Chapter 7 – Chemical Formulas and Chemical Compounds 7-2: Oxidation Numbers

Oxidation Numbers Oxidation numbers don’t have an exact physical meaning. They are numbers assigned to the atoms which make up a compound or ion. They indicate the general distribution of electrons among bonded atoms in a molecular compound or a polyatomic ion They are useful in naming compounds, writing formulas, and in balancing chemical equations.

Oxidation Number Rules 1. The atoms in a pure element have an oxidation number of 0. Na O2 Au F2 S8

Oxidation Number Rules 2. The more electronegative element in a binary molecular compound gets an oxidation number equal to the negative charge it would have as an anion. The less electronegative element is assigned an oxidation number equal to the positive charge it would have as a cation. Ex. As2S5

Oxidation Number Rules Fluorine is always assigned an oxidation number of -1 when it is in a compound Ex. CF4

Oxidation Number Rules 4. Oxygen has an oxidation number of -2 in almost all compounds. Exceptions: Peroxides like H2O2 when it is -1 In compounds with halogens like OF2 when it is +2

Oxidation Number Rules 5. Hydrogen has an oxidation number of +1 in all compounds containing more electronegative elements. It has an oxidation number of -1 in compounds with metals Example: H2O KH

Oxidation Number Rules 6. The algebraic sum of the oxidation numbers of all atoms in a neutral compound is zero Ex. CF4

Oxidation Number Rules 7. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion Example: SO42-

Oxidation Number Rules 8. Rules 1-7 apply to covalently bonded atoms, but oxidation numbers can also be assigned in ionic compounds. In monatomic ions, the oxidation number is equal to the charge

Oxidation Number Examples HCl PCl3 SO2 HNO3 P4O10 HClO3 N2O5

Using Oxidation Numbers for Formulas and Names Many nonmetals can have more than one oxidation number. These numbers can be used in the same manner as ionic charges to determine formulas The Stock System can be used as an alternative to the prefix system in naming binary molecular compounds.

Naming binary molecular compounds Name the following compounds according to the prefix system and using the Stock system: CI4 SO3 As2S3 NCl3