Oxidation Number Formula
Systematic rules to assign oxidation states and identify redox reactions
Oxidation Number Rules (in Priority Order)
Elements in Free State
Oxidation number = 0
Examples: Na, O₂, H₂, Cl₂, P₄, S₈
Monatomic Ions
Oxidation number = charge of ion
Examples: Na⁺ = +1, Ca²⁺ = +2, Cl⁻ = -1, S²⁻ = -2
Hydrogen
Usually +1
Exception: In metal hydrides (NaH, CaH₂), H = -1
Oxygen
Usually -2
Exceptions:
- In peroxides (H₂O₂, Na₂O₂): O = -1
- In superoxides (KO₂): O = -½
- In OF₂: O = +2 (F is more electronegative)
Group 1 Metals (Alkali)
Always +1
Li, Na, K, Rb, Cs in compounds
Group 2 Metals (Alkaline Earth)
Always +2
Be, Mg, Ca, Sr, Ba in compounds
Fluorine
Always -1
Most electronegative element - no exceptions!
Sum Rule
Sum of all oxidation numbers:
- • In neutral compounds = 0
- • In polyatomic ions = charge of ion
Worked Examples
Example 1: H₂SO₄
Find oxidation number of S
Step 1: Assign known values
H = +1 (rule 3)
O = -2 (rule 4)
Step 2: Set up equation
2(+1) + S + 4(-2) = 0 (neutral compound)
2 + S - 8 = 0
S - 6 = 0
S = +6
Example 2: Cr₂O₇²⁻
Find oxidation number of Cr
Setup:
2(Cr) + 7(-2) = -2 (ion charge)
2Cr - 14 = -2
2Cr = +12
Cr = +6
Each Cr = +6
Example 3: NH₄⁺
Find oxidation number of N
Setup:
N + 4(+1) = +1 (ion charge)
N + 4 = +1
N = -3
N = -3
Example 4: Fe₃O₄ (Mixed Oxidation States)
Find average oxidation number of Fe
Setup:
3(Fe) + 4(-2) = 0
3Fe - 8 = 0
Feavg = +8/3 = +2.67
Average Fe = +8/3
Actually: Fe₃O₄ = FeO·Fe₂O₃ (1 Fe²⁺ and 2 Fe³⁺)
Example 5: Identify Redox Reaction
Zn + Cu²⁺ → Zn²⁺ + Cu
Assign oxidation numbers:
| Zn: | 0 → +2 | Oxidation (loses 2e⁻) |
| Cu: | +2 → 0 | Reduction (gains 2e⁻) |
Conclusion:
Zn is oxidized (reducing agent)
Cu²⁺ is reduced (oxidizing agent)
Common Oxidation States
Halogens
F: always -1
Cl, Br, I: usually -1
Exceptions: +1, +3, +5, +7 in oxoacids/oxoanions
ClO⁻: Cl = +1
ClO₄⁻: Cl = +7
Transition Metals
Variable oxidation states
Fe: +2, +3 (most common)
Cr: +2, +3, +6
Mn: +2, +4, +7
Cu: +1, +2
Non-metals
N: -3 to +5
S: -2, +4, +6
P: -3, +3, +5
C: -4 to +4
Common Mistakes
Confusing Oxidation Number with Charge
In H₂O, O has oxidation number -2, but no actual -2 charge! It's a bookkeeping tool.
Forgetting Exceptions for O
In H₂O₂, oxygen is -1 (peroxide), not -2!
Wrong Sum Value
Sum = 0 for neutral, sum = charge for ions. Don't mix up!
OIL RIG Memory Aid
Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)