In Chemistry, the state of oxidation , described by the number of oxidation (n.o.) or the step oxidation (d.o.) is a concept being used to characterize the electronic state of a Atome in a chemical Espèce (Molécule, Ion, radical) compared to the elementary state neutral being used as reference (n.o = 0).
In the case of monoatomic ions, it corresponds to the value of the load carried by the ion (example: Na+; n.o = +1).
On the other hand, in the case of the other chemical species, it indicates the formal number of electron S which each atom would have given or received compared to the neutral state if the connections in which these atoms are implied were of purely ionic nature (see ionic Connection). Actually, except in the case or the difference of electronegativity between two elements is very important, the connections are covalent and present a partial character ionic, which means that there is a partial transfer of load between the dependant atoms. If the electronegativity of the two dependant atoms is the same one (for example, if the dependant atoms are the same element), then the connection does not contribute to the calculation of the n.o. Le number of oxidation is thus in this case a convenient conceptual approximation when one considers the reactions of Oxydo-réduction and the electrochemical reactions . It facilitates the follow-up of the electrons and assistance to check that they are well preserved.
The atoms are defined like having a number of null oxidation, meaning that they are electrically neutral. The positive loads of the Proton S of the core balance the loads of the cloud of negative electrons which surrounds it.
If an atom loses an electron, it has more protons than of electrons and becomes a positive ion. It is said that this Ion has a number of oxidation n.o.= +1.
Reciprocally, if an atom accepts an electron, it becomes negatively charged and its number of oxidation becomes n.o.= - 1.
In short, if an atom or an ion gives an electron in a reaction its state of oxidation is increased by 1; if an atom accepts an electron, its state of oxidation is decreased by 1.
In the neutral or ionic chemical species, the numbers of oxidation are noted by Roman numerals between brackets, just placed after the element concerned in order to take account of the transfer partial of electrons.
For example, the iron oxide (III) corresponds to the Fe2O3 formula, different from iron oxide (II) of FeO formula.
In the same way, the ion tétraoxomanganate (VII) corresponds to manganese mn (VII) and to the MnO4- formula, still called Permanganate.
Rules defining the number of oxidation: n.o.
In a heteropolyatomic chemical species (made up of atoms of different nature), the atom having the most affinity for the electrons, i.e. more electronegative, is regarded as receiving the electrons.
Example: H2O corresponds to 2:00 (+I) and O (- II)
In a neutral chemical species (molecule or radical), the sum of the n.o of the constitutive atoms is null. On the other hand if the compound is ionic, this sum is equal to the load of the ion.
Example: SO42- (ion Sulfate) corresponds to S (VI) and 4 O (- II)
In a neutral homopolyatomic chemical species (made up of atoms of comparable nature), the n.o of each atom is null.
Example: O2 (Dioxygène); O3 (Ozone); N2 (Diazotizes)
Particular case of an ionic homopolyatomic species: Hg22+ (ion dimercure (+I) constituting Calomel); n.o (Hg) = +1.
In a heteropolyatomic chemical species, if there are covalent bonds between atoms of comparable nature, they do not contribute to the calculation of the n.o
Example: C2H6 or H3C-CH3 (ethane), the states of oxidation is H (I) and C (- III).
the hydrogen engaged in a made up body has in the quasi-general information of the cases a n.o. = +1 except for the hydrides of metals such as NaH or LiH (lithium or sodium hydride), for which the n.o. (H) = - 1.
committed oxygen is generally with the state of oxidation n.o. = - 2. There exist some exceptions.
- Example: H2O2 which corresponds to H-O-O-H, more known under the name of Hydrogen peroxide.
Examples of elements to multiple numbers of oxidation
Chlorine can have several numbers of oxidation:
n.o. = -1 Cl- Chloride
n.o. = 0 Cl2 Dichlore
n.o. = +3 ClO2- Chlorite or dioxochlorate (III)
n.o. = +5 ClO3- Chlorate or trioxochlorate (V) known for its explosive properties with the shocks (KClO3: potassium chlorate)
n.o. = +7 ClO4- Perchlorate or tétraoxochlorate (VII) known in the shape of the perchloric acid H ClO4; acid most extremely which exists in water.
It is the same for the others Halogène S, Iode and Brome except for the Fluor which is more electronegative than oxygen.
In the case of the oxides iron:
- in the Wustite FeO, it acts of iron (+II);
- in the Hematite Fe2O3, it acts of iron (+III);
- in the Magnetite Fe3O4, it acts of iron (+II) and iron (+III): Fe (II) Fe (III) 2O4
- valence (out-of-date concept)
- Spectrometry Mössbauer
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