Oxidation Numbers Tutor

This Tutor will help you learn how to assign oxidation numbers in chemical formulas. To get started, click the "New Problem" button and enter your chemical formula in the dialog box. You may enter any formula you wish, or select a formula from the several examples provided on the "Choose a Formula" menu.

Once you have entered your formula, the Tutor will help you assign its oxidation numbers one step at a time. Simply type your work in the box and click "OK" to get feedback from the Tutor. There are two kinds of steps that can be taken in working oxidation number problems. To assign an oxidation number to an element, enter element_symbol = value, such as

Mg = +2

Sometimes it is also necessary to separate an ionic compound into its cation and anion in order to assign all the oxidation numbers. To do this, enter split cation_formula anion_formula, such as

split NH4+ SO4(-2)

If the Tutor does not understand what you typed, it will say so and you can try again.

Also, at any time you can click "Hint" to get a hint towards the next step to take, or "Next Step" to have the Tutor take the best next step for you, building on any work you have done so far.

Rules for Assigning Oxidation Numbers
The rules used for assigning oxidation numbers may differ slightly from those in your textbook. They are arranged in two levels, with the rules that are dependable enough to be used under any circumstances classified as "first priority," and two additional rules that are "second priority." When working a problem, always first check for first priority rules that can be used. Only when the first priority rules do not apply can you use a second priority rule.

Using a second priority rule over an applicable first priority rule is an error!

Oxidation Number Rules
First Priority
Free Element Rule The oxidation number of an atom of a free element equals zero.
Simple Ion Rule The oxidation number of a monatomic ion equals the charge on the ion.
Fluorine Rule The oxidation number of fluorine in compounds equals -1.
Hydrogen Rule The oxidation number of hydrogen in combination with nonmetals equals +1.
Group 1 Metal Rule The oxidation number of Group 1 metals in compounds equals +1.
Group 2 Metal Rule The oxidation number of Group 2 metals in compounds equals +2.
Sum Rule The algebraic sum of the oxidation numbers of all the atoms in a chemical formula equals the net charge on the species.
Separate Ions Rule In ionic compounds, the oxidation numbers in the cation and the anion are independent and can be assigned separately.

Second Priority
Oxygen Rule The oxidation number of oxygen in compounds equals -2.
Nonmetal Rule In binary combinations of nonmetals, the more electronegative element is given a negative oxidation number, equal to the charge on its common monatomic ion.