Summary Oxidation numbers notes

Oxidation Numbers notes
⚡ What is an Oxidation Number?
An oxidation number (or oxidation state) is a number assigned to an atom to show how
many electrons it has gained or lost (or appears to have) in a compound.

It helps us:

 Track electrons in reactions.
 Identify redox reactions (oxidation & reduction).




📜 Basic Oxidation Number Rules
Rule Example Explanation
1. Free elements = 0 O₂, H₂, Fe = 0 Element alone = zero
Na⁺ = +1, Cl⁻ = -
2. Monoatomic ions = ion charge Just use the charge
1
3. Group 1 metals = +1 Na⁺, K⁺ Always +1 in compounds
4. Group 2 metals = +2 Mg²⁺, Ca²⁺ Always +2 in compounds
Except in metal hydrides
5. Hydrogen = +1 HCl → H = +1
(e.g., NaH → H = -1)
Except in peroxides (e.g.,
6. Oxygen = -2 H₂O → O = -2
H₂O₂ → O = -1)
7. Halogens = -1 NaCl → Cl = -1 Unless bonded to oxygen
8. The sum of oxidation numbers = 0 for H₂O → H(+1)×2
neutral compounds, and = charge for ions + O(-2) = 0


🧪 Examples
A. Simple Compounds

Compound Oxidation Numbers Why?
H₂O H = +1, O = -2 2(+1) + (-2) = 0
CO₂ C = +4, O = -2 2(-2) + C = 0 → C = +4
NaCl Na = +1, Cl = -1 Sum = 0

B. Polyatomic Ions

Ion Oxidation Numbers Why?
SO₄²⁻ O = -2, S = +6 4(-2) + S = -2 → S = +6