compound with formula ClF5. This colourless gas is a strong oxidant that was once a candidate oxidizer for rockets. The molecule adopts a square pyramidal structure with C4v symmetry, as confirmed by ...
is produced, along with fluorine gas.


Properties

Typically, interhalogen bonds are more reactive than diatomic halogen bonds—because interhalogen bonds are weaker than diatomic halogen bonds, except for F2. If interhalogens are exposed to water, they convert to halide and
oxyhalide In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), chlor ...
ions. With BrF5, this reaction can be explosive. If interhalogens are exposed to
silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
, or
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
oxides, then silicon or metal respectively bond with one of the types of halogen, leaving free diatomic halogens and diatomic oxygen. Most interhalogens are halogen fluorides, and all but three (IBr, AtBr, and AtI) of the remainder are halogen chlorides. Chlorine and bromine can each bond to five fluorine atoms, and iodine can bond to seven. AX and AX3 interhalogens can form between two halogens whose electronegativities are relatively close to one another. When interhalogens are exposed to metals, they react to form metal halides of the constituent halogens. The oxidation power of an interhalogen increases with the number of halogens attached to the central atom of the interhalogen, as well as with the decreasing size of the central atom of the compound. Interhalogens containing fluorine are more likely to be volatile than interhalogens containing heavier halogens. Interhalogens with one or three halogens bonded to a central atom are formed by two elements whose electronegativities are not far apart. Interhalogens with five or seven halogens bonded to a central atom are formed by two elements whose sizes are very different. The number of smaller halogens that can bond to a large central halogen is guided by the ratio of the
atomic radius The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there ...
of the larger halogen over the atomic radius of the smaller halogen. A number of interhalogens, such as IF7, react with all metals except for those in the
platinum group The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...
. IF7, unlike interhalogens in the XY5 series, does not react with the fluorides of the
alkali metals The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
. ClF3 is the most reactive of the XY3 interhalogens. ICl3 is the least reactive. BrF3 has the highest thermal stability of the interhalogens with four atoms. ICl3 has the lowest. Chlorine trifluoride has a boiling point of −12 Â°C. Bromine trifluoride has a boiling point of 127 Â°C and is a liquid at room temperature. Iodine trichloride melts at 101 Â°C. Most interhalogens are
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
gases. Some interhalogens, especially those containing bromine, are liquids, and most iodine-containing interhalogens are solids. Most of the interhalogens composed of lighter halogens are fairly colorless, but the interhalogens containing heavier halogens are deeper in color due to their higher
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
. In this respect, the interhalogens are similar to the halogens. The greater the difference between the electronegativities of the two halogens in an interhalogen, the higher the boiling point of the interhalogen. All interhalogens are
diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...
. The
bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
of interhalogens in the XY series increases with the size of the constituent halogens. For instance, ClF has a bond length of 1.628  Ã…, and IBr has a bond length of 2.47 Ã….


Production

It is possible to produce larger interhalogens, such as ClF3, by exposing smaller interhalogens, such as ClF, to pure diatomic halogens, such as F2. This method of production is especially useful for generating halogen fluorides. At temperatures of 250 to 300 Â°C, this type of production method can also convert larger interhalogens into smaller ones. It is also possible to produce interhalogens by combining two pure halogens at various conditions. This method can generate any interhalogen save for IF7. Smaller interhalogens, such as ClF, can form by direct reaction with pure halogens. For instance, F2 reacts with Cl2 at 250 Â°C to form two molecules of ClF. Br2 reacts with diatomic fluorine in the same way, but at 60 Â°C. I2 reacts with diatomic fluorine at only 35 Â°C. ClF and BrF can both be produced by the reaction of a larger interhalogen, such as ClF3 or BrF3 and a diatomic molecule of the element lower in the periodic table. Among the hexatomic interhalogens, IF5 has a higher boiling point (97 Â°C) than BrF5 (40.5 Â°C), although both compounds are liquids at room temperature. The interhalogen IF7 can be formed by reacting
palladium iodide Palladium(II) iodide is an inorganic compound of palladium and iodine. It is commercially available, though less common than palladium(II) chloride, the usual entry point to palladium chemistry. Historically, the quantity of palladium in a sample ...
with fluorine.


See also

* Interchalcogen *
Hydrogen halide In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine. ...


Notes


References


Bibliography

* * *


External links

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