Question

How do you calculate the oxidation number of an element in a compound?

Answer 1

You assign oxidation numbers to the elements in a compound by using the Rules for Oxidation Numbers.

Explanation:

1. The oxidation number of a free element is always 0.

2. The oxidation number of a monatomic ion equals the charge of the ion.

3. The oxidation number of `"H"` is +1, but it is -1 in when combined with less electronegative elements.

4. The oxidation number of `"O"` in compounds is usually -2, but it is -1 in peroxides.

5. The oxidation number of a Group 1 element in a compound is +1.

6. The oxidation number of a Group 2 element in a compound is +2.

7. The oxidation number of a Group 17 element in a binary compound is -1.

8. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.

9. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

EXAMPLE:

What is the oxidation number of `"Cr"` in `"CrCl"_3` ?

Solution:

We use what rules we can to determine the oxidation numbers.

Rule 7 states that the oxidation number of Cl is -1.

We write the oxidation number of the element above its symbol and the total for 3 Cl atoms below the symbol.

This gives `"Cr"stackrelcolor(blue)("-1")("Cl")_3`
`color(white)(mmmmmmll)stackrelcolor(blue)("-3"color(white)(mm))`.

Rule 8 states the numbers along the bottom must add up to zero. So the number under `"Cr"` must be +3.

This gives `"Cr"stackrelcolor(blue)("-1")("Cl")_3`
`color(white)(mmmmm)stackrelcolor(blue)("+3"color(white)(ll)"-3"color(white)(mm))`.

There is only one `"Cr"` atom, so its oxidation number is +3.

This gives `stackrelcolor(blue)(+3)("Cr")stackrelcolor(blue)("-1")("Cl")_3`
`color(white)(mmmmm)stackrelcolor(blue)("+3"color(white)(ll)"-3"color(white)(mm))`.

The oxidation number of `"Cr"` in `"CrCl"_3` is +3.

Here is a chart showing the oxidation numbers of the atoms in some common elements and compounds.