Oxidation number

the oxidation number indicates, how a neutral atom has many electrons within a connection formally taken up (negative values) and/or delivered (positive values).

Another definition reads: The oxidation number of an atom in a chemical compound is formally a measure toIndication of conditions of the electron density around this atom. A positive oxidation number indicates that the electron density is degraded in relation to its normal condition, a negative indicates that the electron density is increased by the atom.

Table of contents

graphic regulation of

oxidation numbers

[work on] with redox reactions to recognize the procedures better. The transmission of the electrons of an atom on another shows up in the fact thatthe oxidation number one (delivers electrons) increased, the other one (takes up electrons) degrades itself.

Often becomes only clear by the determination of the oxidation numbers of individual atoms, which chemical reaction runs off.

indication of the oxidation number

of oxidation numbers become inConnections in Roman numbers over the atomic symbols written (ex.s. <math> O^ {- II}< /math>). If the element symbol stands alone, then they are written frequently as Arabic numerals as with ions.

determination of the oxidation number

the oxidation number leaves itself easy with the help of the following rulesdeduce:

  1. Atoms in the elementary condition have always the oxidation number of zero (in addition, zero is possible in connections).
  2. With ionic compounds the oxidation number of an element is identical to its ion charge.
  3. The sum of the oxidation numbers of all atoms of a mehratomigen connection is alikeZero.
  4. The sum of the oxidation numbers of all atoms of a mehratomigen ion is equal to the total load of this ion.
  5. With kovalent formulated connections (line formulas so mentioned, Lewis formulas) the connection is formally divided in ions. It is accepted that in a connection took partElectrons by the more electronegative atom to be completely taken over.
  6. Most elements arise in several Oxidationsstufen.

in practice

it proved auxiliary rules as helpful to formulate for the determination of the oxidation numbers some rules:

  1. Metal atoms get inConnections and as ions always a positive oxidation number.
  2. Alkali metals have always +I and alkaline earth metals always +II as oxidation number.
  3. The fluorine atom (F) always gets the oxidation number in connections except with itself - I.
  4. A hydrogen atom has generally the oxidation number +I.(Does not apply, if hydrogen is directly connected metals (hydrides) or with „electricalmore positive “atoms such as.)
  5. an oxygen atom has generally the oxidation number - II. (Oxygen does not apply directly with itself or with that in connections, in those more electronegative Fluor, sees auxiliary rule 9 is connected.)
  6. Halogen atoms (Fluor, Chlor, Brom, Iod) have generally the oxidation number - I, except in connection with oxygen or a halogen, which stands in the periodic system more highly.
  7. elementary condition: Oxidation number is zero (z.B. I 2, C, O 2, P 4, S 8)
  8. ionic compound: Sum of the oxidation numbers is identically to the ion charge
  9. kovalente connection: Bindungselektronen are assigned to the more electronegative connection partner, same connection partners received per half of the Bindungselektronen;the oxidation number corresponds to the assigned Bindungselektronen compared with the number of normally existing outer electrons.

graphic regulation of oxidation numbers

as example is to serve phosphoric acid (H 3 PO 4):


  • First the Lewis formula is noted.
  • Become subsequently,the electrons the atoms after the Elektronegativität assigned
  • on the basis of the electrons actually available in the outermost Valenzschale one can compute then the oxidation number (here for example oxygen actually 6, always two e - to it, thus oxidation number - II).

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