oxidation states
Information about oxidation states
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are represented by Arabic numerals and can be positive, negative, or zero. Thus, H+ would have an oxidation state of +1.
The increase in oxidation state of an atom 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 more about issues with calculating atomic charges, see partial charge.
From a Lewis structure
When a Lewis structure of a molecule is available, the oxidation states may be assigned unambiguously 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 most electronegative atom; electrons in a bond between atoms of the same element are split equally, and electrons in lone pair belong only to the atom with the lone pair.
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 carboxylic acid carbon atom has an oxidation state of +3 (it only gets one valence electron from the C–C bond; the oxygen atoms get all the other electrons because oxygen is more electronegative than carbon). The oxygen atoms both have an oxidation state of –2; they get 8 electrons each (4 from the lone pairs and 4 from the bonds), while a neutral oxygen atom would have 6. The hydrogen atoms all have oxidation state +1, because they surrender their electron to the more electronegative atoms to which they are bonded.
In the reaction of acetaldehyde with the Tollens' reagent to acetic acid it can be seen that in this reaction the carbonyl carbon atoms changes its oxidation state from +1 to +3 (oxidation). At the same time two equivalents of silver Ag+ are reduced to Ago.
Without a Lewis structure The algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. This fact, combined with the fact that some elements almost always have certain oxidation states, allows one to compute the oxidation states for atoms in simple compounds.
With the example, Cr(OH)3, oxygen has an oxidation state of −2 (no fluorine, O-O bonds present), and hydrogen has a state of +1 (bonded to oxygen). So, the triple hydroxide group has a charge of 3 × (−2 + 1) = −3. As the compound is neutral, Cr has an oxidation state of +3.
For an exhaustive list of the possible oxidation states of each element, see the Standard Periodic Table.
The increase in oxidation state of an atom 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.
Formal vs. spectroscopic oxidation states
Although formal oxidation states can be helpful for classifying compounds, they are unmeasureable and their physical meaning can be ambiguous. Formal oxidation states require particular caution for molecules where the bonding is covalent, since the formal oxidation states require the heterolytic removal of ligands, which essentially denies covalency. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghart, are measureables that are bench-marked using spectroscopic and crystallographic data.[1] Like many concepts in chemistry, spectroscopic oxidation states are powerful but require collateral measurements. Formal oxidation states, on the other hand, result from arithmetic rules, not bonding. Skill in assigning formal oxidation states is considered essential, especially in inorganic chemistry.Calculation of formal oxidation states
There are two common ways of computing the oxidation state of an atom in a compound. The first one is used for molecules when one has a Lewis structure, as is often the case for organic molecules, while the second one is used for simple compounds (molecular or not) and does not require a Lewis structure. Note that there is a minor difference between Oxidation number and Oxidation state. For example in case of Manganese in Potassium Permanganate, Oxidation number is +7 while oxidation state is Mn+7. Hence the difference is only in the way of representation. 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, albeit a useful one for the understanding of many chemical reactions.For more about issues with calculating atomic charges, see partial charge.
From a Lewis structure
When a Lewis structure of a molecule is available, the oxidation states may be assigned unambiguously 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 most electronegative atom; electrons in a bond between atoms of the same element are split equally, and electrons in lone pair belong only to the atom with the lone pair.
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 carboxylic acid carbon atom has an oxidation state of +3 (it only gets one valence electron from the C–C bond; the oxygen atoms get all the other electrons because oxygen is more electronegative than carbon). The oxygen atoms both have an oxidation state of –2; they get 8 electrons each (4 from the lone pairs and 4 from the bonds), while a neutral oxygen atom would have 6. The hydrogen atoms all have oxidation state +1, because they surrender their electron to the more electronegative atoms to which they are bonded.
In the reaction of acetaldehyde with the Tollens' reagent to acetic acid it can be seen that in this reaction the carbonyl carbon atoms changes its oxidation state from +1 to +3 (oxidation). At the same time two equivalents of silver Ag+ are reduced to Ago.
Change in oxidation state in Tollens reaction
)
Without a Lewis structure The algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. This fact, combined with the fact that some elements almost always have certain oxidation states, allows one to compute the oxidation states for atoms in simple compounds.
Other rules and guidelines
- Hydrogen has an oxidation state of +1 except when bonded to more electropositive elements such as sodium, aluminium, and boron, as in NaH, NaBH4, LiAlH4
- Oxygen has an oxidation state of −2 except where it is −1 in peroxides, −1/2 in superoxides, and of +2 in oxygen difluoride, OF2,+1 in O2F2.
- 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.
- Halogens have an oxidation state of −1 except when they are bonded to oxygen or with another halogen.
With the example, Cr(OH)3, oxygen has an oxidation state of −2 (no fluorine, O-O bonds present), and hydrogen has a state of +1 (bonded to oxygen). So, the triple hydroxide group has a charge of 3 × (−2 + 1) = −3. As the compound is neutral, Cr has an oxidation state of +3.
Elements with multiple oxidation states
Most elements have more than one possible oxidation state — with carbon having nine, as follows below:- –4: CH4
- –3: C2H6
- –2: CH3F
- –1: C2H2
- 0: CH2F2
- +1: C2H2F4
- +2: CHF3
- +3: C2F6
- +4: CF4
For an exhaustive list of the possible oxidation states of each element, see the Standard Periodic Table.
History
The concept of oxidation state in its current meaning was introduced by W.M. Latimer in 1938. Oxidation itself was first studied by Antoine Lavoisier who then held the belief that oxidation was literally the results of reactions of the elements with oxygen and that the common bond in any salt was based on oxygen [2]References
1. ^ Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam, S.; Ray, K.; Weyhermueller, T.; Neese, F.; Wieghardt, K., "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: An experimental, density functional, and correlated ab initio study", Chemistry--A European Journal, 2005, 11, 204-224.
2. ^ The Origin of the Oxidation-State Concept William B. Jensen J. Chem. Educ. 2007, 84, 1418
2. ^ The Origin of the Oxidation-State Concept William B. Jensen J. Chem. Educ. 2007, 84, 1418
See also
External links
- "Oxidation state"PDF (3.82 KiB) from the IUPAC Gold Book
- "High Oxidation States of 5d Transition Metals"
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Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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atom (Greek ἄτομος or átomos meaning "indivisible") is the smallest particle still characterizing a chemical element.
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Flavour in particle physics
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ionic bond (or electrovalent bond) is a type of chemical bond based on electrostatic forces between two oppositely-charged ions. In ionic bond formation, a metal donates an electron, due to a low electronegativity, to form a positive ion or cation.
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Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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In chemistry, heterolysis or heterolytic fission (from Greek ἑτερος, heteros, "different," and λυσις, lusis,
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In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as
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Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
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Lewis structures, also called Lewis-dot diagrams, electron-dot structures or electron-dot diagrams, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule [1] [2].
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The ionization potential, ionization energy or EI of an atom or molecule is the energy required to remove one mole of electrons from one mole of isolated gaseous atoms or ions.
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A partial charge is a charge with an absolute value of less than one elementary charge unit (that is, smaller than the charge of the electron).
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Partial atomic charges
Partial charges are created due to the asymmetric distribution of electrons in chemical bonds...... Click the link for more information.
Lewis structures, also called Lewis-dot diagrams, electron-dot structures or electron-dot diagrams, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule [1] [2].
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In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less
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Electronegativity, symbol χ, is a chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself.[1] First proposed by Linus Pauling in 1932 as a development of valence bond theory,[2]
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A lone pair is a (valence) electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons.
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Acetic acid, also known as ethanoic acid, is an organic chemical compound best recognized for giving vinegar its sour taste and pungent smell. Its structural formula is represented as CH3COOH.
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In chemistry, a methyl group is a hydrophobic alkyl functional group derived from methane (CH4). It has the formula -CH3 and is very often abbreviated as -Me in the structure of a molecule. This hydrocarbon unit can be found in many organic compounds.
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A carbon-carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond – a bond composed of two electrons, one from each of the two atoms.
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The carbon group is group 14 (IUPAC style) in the periodic table. Once also known as the tetrels (from Latin tetra, four), stemming from the earlier naming convention of this group as Group IVB.
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Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)OH, usually written -COOH or -CO2H. [1] Carboxylic acids are Bronsted acids — they are proton donors.
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Acetaldehyde, sometimes known as ethanal, is an organic chemical compound with the formula CH3CHO or MeCHO. It is a flammable liquid with a fruity smell.
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Tollens' reagent is usually ammoniacal silver nitrate, but can also be other things, as long as there is an aqueous diamminesilver(I) complex.
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Uses
The diaminesilver(I) complex is an oxidizing agent, which is itself reduced to silver metal, which in a clean glass reaction..... Click the link for more information.
Silver (IPA: /ˈsɪlvə(ɹ)/) is a chemical element with the symbol Ag (Latin: argentum) and atomic number 47.
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More
Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous
Thermal conductivity (300 K) 180.
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(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More
Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous
Thermal conductivity (300 K) 180.
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Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
..... Click the link for more information.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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Sodium (IPA: /ˈsəʊdiəm/) is a chemical element which has the symbol Na (Latin: natrium), atomic number 11, atomic mass 22.9898 g/mol, common oxidation number +1.
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Aluminium (IPA: /ˌæljʊˈmɪniəm/, /ˌæljəˈmɪniəm/) or aluminum (IPA: /əˈluːmɪnəm/
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