World Library  
Flag as Inappropriate
Email this Article

Oxidation state

Article Id: WHEBN0000038452
Reproduction Date:

Title: Oxidation state  
Author: World Heritage Encyclopedia
Language: English
Subject: Oxide, Manganese, Molybdenum, Frost diagram, Protactinium
Publisher: World Heritage Encyclopedia

Oxidation state

The oxidation state, often called the oxidation number, is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state, which may be positive, negative or zero, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic, with no covalent component. This is never exactly true for real bonds.

The term "oxidation" was first used by Lavoisier to mean reaction of a substance with oxygen. Much later, it was realized that the substance on being oxidized loses electrons, and the use of the term "oxidation" was extended to include other reactions in which electrons are lost.

Oxidation states are typically represented by small integers. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe
). The highest known oxidation state is reported to be +9 in the cation IrO+
,[1] while the lowest known oxidation state is −4 for some elements in the carbon group. The possibility of +9 and +10 oxidation states in platinum group elements, especially iridium and platinum, has been discussed by Kiselev and Tretiyakov.[2]

The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.

There are various methods for determining oxidation states/numbers.

In inorganic nomenclature the oxidation state is determined and expressed as an oxidation number represented by a Roman numeral placed after the element name.

In coordination chemistry, oxidation number is defined differently from oxidation state.

IUPAC definitions of oxidation state and oxidation number

Oxidation state

A IUPAC technical report Toward a comprehensive definition of oxidation state has been published.[3] The current Gold Book definition of oxidation state listed by IUPAC is as follows:[4]

Determining the oxidation state or number

There are two different methodologies for determining the oxidation state of elements in chemical compounds. First, a rule-based approach to determine how the electrons are allocated and this method is based on the rules in the IUPAC definition (see above), and this approach is widely taught. Second, a method-based on the relative electronegativity of the elements in the compound, where in simple terms the more electronegative element is assumed to take the negative charge.

Simple examples using IUPAC definition

  • Any pure element—even if it forms diatomic molecules like chlorine (Cl2)—has an oxidation state of zero. Examples of this are Cu or O2.
  • For monatomic ions, the oxidation state is the same as the charge of the ion. For example, the sulfide anion (S2−) has an oxidation state of −2, whereas the lithium cation (Li+) has an oxidation state of +1.
  • The sum of oxidation states for all atoms in a molecule or polyatomic ion is equal to the charge of the molecule or ion. Thus, the oxidation state of one element can be calculated from the oxidation states of the other elements.
  1. An application of this rule is that the sum of the oxidation states of all atoms in a neutral molecule must be zero. Consider a neutral molecule of carbon dioxide, CO2. Oxygen is assumed to have its usual oxidation state of −2, and so the sum of the oxidation states of all the atoms can be expressed as X + 2(−2) = 0, or X − 4 = 0, where X is the unknown oxidation state of carbon. Thus, it can be seen that the oxidation state of carbon in the molecule is +4.
  2. In polyatomic ions, the sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. As an example, consider the sulfate anion, which has the formula SO42-. As indicated by the formula, the total charge of this ion is −2. Because all four oxygen atoms are assumed to have their usual oxidation state of −2, and the sum of the oxidation states of all the atoms is equal to the charge of the ion, the sum of the oxidation states can be represented as Y + 4(−2) = −2, or Y − 8 = −2, where Y is the unknown oxidation state of sulfur. Thus, it can be computed that Y = +6.

These facts, combined with some elements almost always having certain oxidation states (due to their very high electropositivity or electronegativity), allows one to compute the oxidation states for the remaining atoms (such as transition metals) in simple compounds.

Example for a complex salt: In Cr(OH)
, oxygen has an oxidation state of −2 (no fluorine or O–O bonds present), and hydrogen has a state of +1 (bonded to oxygen). So, each of the three hydroxide groups has an oxidation state of −2 + 1 = −1. As the compound is neutral, chromium has an oxidation state of +3.

Using electronegativity

The use of electronegativity in this way was introduced by Pauling in 1947.[5] This method of determining oxidation state is found in some recent text books.[6] This method allows the oxidation state of all atoms in a molecule to be determined whereas the IUPAC 1990/2005 definition does not.[7] In the 1970[8] rules IUPAC recommended that oxidation state was used in nomenclature and elsewhere in inorganic chemistry as the "charge that would be present on an atom if the electrons were assigned to the more electronegative atom", but with a convention that hydrogen is considered to be positive in combination with non-metals and a bond between like atoms makes no contribution to the oxidation number.

In practise the IUPAC 1990/2005 definition is usually extended by adding additional rules based on electronegativity.

  • Fluorine has an oxidation state of −1 when bonded to any other element, since it has the highest electronegativity of all reactive elements.
  • Halogens other than fluorine have an oxidation state of −1 except when they are bonded to oxygen, to nitrogen, or to another halogen that is more electronegative. For example, the oxidation state of chlorine in chlorine monofluoride (ClF) is +1. However, in bromine monochloride (BrCl), the oxidation state of Cl is −1.
  • Hydrogen has an oxidation state of +1 except when bonded to more electropositive elements such as sodium, aluminium, and boron, as in NaH, NaBH
    , LiAlH
    , where each H has an oxidation state of −1.
  • In compounds, oxygen typically has an oxidation state of −2, though there are exceptions that are listed below, such as peroxides (e.g. hydrogen peroxide H2O2), where oxygen has an oxidation state of −1.
  • Alkali metals have an oxidation state of +1 in virtually all of their compounds (exception, see alkalide).
  • Alkaline earth metals have an oxidation state of +2 in virtually all of their compounds.

Ionic compounds

If an ionic compound has two ions with a common element, such as ammonium nitrate, NH
, it is usual to consider the oxidation states for each ion separately. The overall empirical formula of ammonium nitrate is N
, which leads to an average nitrogen oxidation state of +1, but it is much more useful to consider separately the ions NH+
and NO
with nitrogen oxidation states of -3 and +5 respectively.[9]

Calculation of oxidation states with a Lewis structure

This method can be used for molecules when one has a Lewis structure.

It should be remembered that the oxidation state of an atom does not represent the "real" charge on that atom: This is particularly true of high oxidation states, where the ionization energy required to produce a multiply positive ion are far greater than the energies available in chemical reactions. The assignment of electrons between atoms in calculating an oxidation state is purely a formalism, but is a useful one for the understanding of many chemical reactions.

For more about issues with calculating atomic charges, see partial charge.

The Lewis structure

When a Lewis structure of a molecule is available, the oxidation states may be assigned by computing the difference between the number of valence electrons that a neutral atom of that element would have and the number of electrons that "belong" to it in the Lewis structure. For purposes of computing oxidation states, electrons in a bond between atoms of different elements belong to the more electronegative atom; electrons in a bond between atoms of the same element are split equally, and electrons in a lone pair belong only to the atom with the lone pair.[10]

For example, consider acetic acid:

The methyl group carbon atom has 6 valence electrons from its bonds to the hydrogen atoms because carbon is more electronegative than hydrogen. Also, 1 electron is gained from its bond with the other carbon atom because the electron pair in the C–C bond is split equally, giving a total of 7 electrons. A neutral carbon atom would have 4 valence electrons, because carbon is in group 14 of the periodic table. The difference, 4 – 7 = –3, is the oxidation state of that carbon atom. That is, if it is assumed that all the bonds were 100% ionic (which in fact they are not), the carbon would be described as C3-.

Following the same rules, the formal charges (calculated with all bonds covalent).

Inequivalent atoms of an element

Structure of the thiosulfate anion
An example of a molecule with inequivalent atoms of the same element is the thiosulfate ion (S2O32−), for which the algebraic sum rule yields the average value +2 for sulfur, where the two ionizing electrons are assigned to the terminal sulfur atom. However, the use of a Lewis structure and electron counting shows that the two sulfur atoms are different. The central sulfur is assigned only one valence electron from the S-S bond and no valence electrons from the S-O bonds, compared to six valence electrons for a free sulfur atom, so the oxidation state of the central sulfur is +5. The terminal sulfur atom is assigned the other electron from the S-S bond plus three lone pairs for a total of seven valence electrons, so its oxidation state is −1.

Redox reactions

Oxidation states can be useful for balancing chemical equations for oxidation-reduction (or 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 with the Tollens' reagent to acetic acid (shown below), the carbonyl carbon atom changes its oxidation state from +1 to +3 (oxidation). This oxidation is balanced by reducing two equivalents of silver from Ag+ to Ag0.

Change in oxidation state in Tollens reaction

Elements with multiple oxidation states

Most elements have more than one possible oxidation state. For example, carbon has nine integer oxidation states:
Integer oxidation states of carbon
Oxidation state Example compound
–4 CH
–3 C
–2 CH
–1 C
0 CH
+1 CHCl
+2 CHCl
+3 C
+4 CCl

Fractional oxidation states

Fractional oxidation states are often used to represent the average oxidation states of several atoms in a structure. For example, the formula of magnetite is Fe
, implying an oxidation state for iron of +83.[9] However this average value may not be representative if the atoms are not equivalent. In Fe
, two-thirds of the iron ions are Fe3+ and one-third Fe2+, and the formula may be better represented as FeO•Fe

Likewise, propane, C
, has been described as having a carbon oxidation state of -8/3.[11] Again this is an average value since the structure of the molecule is H3C-CH2-CH3 and the central carbon is not equivalent to the other two.

An example with fractional oxidation states for equivalent atoms is potassium superoxide, KO
. The diatomic superoxide ion has an overall charge of −1, so each of its two oxygen atoms is assigned an oxidation state of −½, This ion can be described as a resonance hybrid of two Lewis structures, where each oxygen has oxidation state 0 in one structure and −1 in the other.

For the cyclopentadienyl ion C
, the oxidation state of C is (−1) + (−15) = −65. The −1 occurs because each C is bonded to one hydrogen (a less electronegative element), and the −15 because the total ionic charge is divided among five equivalent C.

Examples of fractional oxidation states for carbon
Oxidation state Example species
65 C
67 C
54 C

Oxidation state and formal charge

The oxidation state of an atom is often different from the formal charge often included in Lewis structures (when it is non-zero). The oxidation state is calculated by assuming that each chemical bond (except between identical atoms) is ionic so that both electrons are assigned to the more electronegative bonded atom. In contrast, the formal charge is calculated by assuming that each bonds is covalent so that one electron is assigned to each bonded atom. For example, in ammonium ion (NH4+) the oxidation state of nitrogen is -3, as all eight valence electrons are assigned to the nitrogen atom that is more electronegative than hydrogen. However, the formal charge is +1, calculated by assigning only four valence electrons (one per bond) to nitrogen. For comparison, the nitrogen in ammonia (NH3) has oxidation state -3 also but a formal charge of zero. On protonation of ammonia to form ammonium, the formal charge on nitrogen changes, but its oxidation state does not.

Oxidation number in naming of inorganic compounds

In the Roman numeral. The oxidation number is equal to the oxidation state using the rules, although they acknowledge other methods can be used. Oxidation numbers must be positive or negative integers, fractional oxidation numbers should not be used and in the event of any uncertainty alternative naming conventions should be used.

Use in nomenclature

In older literature the term is referred to as [13]

Oxidation number in coordination compounds

Whist oxidation state and oxidation number are often used interchangeably, oxidation number is used in coordination chemistry with a slightly different meaning. In coordination chemistry, the rules used for counting electrons are different. Every electron in a metal-ligand bond belongs to the ligand, regardless of electronegativity, so that the oxidation number is the charge that would remain if all ligands were removed together with the electron pairs shared with the central atom.[14]

The current IUPAC definition of the oxidation number in a coordination compound is as follows:[15]

For most coordination complexes, the metal atom is the less electronegative end of each metal-ligand bond, so that this rule gives the same result as the electronegativity-based rule[7] There are exceptions, however, such as Wilkinson's catalyst RhCl(PPh3)3 (Ph = phenyl), in which the rhodium atom is more electronegative than phosphorus. Nevertheless the oxidation number of rhodium in this molecule is considered to be +1 and the molecule’s systematic name is chlorotris(triphenylphosphine)rhodium(I), as the electrons of each Rh-P bond are assigned to the P atom of the ligand. The electronegativity rule would assign them instead to the Rh with an oxidation state of −5.

Spectroscopic oxidation states vs. oxidation numbers

Although oxidation numbers can be helpful for classifying compounds, they are unmeasurable and their physical meaning can be ambiguous. Oxidation numbers require particular caution for molecules where the bonding is covalent, since the oxidation numbers require the spectroscopic and crystallographic data.[16]

Oxidation state can also have effect on spectroscopic studies of compounds. In infrared spectroscopy of metal carbonyls this effect is illustrated by using spectroscopic studies on metals from oxidation states of –2 to +2.

Unusual oxidation states

Unusual oxidation states of metals are important in biochemical processes, the notable ones being Fe(IV) and Fe(V) in Cytochrome P450-containing systems.

History of the oxidation number concept

First study

Oxidation itself was first studied by Antoine Lavoisier, who believed that what we now call oxidation was always the result of reactions with oxygen,[17] thus the name. Although Lavoisier's idea has been shown to be incorrect, the name he proposed is still used, albeit more generally.

Oxidation states were one of the intellectual "stepping stones" that Mendeleev used to derive the periodic table.

The Stock nomenclature (named for Alfred Stock who suggested it in 1919) was intended to replace the naming that was prevalent at the time. Under the Stock system FeCl2 came to be called iron(II) chloride rather than ferrous chloride.

Current concept

The current concept of "oxidation state" was introduced by W. M. Latimer in 1938.[18] In 1940 IUPAC recommended that the term Stock number should be replaced by the term oxidation number. In 1947 Pauling proposed that the oxidation number could be determined using the electronegativity of the atoms to determine the "ions" in the formal determination of oxidation number.[5] In 1970[8] IUPAC defined oxidation number in terms of electronegativity. In 1990 IUPAC changed course and adopted a rule based determination for the "central atom" rather than using electronegativity. This is the definition in the current gold book for "oxidation state". They also introduced the definition of oxidation number, shown in the current gold book, that appears to make oxidation number specific to coordination chemistry. This may not have been their intention, as in 2005 they issued new recommendations for inorganic nomenclature that define oxidation number in the same terms as the 1990 definition of oxidation state, and that oxidation number is, as in the earlier recommendations, used in the naming of inorganic compounds.

Oxidation number versus oxidation state

In general, in the wider field of chemistry the IUPAC definitions have not been adhered to and both terms are used interchangeably, as they were when Latimer introduced the concept in 1938.[18] For example, two well-known textbooks[19][20] use the term oxidation state and represent it in Roman numerals in chemical formulae. The point has been made that, if there is any semantic difference between the terms, then oxidation number refers to the specific numerical value assigned to the entity known as oxidation state, much as IUPAC use the term charge number to refer to the numerical value assigned to the entity know as ionic charge.[21] The [24]

See also


  1. ^ Wang, Guanjun; Zhou, Mingfei; Goettel, James T.; Schrobilgen, Gary G.; Su, Jing; Li, Jun; Schlöder, Tobias; Riedel, Sebastian (21 August 2014). "Identification of an iridium-containing compound with a formal oxidation state of IX". Nature 514: 475–477.  
  2. ^ Kiselev, Yurii M. and Tretiyakov, Yurii D. (1999), Russian Chemical Reviews, 68, 365-379 "The problem of oxidation state stabilisation and some regularities of a Periodic system of the elements" [1]
  3. ^ a b Karen, Pavel; McArdle, Patrick; Takats, Josef (2014). "Toward a comprehensive definition of oxidation state (IUPAC Technical Report)". Pure and Applied Chemistry 86 (6).  
  4. ^ IUPAC Gold Book definition: oxidation state
  5. ^ a b General Chemistry: An Introduction to Descriptive Chemistry and Modern Chemical Theory, Linus Pauling, W.H Freeman, 1947
  6. ^ Basic Concepts of Chemistry, 8th Edition, Leo J. Malone, Theodore Dolter, John Wiley & Sons, 2008, ISBN 047174154X , ISBN 978-0471741541
  7. ^ a b Loock, Hans-Peter (2011). "Expanded Definition of the Oxidation State". Journal of Chemical Education 88 (3): 282–283.  
  8. ^ a b Nomenclature of Inorganic chemistry, 2d Edition, Definitive rules 1970, Butterworths
  9. ^ a b c Petrucci R.H., Harwood W.S. and Herring F.G. General Chemistry (8th ed., Prentice-Hall 2002) pp.81-82
  10. ^ Spencer J.N., Bodner G.M. and Rickard L.H. Chemistry: Structure and Dynamics (5th ed., Wiley 2010), p.543. This reference uses 1-propanol as an example.
  11. ^ Whitten K.W., Galley K.D. and Davis R.E. General Chemistry (4th ed., Saunders 1992) p.147
  12. ^ IUPAC Gold Book definition: Stock number
  13. ^ Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing
  14. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "oxidation number".
  15. ^ IUPAC Gold Book definition: oxidation number
  16. ^ Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam, S.; Ray, K.; Weyhermueller, T.; Neese, F.; Wieghardt, K. (2005). "Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels functional, and correlated ab initio study". Chemistry - A European Journal 11 (1): 204–224.  
  17. ^ The Origin of the Oxidation-State Concept William B. Jensen J. Chem. Educ. 2007, 84, 1418
  18. ^ a b Jensen, William B. (2007). "The Origin of the Oxidation-State Concept". Journal of Chemical Education 84 (9): 1418.  
  19. ^  
  20. ^  
  21. ^ Jensen, William B. (2011). "Oxidation States versus Oxidation Numbers". Journal of Chemical Education 88 (12): 1599–1600.  
  22. ^ Red Book: IUPAC Nomenclature of Inorganic Chemistry. Third Edition, Blackwell Scientific Publications, Oxford, 1990.
  23. ^ Calvert, J. G. (1990). "Glossary of atmospheric chemistry terms (Recommendations 1990)". Pure and Applied Chemistry 62 (11): 2167–2219.  
  24. ^ "Project Details: Towards a comprehensive definition of oxidation state".  
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Hawaii eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.