Understanding Oxidation Numbers
* Definition: An oxidation number (or oxidation state) represents the hypothetical charge an atom would have if all its bonds were completely ionic.
* Key Points:
* They can be positive, negative, or zero.
* They are used to track electron transfer in chemical reactions.
* The sum of oxidation numbers in a neutral molecule is always zero.
* The sum of oxidation numbers in a polyatomic ion equals the charge of the ion.
Rules for Assigning Oxidation Numbers
1. Elements in their Elemental State: Oxidation number is always zero.
* Examples: Na (sodium), O₂ (oxygen gas), Fe (iron)
2. Monatomic Ions: The oxidation number is equal to the charge of the ion.
* Examples: Na⁺ (+1), Cl⁻ (-1), Mg²⁺ (+2)
3. Group 1 (Alkali Metals): Always have a +1 oxidation number.
4. Group 2 (Alkaline Earth Metals): Always have a +2 oxidation number.
5. Fluorine: Always has a -1 oxidation number.
6. Oxygen: Usually has a -2 oxidation number. Exceptions include peroxides (O₂²⁻, where oxygen is -1) and superoxides (O₂⁻, where oxygen is -1/2).
7. Hydrogen: Usually has a +1 oxidation number, except in metal hydrides (e.g., NaH), where it has a -1 oxidation number.
8. Halogens: Usually have a -1 oxidation number, except when bonded to oxygen or another halogen higher up in the group.
9. Sum of Oxidation Numbers: In a neutral molecule, the sum of oxidation numbers must equal zero. In a polyatomic ion, the sum must equal the charge of the ion.
Examples
1. H₂O (Water)
* Oxygen (O) has a -2 oxidation number (rule 6).
* Hydrogen (H) has a +1 oxidation number (rule 7).
* Total: (+1) + (+1) + (-2) = 0
2. SO₄²⁻ (Sulfate Ion)
* Oxygen (O) has a -2 oxidation number (rule 6).
* Since there are four oxygen atoms, the total is -8.
* The overall charge of the ion is -2.
* To balance, sulfur (S) must have a +6 oxidation number.
3. KMnO₄ (Potassium Permanganate)
* Potassium (K) has a +1 oxidation number (rule 3).
* Oxygen (O) has a -2 oxidation number (rule 6).
* Total for the four oxygens: -8
* To balance, manganese (Mn) must have a +7 oxidation number.
Tips
* Start with the elements that have fixed oxidation numbers.
* Use the sum of oxidation numbers rule to solve for the unknown.
* Remember to account for the number of atoms of each element.
Important Note: Oxidation numbers are a theoretical concept. They don't always reflect the actual charges on atoms in a molecule. They are primarily used to help understand and predict the behavior of chemical reactions.
