Oxidation State Calculator
Determine oxidation numbers of elements in compounds and ions
Enter the formula without the charge symbol (e.g., Cr2O7 for Cr₂O₇²⁻)
Specify one element symbol, or leave blank to calculate all oxidation states
Quick Examples
Oxidation State(s)
Compound Information
Formula:
Type:
Charge:
Calculation Steps
Rules Applied
Rules for Determining Oxidation States
Rule 1: Elements in Their Natural State
The oxidation state of an element in its elemental form is always 0.
Examples: O₂, N₂, Fe, Na all have oxidation state 0
Rule 2: Monatomic Ions
The oxidation state of a monatomic ion equals its charge.
Examples: Na⁺ = +1, Cl⁻ = -1, Fe³⁺ = +3, O²⁻ = -2
Rule 3: Hydrogen
Hydrogen is usually +1, except in metal hydrides where it is -1.
Examples: H₂O (H = +1), NaH (H = -1)
Rule 4: Oxygen
Oxygen is usually -2, except in peroxides (-1) and superoxides (-½).
Examples: H₂O (O = -2), H₂O₂ (O = -1), KO₂ (O = -½)
Rule 5: Group 1 (Alkali Metals)
Always +1 in compounds.
Examples: Li, Na, K, Rb, Cs
Rule 6: Group 2 (Alkaline Earth Metals)
Always +2 in compounds.
Examples: Be, Mg, Ca, Sr, Ba
Rule 7: Halogens
Usually -1, except when bonded to oxygen or more electronegative halogens.
Examples: NaCl (Cl = -1), HOCl (Cl = +1)
Rule 8: Sum of Oxidation States
• In a neutral compound: sum = 0
• In a polyatomic ion: sum = charge of the ion
Worked Examples
Example 1: Sulfuric Acid (H₂SO₄)
Find the oxidation state of S:
1. H₂SO₄ is a neutral compound, so sum of oxidation states = 0
2. H is +1 (rule 3), O is -2 (rule 4)
3. Let x = oxidation state of S
4. 2(+1) + x + 4(-2) = 0
5. 2 + x - 8 = 0
6. x = +6
Answer: S has oxidation state +6
Example 2: Permanganate Ion (MnO₄⁻)
Find the oxidation state of Mn:
1. MnO₄⁻ has a charge of -1, so sum of oxidation states = -1
2. O is -2 (rule 4)
3. Let x = oxidation state of Mn
4. x + 4(-2) = -1
5. x - 8 = -1
6. x = +7
Answer: Mn has oxidation state +7
Example 3: Dichromate Ion (Cr₂O₇²⁻)
Find the oxidation state of Cr:
1. Cr₂O₇²⁻ has a charge of -2, so sum of oxidation states = -2
2. O is -2 (rule 4)
3. Let x = oxidation state of Cr
4. 2(x) + 7(-2) = -2
5. 2x - 14 = -2
6. 2x = 12
7. x = +6
Answer: Each Cr has oxidation state +6
Example 4: Hydrogen Peroxide (H₂O₂)
Find the oxidation state of O (special case):
1. H₂O₂ is a neutral compound, so sum of oxidation states = 0
2. H is +1 (rule 3)
3. This is a peroxide, so O follows the exception (rule 4)
4. Let x = oxidation state of O
5. 2(+1) + 2(x) = 0
6. 2 + 2x = 0
7. x = -1
Answer: O has oxidation state -1 (peroxide exception)
Common Oxidation States
Transition Metals
- • Fe: +2, +3
- • Cu: +1, +2
- • Cr: +2, +3, +6
- • Mn: +2, +4, +7
- • V: +2, +3, +4, +5
Non-Metals
- • N: -3, +3, +5
- • S: -2, +4, +6
- • P: -3, +3, +5
- • C: -4 to +4
- • Cl: -1, +1, +5, +7
Main Group Metals
- • Al: +3
- • Zn: +2
- • Ag: +1
- • Sn: +2, +4
- • Pb: +2, +4
Applications of Oxidation States
1. Balancing Redox Reactions
Oxidation states help identify which species are oxidized (increase in oxidation state) and which are reduced (decrease in oxidation state), making it easier to balance redox equations.
2. Naming Compounds
IUPAC nomenclature uses oxidation states to distinguish between different oxidation states of the same element (e.g., Iron(II) chloride vs. Iron(III) chloride).
3. Predicting Reactivity
Elements in high or low oxidation states tend to be more reactive. Understanding oxidation states helps predict which compounds will act as oxidizing or reducing agents.
4. Electrochemistry
Oxidation states are fundamental in understanding electrode reactions, battery chemistry, and corrosion processes.
5. Coordination Chemistry
In complex ions and coordination compounds, determining the oxidation state of the central metal ion is crucial for understanding structure and reactivity.
References
The oxidation state rules and calculations are based on fundamental inorganic chemistry principles from reputable sources:
Note: This calculator uses standard oxidation state rules for determining oxidation numbers. For complex compounds or unusual bonding situations, consult advanced inorganic chemistry references. Oxidation states are formal charges and don't always represent actual electron distribution in covalent bonds.
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