Rule 1
Free elements have an oxidation number of 0

• Any element in its pure form (not in a compound) has an oxidation number of 0.

• Examples: O₂, H₂, Na, Fe all have 0 oxidation numbers.

Rule 2
Monoatomic ions have oxidation numbers equal to their charge

• The oxidation number of a single-atom ion is the same as its charge.

• Examples: Na⁺ = +1, Cl⁻ = -1, Mg²⁺ = +2

Rule 3
Oxygen is usually -2, except in Peroxides (-1) or with Fluorine (+2)

• Oxygen is normally -2 (e.g., H₂O, CO₂).

• In peroxides (H₂O₂), oxygen is -1.

• With fluorine (OF₂), oxygen is +2.The oxidation number of a single-atom ion is the same as its charge.

Rule 4
Hydrogen is +1 with nonmetals and -1 with metals

• In compounds like HCl or H₂O, hydrogen is +1.

• In metal hydrides like NaH, hydrogen is -1.

Rule 5
The sum of oxidation numbers in a neutral compound is 0

• Example: H₂O → H (+1) x 2 + O (-2) = 0

• Example:

• SO₄²⁻ → S (+6) + O₄ (-2 x 4) = -2

• Example:

• NO₃⁻ → N (+5) + O₃ (-2 x 3) = -1

​

​Rule 6
The sum of oxidation numbers in a polyatomic ion equals Its Charge

Use the rules for assigning oxidation numbers to answer the following questions.

​

Let's Play!

Use the rules to answer the following questions AND a few surprise questions. :)

​

​Challenge Round