Determine oxidation states for elements in compounds and understand redox chemistry
Oxidation Number: The charge an atom would have if all bonds were ionic
Sum of oxidation numbers in neutral compound = 0
Sum of oxidation numbers in ion = ion charge
Use capital letters for elements, numbers for subscripts (H2O, not h2o)
0 for neutral molecules, positive/negative for ions (e.g., -2 for SO₄²⁻)
Optional: Specify Known Oxidation State
• Alkali metals (Li, Na, K): Always +1
• Alkaline earth metals (Mg, Ca, Ba): Always +2
• Hydrogen: Usually +1 (except in metal hydrides: -1)
• Oxygen: Usually -2 (except in peroxides: -1, and OF₂: +2)
• Halogens (F, Cl, Br, I): Usually -1 when in compounds
• Free elements: Always 0
• Monatomic ions: Equal to ion charge
An oxidation number (or oxidation state) is a number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element. It helps track electron transfer in redox (reduction-oxidation) reactions.
The oxidation number of any free (uncombined) element is 0.
Examples: O₂, H₂, N₂, Fe, Cu, S₈ → all have oxidation number 0
The oxidation number of a monatomic ion equals its charge.
Examples: Na⁺ = +1, Cl⁻ = -1, Ca²⁺ = +2, O²⁻ = -2, Al³⁺ = +3
Hydrogen is usually +1, except in metal hydrides where it is -1.
H₂O: H = +1 | HCl: H = +1 | NaH: H = -1 | LiH: H = -1
Oxygen is usually -2, except in peroxides (-1) and when bonded to fluorine (+2).
H₂O: O = -2 | CO₂: O = -2 | H₂O₂: O = -1 | OF₂: O = +2
Group 1 elements (Li, Na, K, Rb, Cs) are always +1 in compounds.
NaCl: Na = +1 | KBr: K = +1 | Li₂O: Li = +1
Group 2 elements (Be, Mg, Ca, Sr, Ba) are always +2 in compounds.
MgCl₂: Mg = +2 | CaO: Ca = +2 | BaSO₄: Ba = +2
Halogens (F, Cl, Br, I) are usually -1 in binary compounds.
HCl: Cl = -1 | NaBr: Br = -1 | But in ClO₃⁻: Cl = +5 (not binary)
The sum of oxidation numbers must equal:
H₂SO₄: (+1)×2 + S + (-2)×4 = 0 → S = +6
SO₄²⁻: S + (-2)×4 = -2 → S = +6
Given: H₂SO₄ is a neutral molecule (charge = 0)
Known: H = +1, O = -2
2(+1) + S + 4(-2) = 0
2 + S - 8 = 0
S = +6
Answer: S has oxidation number +6
Given: Cr₂O₇²⁻ has charge = -2
Known: O = -2
2(Cr) + 7(-2) = -2
2Cr - 14 = -2
2Cr = +12
Cr = +6
Answer: Cr has oxidation number +6
Given: NH₃ is neutral (charge = 0)
Known: H = +1
N + 3(+1) = 0
N + 3 = 0
N = -3
Answer: N has oxidation number -3
Oxidation numbers are essential for identifying and balancing redox reactions(reduction-oxidation reactions), where electrons are transferred between species.
Zn + Cu²⁺ → Zn²⁺ + Cu
• Oxidation: Zn (0) → Zn²⁺ (+2) — loses 2e⁻
• Reduction: Cu²⁺ (+2) → Cu (0) — gains 2e⁻
Zinc is oxidized (reducing agent)
Copper is reduced (oxidizing agent)
| Element | Common States | Examples |
|---|---|---|
| Nitrogen (N) | -3, +3, +5 | NH₃ (-3), HNO₂ (+3), HNO₃ (+5) |
| Sulfur (S) | -2, +4, +6 | H₂S (-2), SO₂ (+4), H₂SO₄ (+6) |
| Chlorine (Cl) | -1, +1, +3, +5, +7 | HCl (-1), ClO₃⁻ (+5), ClO₄⁻ (+7) |
| Iron (Fe) | +2, +3 | FeCl₂ (+2), Fe₂O₃ (+3) |
| Manganese (Mn) | +2, +4, +7 | MnCl₂ (+2), MnO₂ (+4), KMnO₄ (+7) |
| Chromium (Cr) | +3, +6 | Cr₂O₃ (+3), K₂Cr₂O₇ (+6) |