To determine oxidation numbers, you follow a step-by-step method using the rules. Here’s a simple guide:
Steps to Find Oxidation Numbers
- Identify the type of substance
- If it’s a pure element → oxidation number is 0.
- If it’s an ion → oxidation number is equal to the ion’s charge.
- Apply fixed rules for common elements
- Group 1 metals = +1
- Group 2 metals = +2
- Fluorine = –1
- Oxygen = –2 (except in peroxides, superoxides, or OF₂)
- Hydrogen = +1 (except in metal hydrides, where it’s –1)
- Use the sum rule
- In a neutral compound, the sum of all oxidation numbers = 0.
- In a polyatomic ion, the sum = the ion’s charge.
- Assign unknown oxidation numbers
- After applying fixed rules, solve for the unknown oxidation number like in a simple math equation.
Examples in words
- In water: Oxygen is –2, hydrogen is +1 each, and the total = 0.
- In carbon dioxide: Each oxygen is –2, total for oxygen is –4, so carbon must be +4.
- In sulfate ion: Oxygen is –2 (×4 = –8), total charge is –2, so sulfur must be +6.
Simple trick: Start with known elements (like oxygen, hydrogen, fluorine, alkali/alkaline metals), then balance the rest so that the total matches the charge of the compound or ion.