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Oxidation States

Numbers representing electron transfer in chemical compounds

What are Oxidation States?

Oxidation state (oxidation number) is a number assigned to an atom in a chemical compound that represents the number of electrons lost (positive) or gained (negative). It's a bookkeeping tool to track electron transfer in redox reactions. Rules are somewhat arbitrary but consistent. Oxidation/reduction involve gaining or losing electrons.

📋 Rules for Assigning Oxidation States

1. Elemental Form

Atoms in elemental form = 0

Examples: O₂ = 0, H₂ = 0, Na = 0, Cl₂ = 0

2. Monatomic Ions

Ion's charge = oxidation state

Examples: Na⁺ = +1, O²⁻ = -2, Al³⁺ = +3, Cl⁻ = -1

3. Oxygen (Usual)

Usually -2 (except in peroxides = -1)

Examples: H₂O (O = -2), CO₂ (O = -2), H₂O₂ (O = -1)

4. Hydrogen (Usual)

Usually +1 (except in metal hydrides = -1)

Examples: H₂O (H = +1), NaH (H = -1), HCl (H = +1)

5. Halogens (Group 17)

Usually -1 (except with O or other halogens)

Examples: NaCl (Cl = -1), Cl₂ (Cl = 0), ClF (F = -1, Cl = +1)

6. Neutral Compounds

Sum of all oxidation states = 0

Example: CaCl₂: Ca = +2, Cl = -1 each, total = 0 ✓
Remember: These are RULES, not "real" charges. Real charges (formal charges, partial charges) are different. Oxidation states are a bookkeeping convention.

📊 Common Oxidation States by Element

Element Common Oxidation States Examples Notes
Alkali Metals (Group 1) +1 (always) NaCl, KOH, Li₂O Always lose 1 electron
Alkaline Earth (Group 2) +2 (always) CaCl₂, MgO, BaCl₂ Always lose 2 electrons
Aluminum (Group 13) +3 (usually) AlCl₃, Al₂O₃ Loses 3 valence electrons
Carbon (Group 14) -4, -2, 0, +2, +4 CH₄ (-4), CO (+2), CO₂ (+4) Variable, very important in organics
Nitrogen (Group 15) -3, -2, 0, +1, +2, +3, +4, +5 NH₃ (-3), N₂ (0), NO₂ (+4), HNO₃ (+5) VERY variable, important in redox
Sulfur (Group 16) -2, 0, +4, +6 H₂S (-2), S₈ (0), SO₂ (+4), H₂SO₄ (+6) Common in redox reactions
Halogens (Group 17) -1, 0, +1, +3, +5, +7 Cl⁻ (-1), Cl₂ (0), ClO₄⁻ (+7) Vary widely in oxycompounds

🔄 Redox Processes

Oxidation: Loss of electrons = increase in oxidation state (becomes more positive)
Reduction: Gain of electrons = decrease in oxidation state (becomes more negative)
Example: 2Na + Cl₂ → 2NaCl
Na: 0 → +1 (loses 1 electron, OXIDIZED)
Cl: 0 → -1 (gains 1 electron, REDUCED)
Na is reducing agent (causes reduction), Cl₂ is oxidizing agent (causes oxidation)

🔬 Why This Matters