Chemical Formulas and
Chemical Compounds
Oxidation Numbers
Oxidation number
• A number assigned to an element in a
compound that indicates the general
distribution of electrons in the compound.
Rules for Assigning Oxidation Numbers
1. Atoms in a pure element (Na or O2) have an oxidation
number equal to zero
2. In a binary compound the more electronegative element
gets a negative charge and the other gets a positive charge.
3. Fluorine always has an oxidation # = -1
4. Oxygen usually has an oxidation # = -2 Peroxides are an
exception (H2O2) where oxidation # = -1
5. Hydrogen has an oxidation # = +1 when bonded to more
electronegative elements or -1 when bonded to metals
6. The sum of oxidation numbers is 0 for a neutral compound
7. The sum of oxidation numbers for a polyatomic ion is equal
to the charge of the ion
Sample Problems
a. KClO4
b. SO32-
c. NF3
d. CO2
e. NO3f. NH4Cl