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 bio ...
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 is ...
,
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 simil ...
,
iodine
Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
, 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 hydrolysis ...
, 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 pola ...
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 azide
In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant ...
s (, , , 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 molec ...
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 atom ...
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 (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 °C.
* Bromine monofluoride (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 simil ...
. It is created according to the following equation:
::Br2(l) + F2(g) → 2 BrF(g)
Bromine monofluoride dissociates like this:
::3 BrF → Br2 + BrF3
![Astatine-chloride-3D-vdW](https://upload.wikimedia.org/wikipedia/commons/a/ab/Astatine-chloride-3D-vdW.png)
![Astatine-bromide-3D-vdW](https://upload.wikimedia.org/wikipedia/commons/e/ed/Astatine-bromide-3D-vdW.png)
*Iodine monofluoride
Iodine monofluoride is an interhalogen compound of iodine and fluorine with formula IF. It is a chocolate-brown solid that decomposes at 0 °C, disproportionating to elemental iodine and iodine pentafluoride:
:5 IF → 2 I2 + IF5
However, ...
(IF) is unstable and decomposes at 0 °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 °C and melting point â ...
(BrCl) is a yellow-brown gas with a boiling point of 5 °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 pola ...
(ICl) exists as red transparent crystals that melt at 27.2 °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 simil ...
). 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 t ...
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 bi ...
(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
Iodine monobromide is an interhalogen compound with the chemical symbol IBr. It is a dark red solid that melts near room temperature. Like iodine monochloride, IBr is used in some types of iodometry. It serves as a source of I+. Its Lewis acid ...
(IBr) is made by the direct combination of the elements to form a dark red crystalline solid. It melts at 42 °C and boils at 116 °C to form a partially dissociated vapour.
*Astatine monobromide
Astatine bromide is an interhalogen, interhalogen compound with the chemical formula Astatine, AtBromine, Br.
Production
Astatine bromide is produced by the reaction of astatine with an aqueous solution of iodine monobromide:
:At2 + 2 IBr → 2 ...
(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, interhalogen compound with the chemical formula Astatine, AtIodine, I. It is the second heaviest known interhalogen compound, after iodine tribromide.
Production
Astatine iodide is produced by the direct combi ...
(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 halides.
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 te ...
(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 °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. 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 2">rF2sup>+ and 4">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 we ...
; 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 science, 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.Clay ...
as a fluorinating agent. It has the same molecular shape as chlorine trifluoride.
*Iodine trifluoride
Iodine trifluoride is an interhalogen compound with the chemical formula IF3. It is a yellow solid which decomposes above −28 °C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5.
Reactions
F2 r ...
(IF3) is a yellow solid that decomposes above −28 °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 °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 (ICl3)2, with each iodine atom surrounded by four chlorine atoms.
* Iodine tribromide (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 (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 μÎταλλον ''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 ...
and nonmetal
In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differen ...
s.
*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 subs ...
(BrF5) is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine at 200 °C. It is physically stable, but decomposes violently on contact with water, organic substances, and most metals and nonmetal
In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differen ...
s.
*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
Silver(II) fluoride is a chemical compound with the formula AgF2. It is a rare example of a silver(II) compound. Silver usually exists in its +1 oxidation state. It is used as a fluorinating agent.
Preparation
AgF2 can be synthesized by fluori ...
. It is highly reactive, even slowly with glass. It reacts with water to form hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...
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 Platoni ...
. 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 subs ...
or chlorine pentafluoride 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
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a f ...
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), ch ...
ions. With BrF5, this reaction can be explosive
An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An exp ...
. 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 μÎταλλον ''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 typi ...
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, th ...
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 to ...
. 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 °C. Bromine trifluoride has a boiling point of 127 °C and is a liquid at room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
. Iodine trichloride melts at 101 °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 atom ...
gases. Some interhalogens, especially those containing bromine, are liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...
s, 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 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 fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ...
s. At temperatures of 250 to 300 °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 °C to form two molecules of ClF. Br2 reacts with diatomic fluorine in the same way, but at 60 °C. I2 reacts with diatomic fluorine at only 35 °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
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ...
. Among the hexatomic interhalogens, IF5 has a higher boiling point (97 °C) than BrF5 (40.5 °C), although both compounds are liquids at room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
. The interhalogen IF7 can be formed by reacting palladium iodide 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 astati ...
Notes
References
Bibliography
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External links
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