HOME

TheInfoList




The oxidation state, or oxidation number, is the hypothetical
charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * Charge (David Ford album), ''Charge'' (David Ford album) * Charge (Machel Montano album), ''Charge'' (Mac ...
of an atom if all of its bonds to different atoms were fully
ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic scale Places and peoples * Ionian, of or from Ionia, an ancient region in western An ...
. It describes the degree of
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

oxidation
(loss of
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
s) of an
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
in a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
. Conceptually, the oxidation state may be positive, negative or zero. While fully ionic bonds are not found in nature, many bonds exhibit strong ionicity, making oxidation state a useful predictor of charge. The oxidation state of an atom does not represent the "real"
formal charge In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
on that atom, or any other actual atomic property. This is particularly true of high oxidation states, where the
ionization energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and forc ...
required to produce a multiply positive ion is far greater than the energies available in chemical reactions. Additionally, the oxidation states of atoms in a given compound may vary depending on the choice of
electronegativity Electronegativity, symbolized as '' χ'', is the tendency for an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume ...

electronegativity
scale used in their calculation. Thus, the oxidation state of an atom in a compound is purely a formalism. It is nevertheless important in understanding the nomenclature conventions of inorganic compounds. Also, several observations regarding chemical reactions may be explained at a basic level in terms of oxidation states. Oxidation states are typically represented by
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
s which may be positive, zero, or negative. In some cases, the average oxidation state of an element is a fraction, such as for iron in
magnetite Magnetite is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure for ...

magnetite
(
see below See or SEE may refer to: Arts, entertainment, and media * Music: ** See (album), ''See'' (album), studio album by rock band The Rascals *** "See", song by The Rascals, on the album ''See'' ** See (Tycho song), "See" (Tycho song), song by Tycho * T ...
). The highest known oxidation state is reported to be +9 in the tetroxoiridium(IX) cation (). It is predicted that even a +12 oxidation state may be achievable by
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
in the unusual hexoxide UO6. The lowest oxidation state is −5, as for
boron Boron is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

boron
in Al3BC. In inorganic nomenclature, the oxidation state is represented by a
Roman numeral Roman numerals are a numeral system A numeral system (or system of numeration) is a writing system for expressing numbers; that is, a mathematical notation for representing numbers of a given set, using Numerical digit, digits or other s ...
placed after the element name inside the parenthesis or as a superscript after the element symbol, e.g.
Iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is ...
. The term ''oxidation'' was first used by
Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ,; 26 August 17438 May 1794), When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air and t ...

Antoine Lavoisier
to signify the reaction of a substance with
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. Much later, it was realized that the substance, upon being oxidized, loses electrons, and the meaning was extended to include other
reactions
reactions
in which electrons are lost, regardless of whether oxygen was involved. The increase in the oxidation state of an atom, through a chemical reaction, is known as oxidation; a decrease in oxidation state is known as a
reduction
reduction
. Such reactions involve the formal transfer of electrons: a net gain in electrons being a reduction, and a net loss of electrons being oxidation. For pure elements, the oxidation state is zero.


IUPAC definition

IUPAC has published a "Comprehensive definition of the term oxidation state (IUPAC Recommendations 2016)". It is a distillation of an
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative power over chemist ...
technical report "Toward a comprehensive definition of oxidation state" from 2014. The current IUPAC ''
Gold Book The International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a m ...
'' definition of oxidation state is: and the term ''oxidation number'' is nearly synonymous. The underlying principle is that the ionic charge is "the oxidation state of an atom, after ionic approximation of its bonds", where ionic approximation means, hypothesizing that all bonds are ionic. Several criteria were considered for the ionic approximation: # Extrapolation of the bond’s polarity; # Assignment of electrons according to the atom’s contribution to the bonding MO/ the electron's allegiance in a LCAO–MO model. In a bond between two different elements, the bond's electrons are assigned to its main atomic contributor/higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. This is because most electronegativity scales depend on the atom's bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as -6 for
platinum Platinum is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

platinum
in PtH4−2, for
PaulingPauling is a surname In some cultures, a surname, family name, or last name is the portion of one's personal name that indicates their family, tribe or community. Practices vary by culture. The family name may be placed at either the start of ...
and Mulliken scales. The dipole moments would, sometimes, also turn out abnormal oxidation numbers, such as in and NO, which are oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to the bonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can use Allen electronegativities, as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom:


Determination

While introductory levels of chemistry teaching use
postulate An axiom, postulate or assumption is a statement that is taken to be truth, true, to serve as a premise or starting point for further reasoning and arguments. The word comes from the Greek ''axíōma'' () 'that which is thought worthy or fit' or ...
d oxidation states, the IUPAC recommendation and the ''Gold Book'' entry list two entirely general algorithms for the calculation of the oxidation states of elements in chemical compounds.


Simple approach without bonding considerations

Introductory chemistry uses postulates: the oxidation state for an element in a chemical formula is calculated from the overall charge and postulated oxidation states for all the other atoms. A simple example is based on two postulates, # OS = +1 for
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
# OS = −2 for
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
where OS stands for oxidation state. This approach yields correct oxidation states in oxides and hydroxides of any single element, and in acids such as or H2Cr2O7. Its coverage can be extended either by a list of exceptions or by assigning priority to the postulates. The latter works for where the priority of rule 1 leaves both oxygens with oxidation state −1. Additional postulates and their ranking may expand the range of compounds to fit a textbook's scope. As an example, one postulatory algorithm from many possible; in a sequence of decreasing priority: # An element in a free form has OS = 0. # In a compound or ion, the sum of the oxidation states equals the total charge of the compound or ion. #
Fluorine Fluorine is a chemical element with the Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard conditions for temperature and pressure, standard conditions as a highly toxic, pale yellow Diatomic molecule ...

Fluorine
in compounds has OS = −1; this extends to
chlorine Chlorine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemica ...

chlorine
and
bromine Bromine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

bromine
only when not bonded to a lighter halogen, oxygen or nitrogen. #
Group 1Group 1 may refer to: * Alkali metal, a chemical element classification for Alkali metal * Group 1 (racing), a historic (until 1981) classification for Touring car racing, applied to standard touring cars. Comparable to modern FIA Group N * Group One ...
and
group 2The term Group 2 may refer to: * Alkaline earth metal, a chemical element classification * Astronaut Group 2, also known as The New Nine, the second group of astronauts selected by NASA in 1962 * Group 2 (racing), an FIA classification for cars in au ...
metals in compounds have OS = +1 and +2, respectively. # Hydrogen has OS = +1 but adopts −1 when bonded as a
hydride In chemistry, a hydride is formally the anion of hydrogen, H−. The term is applied loosely. At one extreme, all chemical compound, compounds containing covalent bond, covalently bound H atoms are called hydrides: water (H2O) is a hydride of o ...

hydride
to metals or metalloids. # Oxygen in compounds has OS = −2 but only when not bonded to oxygen (e.g. in peroxides) or fluorine. This set of postulates covers oxidation states of fluorides, chlorides, bromides, oxides, hydroxides, and hydrides of any single element. It covers all
oxoacids An oxyacid, oxoacid, or ternary acid is an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the speci ...
of any central atom (and all their fluoro-, chloro-, and bromo-relatives), as well as
salts In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
of such acids with group 1 and 2 metals. It also covers
iodide An iodide ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that c ...

iodide
s,
sulfide Sulfide (British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and usage a ...

sulfide
s, and similar simple salts of these metals.


Algorithm of assigning bonds

This algorithm is performed on a
Lewis structure#REDIRECT Lewis structure Lewis structures, also known as Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDS), are diagrams that show the bonding between atom An atom is the smallest unit o ...
(a diagram that shows all
valence electron In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s). Oxidation state equals the charge of an atom after each of its
heteronuclear A heteronuclear molecule is a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in posit ...
bonds has been assigned to the more-
electronegative Electronegativity, symbolized as '' χ'', is the tendency for an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume ...
partner of the bond ( except when that partner is a reversibly bonded Lewis-acid ligand) and
homonuclear Homonuclear molecules, or homonuclear species, are molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of ph ...
bonds have been divided equally: : where each "—" represents an electron pair (either shared between two atoms or solely on one atom), and "OS" is the oxidation state as a numerical variable. After the electrons have been assigned according to the vertical red lines on the formula, the total number of valence electrons that now "belong" to each atom is subtracted by the number ''N'' of valence electrons of the neutral atom (such as 5 for nitrogen in
group 15 A pnictogen ( or ; from grc, wikt:πνίγω, πνῑ́γω "to choke" and wikt:-gen#English, -gen, "generator") is any of the chemical elements in group (periodic table), group 15 of the periodic table. This group is also known as the nit ...
) to yield that atom's oxidation state. This example shows the importance of describing the bonding. Its summary formula, HNO3, corresponds to two
structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct chemical bond, bon ...
s; the peroxynitrous acid in the above figure and the more stable
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid. The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into nitroge ...

nitric acid
. With the formula HNO3, the simple approach without bonding considerations yields −2 for all three oxygens and +5 for nitrogen, which is correct for nitric acid. For the peroxynitrous acid, however, the two oxygens in the O–O bond each has OS = −1 and the nitrogen has OS = +3, which requires a structure to understand.
Organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s are treated in a similar manner; exemplified here on
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
s occurring in between CH4 and : : Analogously for transition-metal compounds; on the left has a total of 36 valence electrons (18 pairs to be distributed), and on the right has 66 valence electrons (33 pairs): : A key step is drawing the Lewis structure of the molecule (neutral, cationic, anionic): atom symbols are arranged so that pairs of atoms can be joined by single two-electron bonds as in the molecule (a sort of "skeletal" structure), and the remaining valence electrons are distributed such that sp atoms obtain an
octet Octet may refer to: Music * Octet (music) In music, an octet is a musical ensemble consisting of eight Musical instrument, instruments or voices, or a musical composition written for such an ensemble. Octets in classical music Octets in Europea ...
(duet for hydrogen) with a priority that increases in proportion with electronegativity. In some cases, this leads to alternative formulae that differ in bond orders (the full set of which is called the resonance formulas). Consider the
sulfate The sulfate or sulphate ion is a polyatomic anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...

sulfate
anion ( with 32 valence electrons; 24 from oxygens, 6 from sulfur, 2 of the anion charge obtained from the implied cation). The
bond order Bond order, as introduced by Linus Pauling Linus Carl Pauling (; February 28, 1901 – August 19, 1994) was an American , , , , author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific topics ...
s to the terminal oxygens do not affect the oxidation state so long as the oxygens have octets. Already the skeletal structure, top left, yields the correct oxidation states, as does the Lewis structure, top right (one of the resonance formulas): : The bond-order formula at the bottom is closest to the reality of four equivalent oxygens each having a total bond order of 2. That total includes the bond of order to the implied cation and follows the 8 − ''N'' rule requiring that the main-group atom's bond order equals 8 minus ''N'' valence electrons of the neutral atom, enforced with a priority that proportionately increases with electronegativity. This algorithm works equally for molecular cations composed of several atoms. An example is the
ammonium The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

ammonium
cation of 8 valence electrons (5 from nitrogen, 4 from hydrogens, minus 1 electron for the cation's positive charge): : Drawing Lewis structures with electron pairs as dashes emphasizes the essential equivalence of bond pairs and lone pairs when counting electrons and moving bonds onto atoms. Structures drawn with electron dot pairs are of course identical in every way: :


The algorithm's caveat

The algorithm contains a caveat, which concerns rare cases of transition-metal complexes with a type of
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
that is reversibly bonded as a
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty Non-bonding orbital, orbital which is capable of accepting an electron pair from a Lewis Base (chemistry), base to form a Lewis ad ...

Lewis acid
(as an acceptor of the electron pair from the transition metal); termed a "Z-type" ligand in Green's
covalent bond classification methodThe covalent bond classification (CBC) method is also referred to as the LXZ notation. It was published by Malcolm Green (chemist), M. L. H. Green in the mid-1990s as a solution for the need to describe covalent compounds such as organometallic comp ...
. The caveat originates from the simplifying use of electronegativity instead of the MO-based electron allegiance to decide the ionic sign. One early example is the O2S−RhCl(CO)()2 complex with SO2 as the reversibly-bonded acceptor ligand (released upon heating). The Rh−S bond is therefore extrapolated ionic against Allen electronegativities of
rhodium Rhodium is a chemical element with the Symbol (chemistry), symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion, corrosion-resistant, and chemically inert transition metal. It is a noble metal and a member of the pla ...

rhodium
and sulfur, yielding oxidation state +1 for rhodium: :


Algorithm of summing bond orders

This algorithm works on Lewis structures and bond graphs of extended (non-molecular) solids:


Applied to a Lewis structure

An example of a Lewis structure with no formal charge, : illustrates that, in this algorithm, homonuclear bonds are simply ignored (the bond orders are in blue). Carbon monoxide exemplifies a Lewis structure with
formal charges In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
: : To obtain the oxidation states, the formal charges are summed with the bond-order value taken positively at the carbon and negatively at the oxygen. Applied to molecular ions, this algorithm considers the actual location of the formal (ionic) charge, as drawn in the Lewis structure. As an example, summing bond orders in the
ammonium The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

ammonium
cation yields −4 at the nitrogen of formal charge +1, with the two numbers adding to the oxidation state of −3: : The sum of oxidation states in the ion equals its charge (as it equals zero for a neutral molecule). Also in anions, the formal (ionic) charges have to be considered when nonzero. For sulfate this is exemplified with the skeletal or Lewis structures (top), compared with the bond-order formula of all oxygens equivalent and fulfilling the octet and 8 − ''N'' rules (bottom): :


Applied to bond graph

A
bond graph A bond graph is a Graph (discrete mathematics), graphical representation of a physical Dynamical system, dynamic system. It allows the conversion of the system into a state-space representation. It is similar to a block diagram or signal-flow grap ...
in
solid-state chemistry Solid-state chemistry, also sometimes referred as materials chemistry, is the study of the synthesis, structure, and properties of solid phase materials, particularly, but not necessarily exclusively of, non-molecular solids. It therefore has a str ...
is a chemical formula of an extended structure, in which direct bonding connectivities are shown. An example is the AuORb3
perovskite Perovskite (pronunciation: ) is a calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuc ...

perovskite
, the unit cell of which is drawn on the left and the bond graph (with added numerical values) on the right: : We see that the oxygen atom bonds to the six nearest
rubidium Rubidium is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science th ...

rubidium
cations, each of which has 4 bonds to the auride anion. The bond graph summarizes these connectivities. The bond orders (also called bond valences) sum up to oxidation states according to the attached sign of the bond's ionic approximation (there are no formal charges in bond graphs). Determination of oxidation states from a bond graph can be illustrated on
ilmenite Ilmenite is a titanium-iron oxide mineral The oxide mineral class includes those mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composi ...
, FeTiO3. We may ask whether the mineral contains Fe2+ and Ti4+, or Fe3+ and Ti3+. Its crystal structure has each metal atom bonded to six oxygens and each of the equivalent oxygens to two
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
s and two
titanium Titanium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

titanium
s, as in the bond graph below. Experimental data show that three metal-oxygen bonds in the octahedron are short and three are long (the metals are off-center). The bond orders (valences), obtained from the bond lengths by the bond valence method, sum up to 2.01 at Fe and 3.99 at Ti; which can be rounded off to oxidation states +2 and +4, respectively: :


Balancing redox

Oxidation states can be useful for balancing chemical equations for oxidation-reduction (or
redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

redox
) reactions, because the changes in the oxidized atoms have to be balanced by the changes in the reduced atoms. For example, in the reaction of
acetaldehyde Ethanal (common name acetaldehyde) is an organic chemical compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
with
Tollens' reagent Tollens' reagent (chemical formula Ag(NH3)2OH ) is a chemical reagent A reagent is a substance or compound added to a system to cause a chemical reaction A chemical reaction is a process that leads to the chemical transformation of on ...
to form
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
(shown below), the
carbonyl In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

carbonyl
carbon atom changes its oxidation state from +1 to +3 (loses two electrons). This oxidation is balanced by reducing two Ag+ cations to Ag0 (gaining two electrons in total). : An inorganic example is the Bettendorf reaction using SnCl2 to prove the presence of
arsenite In chemistry, an arsenite is a chemical compound containing an arsenic Arsenic is a chemical element with the Symbol (chemistry), symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and ...
ions in a concentrated
HCl The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with ...

HCl
extract. When arsenic(III) is present, a brown coloration appears forming a dark precipitate of
arsenic Arsenic is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

arsenic
, according to the following simplified reaction: : 2 As3+ + 3 Sn2+ → 2 As0 + 3 Sn4+ Here three
tin Tin is a with the Sn (from la, ) and  50. Tin is a silvery-colored metal that characteristically has a faint yellow hue. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent ...

tin
atoms are oxidized from oxidation state +2 to +4, yielding six electrons that reduce two arsenic atoms from oxidation state +3 to 0. The simple one-line balancing goes as follows: the two redox couples are written down as they react; :As3+ + Sn2+ As0 + Sn4+. One tin is oxidized from oxidation state +2 to +4, a two-electron step, hence 2 is written in front of the two arsenic partners. One arsenic is reduced from +3 to 0, a three-electron step, hence 3 goes in front of the two tin partners. An alternative three-line procedure is to write separately the
half-reaction A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads t ...
s for oxidation and reduction, each balanced with electrons, and then to sum them up such that the electrons cross out. In general, these redox balances (the one-line balance or each half-reaction) need to be checked for the ionic and electron charge sums on both sides of the equation being indeed equal. If they are not equal, suitable ions are added to balance the charges and the non-redox elemental balance.


Appearances


Nominal oxidation states

A nominal oxidation state is a general term for two specific purpose-oriented values: *
Electrochemical Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...

Electrochemical
oxidation state; it represents a molecule or ion in the
Latimer diagram A Latimer diagram of a chemical element is a summary of the standard electrode potential data of that element. This type of diagram is named after Wendell Mitchell Latimer, an American chemist. Construction In a Latimer diagram, the most highly ox ...
or
Frost diagram A Frost diagram or Frost–Ebsworth diagram is a type of graph used by inorganic chemists in electrochemistry to illustrate the relative stability of a number of different oxidation states of a particular substance. The graph illustrates the Gibbs ...
for its redox-active element. An example is the Latimer diagram for
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
at pH 0 where the electrochemical oxidation state +2 for sulfur puts between S and : :: * Systematic oxidation state; it is chosen from close alternatives for pedagogical reasons of descriptive chemistry. An example is the oxidation state of phosphorus in (which is in fact the
diprotic An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid). The fir ...
HPO(OH)2) taken nominally as +3, while Allen electronegativities of
phosphorus Phosphorus is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
suggest +5 by a narrow margin that makes the two alternatives almost equivalent: : Both alternative oxidation states of phosphorus make chemical sense, depending on the chemical property or reaction we wish to emphasize. In contrast, any mathematical alterations, such as the average (+4) does not.


Ambiguous oxidation states

Lewis formulae are fine rule-based approximations of chemical reality, as indeed are Allen electronegativities. Still, oxidation states may seem ambiguous when their determination is not straightforward. Rule-based oxidation states feel ambiguous when only experiments can decide. There are also truly
dichotomous A dichotomy is a partition of a whole (or a set) into two parts (subsets). In other words, this couple of parts must be * jointly exhaustive: everything must belong to one part or the other, and * mutually exclusive In logic and probabilit ...

dichotomous
values to be decided by mere convenience.


Oxidation-state determination from resonance formulas is not straightforward

Seemingly ambiguous oxidation states are obtained on a set of
resonance Resonance describes the phenomenon of increased amplitude The amplitude of a ic is a measure of its change in a single (such as or ). There are various definitions of amplitude (see below), which are all s of the magnitude of the differ ...

resonance
formulas of equal weights for a molecule of heteronuclear bonds where the atom connectivity does not correspond to the number of two-electron bonds dictated by the 8 − ''N'' rule. An example is S2N2 where four resonance formulas featuring one S=N double bond have oxidation states +2 and +4 on the two sulfur atoms, to be averaged to +3 because the two sulfur atoms are equivalent in this square-shaped molecule.


A physical measurement is needed to decide the oxidation state

* This happens when a non-innocent
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
is present, of hidden or unexpected redox properties that could otherwise be assigned to the central atom. An example is the
nickel Nickel is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

nickel
dithiolate complex, . * When the redox ambiguity of a central atom and ligand yields dichotomous oxidation states of close stability, thermally induced
tautomerism Tautomers () are structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with ...
may result, as exemplified by
manganese Manganese is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical e ...

manganese
catecholate, . Assignment of such oxidation states in general requires spectroscopic, magnetic or structural data. * When the bond order has to be ascertained along with an isolated tandem of a heteronuclear and a homonuclear bond. An example is thiosulfate with two oxidation-state alternatives (bond orders are in blue and formal charges in green): :: :The S–S distance in thiosulfate is needed to reveal that this bond order is very close to 1, as in the formula on the left.


Truly ambiguous oxidation states occur

* When the electronegativity difference between two bonded atoms is very small (as in above). Two almost equivalent pairs of oxidation states, open for a choice, are obtained for these atoms. * When an electronegative p-block atom forms solely homonuclear bonds, the number of which differs from the number of two-electron bonds suggested by Octet rule, rules. Examples are homonuclear finite chains like azide, (the central nitrogen connects two atoms with four two-electron bonds while only three two-electron bonds are required by Octet rule, 8 − ''N'' rule) or triiodide, (the central iodine connects two atoms with two two-electron bonds while only one two-electron bond fulfills the 8 − ''N'' rule). A sensible approach is to distribute the ionic charge over the two outer atoms. Such a placement of charges in a polysulfide (where all inner sulfurs form two bonds, fulfilling the 8 − ''N'' rule) follows already from its Lewis structure. * When the isolated tandem of a heteronuclear and a homonuclear bond leads to a bonding compromise in between two Lewis structures of limiting bond orders. An example here is nitrous oxide, N2O: :: :The typically-used oxidation state of nitrogen in N2O is +1, which also obtains for both nitrogens by a molecular orbital approach. The formal charges on the right comply with electronegativities, and this implies an added ionic bonding contribution. Indeed, the estimated N−N and N−O bond orders are 2.76 and 1.9, respectively, approaching the formula of integer bond orders that would include the ionic contribution explicitly as a bond (in green): :: :Conversely, formal charges against electronegativities in a Lewis structure decrease the bond order of the corresponding bond. An example is carbon monoxide with a bond-order estimate of 2.6.


Fractional oxidation states

Fractional oxidation states are often used to represent the average oxidation state of several atoms of the same element in a structure. For example, the formula of
magnetite Magnetite is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure for ...

magnetite
is , implying an average oxidation state for iron of +. However, this average value may not be representative if the atoms are not equivalent. In a crystal below , two-thirds of the cations are and one-third are , and the formula may be more specifically represented as FeO·. Likewise, propane, , has been described as having a carbon oxidation state of −. Again, this is an average value since the structure of the molecule is , with the first and third carbon atoms each having an oxidation state of −3 and the central one −2. An example with true fractional oxidation states for equivalent atoms is potassium superoxide, . The diatomic superoxide ion has an overall charge of −1, so each of its two equivalent oxygen atoms is assigned an oxidation state of −. This ion can be described as a resonance (chemistry), resonance hybrid of two Lewis structures, where each oxygen has an oxidation state of 0 in one structure and −1 in the other. For the cyclopentadienyl anion , the oxidation state of C is −1 + − = −. The −1 occurs because each carbon is bonded to one hydrogen atom (a less electronegative element), and the − because the total ionic charge of −1 is divided among five equivalent carbons. Again this can be described as a resonance hybrid of five equivalent structures, each having four carbons with oxidation state −1 and one with −2. : Finally, fractional oxidation numbers should not be used in naming. Lead(II,IV) oxide, Red lead, , is represented as lead(II,IV) oxide, showing the actual two oxidation states of the nonequivalent lead (metal), lead atoms.


Elements with multiple oxidation states

Most elements have more than one possible oxidation state. For example, carbon has nine possible integer oxidation states from −4 to +4: :


Oxidation state in metals

Many compounds with Lustre (mineralogy), luster and electrical conductivity maintain a simple stoichiometric formula; such as the golden titanium monoxide, TiO, blue-black ruthenium dioxide, RuO2 or coppery rhenium trioxide, ReO3, all of obvious oxidation state. Ultimately, however, the assignment of the free metallic electrons to one of the bonded atoms has its limits and leads to unusual oxidation states. Simple examples are the LiPb and Cu3Au ordered alloys, the composition and structure of which are largely determined by Atomic radius, atomic size and Atomic packing factor, packing factors. Should oxidation state be needed for redox balancing, it is best set to 0 for all atoms of such an alloy.


List of oxidation states of the elements

This is a list of known oxidation states of the chemical elements, excluding oxidation state#Fractional oxidation states, nonintegral values. The most common states appear in bold. The table is based on that of Greenwood and Earnshaw, with additions noted. Every element exists in oxidation state 0 when it is the pure non-ionized element in any phase, whether monatomic or polyatomic allotrope. The column for oxidation state 0 only shows elements known to exist in oxidation state 0 in compounds.


Early forms (octet rule)

A figure with a similar format was used by Irving Langmuir in 1919 in one of the early papers about the octet rule. The periodicity of the oxidation states was one of the pieces of evidence that led Langmuir to adopt the rule. :


Use in nomenclature

The oxidation state in compound naming for transition metals and lanthanides and actinides is placed either as a right superscript to the element symbol in a chemical formula, such as FeIII or in parentheses after the name of the element in chemical names, such as iron(III). For example, is named iron(III) sulfate and its formula can be shown as Fe. This is because a sulfate ion has a charge of −2, so each iron atom takes a charge of +3.


History of the oxidation state concept


Early days

Oxidation itself was first studied by
Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ,; 26 August 17438 May 1794), When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air and t ...

Antoine Lavoisier
, who defined it as the result of reactions with
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
(hence the name). The term has since been generalized to imply a ''formal'' loss of electrons. Oxidation states, called ''oxidation grades'' by Friedrich Wöhler in 1835, were one of the intellectual stepping stones that Dmitri Mendeleev used to derive the periodic table. William B. Jensen gives an overview of the history up to 1938.


Use in nomenclature

When it was realized that some metals form two different binary compounds with the same nonmetal, the two compounds were often distinguished by using the ending ''-ic'' for the higher metal oxidation state and the ending ''-ous'' for the lower. For example, FeCl3 is ferric chloride and FeCl2 is ferrous chloride. This system is not very satisfactory (although sometimes still used) because different metals have different oxidation states which have to be learned: ferric and ferrous are +3 and +2 respectively, but cupric and cuprous are +2 and +1, and stannic and stannous are +4 and +2. Also, there was no allowance for metals with more than two oxidation states, such as vanadium with oxidation states +2, +3, +4, and +5. This system has been largely replaced by one suggested by Alfred Stock in 1919 and adopted by IUPAC in 1940. Thus, FeCl2 was written as iron(II) chloride rather than ferrous chloride. The Roman numeral II at the central atom came to be called the "Stock nomenclature, Stock number" (now an obsolete term), and its value was obtained as a charge at the central atom after removing its ligands along with the electron pairs they shared with it.


Development towards the current concept

The term "oxidation state" in English chemical literature was popularized by Wendell Mitchell Latimer in his 1938 book about electrochemical potentials. He used it for the value (synonymous with the German term ''Wertigkeit'') previously termed "valence", "polar valence" or "polar number" in English, or "oxidation stage" or indeed the "state of oxidation". Since 1938, the term "oxidation state" has been connected with electrochemical potentials and electrons exchanged in redox couples participating in redox reactions. By 1948, IUPAC used the 1940 nomenclature rules with the term "oxidation state", instead of the original ''valency''. In 1948 Linus Pauling proposed that oxidation number could be determined by extrapolating bonds to being completely ionic in the direction of
electronegativity Electronegativity, symbolized as '' χ'', is the tendency for an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume ...

electronegativity
. A full acceptance of this suggestion was complicated by the fact that the Electronegativity#Pauling electronegativity, Pauling electronegativities as such depend on the oxidation state and that they may lead to unusual values of oxidation states for some transition metals. In 1990 IUPAC resorted to a postulatory (rule-based) method to determine the oxidation state. This was complemented by the synonymous term oxidation number as a descendant of the Stock number introduced in 1940 into the nomenclature. However, the terminology using "ligands" gave the impression that oxidation number might be something specific to coordination complexes. This situation and the lack of a real single definition generated numerous debates about the meaning of oxidation state, suggestions about methods to obtain it and definitions of it. To resolve the issue, an IUPAC project (2008-040-1-200) was started in 2008 on the "Comprehensive Definition of Oxidation State", and was concluded by two reports and by the revised entries "Oxidation State" and "Oxidation Number" in the IUPAC Gold Book. The outcomes were a single definition of oxidation state and two algorithms to calculate it in molecular and extended-solid compounds, guided by Allen electronegativities that are independent of oxidation state.


See also

* Electronegativity * Electrochemistry * Atomic orbital * Atomic shell * Quantum numbers ** Azimuthal quantum number ** Principal quantum number ** Magnetic quantum number ** Spin quantum number * Aufbau principle ** Wiswesser's rule * Ionization energy * Electron affinity * Ionic potential * Ions ** Cations and Anions ** Polyatomic ions * Covalent bonding * Metallic bonding * Orbital hybridisation, Hybridization


References

{{Oxide Chemical properties Chemical nomenclature Coordination chemistry Dimensionless numbers of chemistry