Fluorine is assigned an oxidation number of -1 in compounds

Oxygen is assigned an oxidation number of -2 in compounds

Except in

Peroxides

Superoxides

Oxygen fluorides

Hydrogen is assigned an oxidation of +1 in compounds

Group 1 elements (Alkali Metals) are assigned an oxidation number of +1 in compounds

Group 2 elements (Alkaline-earth metals) are assigned an oxidation number of +2 in compounds

An atom of any element in the free state has an oxidation number of 0

Any monatomic ion has an oxidation number equal to its charge

The sum of the oxidation numbers of all the atoms in formula equals the electrical charge shown with the formula

Examples: Calculating an Oxidation Number (oxidation state)

Find the oxidation number (state) of carbon in CO_{2}

Write a general equation to calculate the oxidation number (state) for the unknown atom:
charge on molecule = carbon's oxidation number + 2 x oxygen's oxidation number

Use the Rules above to list the oxidation numbers (states) for the molecule and each element:
CO_{2} is a neutral molecule so its overall electrical charge is 0 (rule h above)
The oxidation number (state) for oxygen is -2 (rule b above)
let x be the unknown oxidation number (state) of carbon

Substitute in the values for the known oxidation numbers (states) in the general equation
charge on molecule = carbon's oxidation number + 2 x oxygen's oxidation number
0 = x + (2 x -2)

Solve the equation to determine the oxidation number (state) of the unknown atom:
x (oxidation number of C) = 0 + 4 = +4
The oxidation number (state) of carbon in CO_{2} is +4

Find the oxidation number (state) for chromium in Cr_{2}O_{7}^{2-}

Write a general equation to calculate the oxidation number (state) for the unknown atom:
charge on ion = 2 x chromium's oxidation number + 7 x oxygen's oxidation number

Use the Rules above to list the oxidation numbers (states) for the molecule and each element:
Cr_{2}O_{7}^{2-} is an ion with an overall electrical charge of 2- (rule h above)
The oxidation number (state) for oxygen is -2 (rule b above)
let x be the unknown oxidation number (state) of chromium

Substitute in the values for the known oxidation numbers (states) in the general equation
charge on ion = 2 x chromium's oxidation number + 7 x oxygen's oxidation number
2- = 2x + (7 x -2)
2- = 2x - 14

Rearrange the equation and solve it to determine the oxidation number (state) of the unknown atom:
2x = 2- + 14
2x = 12
x = 12 ÷ 2 = +6
The oxidation number (state) of chromium in Cr_{2}O_{7}^{2-} is +6