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

2. 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.

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

4. 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

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

6. 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

7. 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

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

9. 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-

10. 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

11. Oxidation Number Examples
HCl
PCl3
SO2
HNO3
P4O10
HClO3
N2O5

12. 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.

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