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Oxidation Numbers in Chemistry

Discover how oxidation numbers describe electron distribution in bonded atoms, learn how to assign them with examples and guidelines.

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Oxidation Numbers in Chemistry

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  1. Oxidation numbers

  2. Oxidation numbers • Oxidation numbers are used to describe the distribution of electrons among bonded atoms. • Covalent bonds involve sharing of electrons, but oxidation numbers show what the distribution would be if the electrons were completely transferred.

  3. Guidelines for Assigning Oxidation Numbers ( see p. 180 for a complete list) • The oxidation of any free (uncombined) element is zero. • The oxidation number of a monatomic ion is equal to the charge of the ion. • e.g.The oxidation number of K+ is +1. • The oxidation number of each hydrogen atom is +1, unless it is combined with a metal, then it has a state of -1. • The oxidation number of fluorine is always -1. • The oxidation number of each oxygen atom in most of its compounds is -2.

  4. Guidelines for Assigning Oxidation Numbers ( see p. 180 for a complete list) • The algebraic sum of the oxidation numbers for all the atoms in a compound is zero. • The algebraic sum of the oxidation numbers for all the atoms in a polyatomic ion is equal to the charge on that ion.

  5. Example: Determine the oxidation numbers for each atom in KMnO4 • This compound is made up of a K+ cation and an MnO4- anion. • The K+ is a monatomic ion with a charge of +1, so its oxidation number is +1. • Assume that each O atom has an oxidation number of -2. • The MnO4- has a total charge of -1. There are 4 O atoms, each with an oxidation number of -2 • The oxidation number of the Mn may be found by the equation: • Mn + 4(-2) = -1 • Therefore, the oxidation number of Mn in this compound is +7.

  6. Example: Determine the oxidation numbers for each atom in Co(NO2)2 • The anion is the nitrite ion, NO2-. • Assume that each O atom in the nitrite ion has an oxidation number of -2. • The NO2- has a total charge of -1. There are 2 O atoms, each with an oxidation number of -2 • The oxidation number of the N may be found by the equation: • N + 2(-2) = -1 • Therefore, the oxidation number of N in this compound is +3. • Since there are two nitrate ions, each with a charge of -1, the charge on the Co must be +2. As an ion, its oxidation number is equal to its charge.

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