Oxidation Number Calculator

Author: Henrick Yau
- 8 October 2026
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Calculators

Calculate oxidation numbers (oxidation states) for elements in chemical compounds and ions. This calculator helps chemistry students and professionals determine oxidation states using established rules and principles.

Chemical Formula Input

Enter compound or ion formula (use ^ for superscripts: SO4^2-)

Calculation Settings

Leave blank to calculate all elements

Advanced Options

Quick Examples

Oxidation Number Theory

Oxidation numbers (oxidation states) represent the hypothetical charge an atom would have if all bonding electrons were assigned to the more electronegative atom. They are essential for understanding redox reactions and electron transfer.

Fundamental Rules for Assigning Oxidation Numbers
  1. Free Elements: Oxidation number is 0 (e.g., H₂, O₂, Na, Cu)
  2. Monatomic Ions: Oxidation number equals the charge (e.g., Na⁺ = +1, Cl⁻ = -1)
  3. Hydrogen: Usually +1 (except in metal hydrides where it's -1)
  4. Oxygen: Usually -2 (except in peroxides -1, and compounds with F)
  5. Fluorine: Always -1 in compounds
  6. Group 1 metals: Always +1 in compounds
  7. Group 2 metals: Always +2 in compounds
  8. Sum Rule: Sum of oxidation numbers equals total charge
Common Oxidation States
  • Carbon: -4 to +4 (most common: -4, 0, +2, +4)
  • Nitrogen: -3 to +5 (most common: -3, 0, +3, +5)
  • Sulfur: -2 to +6 (most common: -2, 0, +4, +6)
  • Phosphorus: -3 to +5 (most common: -3, +3, +5)
  • Transition Metals: Variable states (e.g., Fe: +2, +3; Cr: +3, +6)
  • Halogens: -1 to +7 (most common: -1, +1, +3, +5, +7)
Special Cases and Exceptions
  • Peroxides: Oxygen has -1 oxidation state (e.g., H₂O₂, Na₂O₂)
  • Superoxides: Oxygen has -½ oxidation state (e.g., KO₂)
  • Metal Hydrides: Hydrogen has -1 oxidation state (e.g., NaH, CaH₂)
  • Oxygen Fluorides: Oxygen can be positive (e.g., OF₂: O = +2)
  • Complex Ions: Consider ligand charges separately
  • Organometallics: Special consideration for metal-carbon bonds
Applications of Oxidation Numbers
  • Balancing Redox Equations: Track electron transfer
  • Identifying Redox Reactions: Changes in oxidation states indicate redox
  • Predicting Reaction Products: Common oxidation states help predict products
  • Naming Compounds: Roman numerals indicate oxidation states
  • Understanding Bonding: Relate to electronegativity and electron distribution
  • Electrochemistry: Essential for understanding electrode reactions

Disclaimer: This calculator provides oxidation numbers based on standard rules and conventions. In some complex cases, alternative assignments may be possible. Always verify results with chemical literature for critical applications. The calculator is designed for educational purposes and may not cover all exceptional cases in advanced chemistry.

What the oxidation number calculator does

The Oxidation Number Calculator is an interactive tool that helps users identify the oxidation states of elements within chemical compounds and ions. It is especially useful for students, teachers, and chemistry professionals who need to analyse redox reactions, assign oxidation numbers for compounds, or understand chemical bonding and structure.

Oxidation number sum formula:
∑(oxidation number × number of atoms) = total charge

How the calculator determines oxidation states

The calculator uses well-established rules and optionally algebraic methods to determine oxidation numbers for each element in a compound or ion. By inputting a chemical formula, users receive a breakdown of each element’s oxidation state, calculation steps, verification, and common examples.

Entering a formula and setting options

  • Enter a Chemical Formula: Type in a chemical compound or ion (e.g., H2SO4, NH4+, Cr2O7^2-).
  • Select the Formula Type: Choose from Neutral Compound, Cation, Anion, or Complex Ion.
  • Specify Charge (if needed): For ions, set the magnitude and sign of the charge.
  • Optional – Target Element: Focus the calculation on one element or leave blank for all.
  • Choose Calculation Method: Use Standard Rules, Algebraic Method, or Both.
  • Advanced Options: Toggle options to show rules, calculation steps, validation, and similar examples.
  • Click "Calculate Oxidation Numbers": Results appear instantly below.

What the calculator handles and shows

  • Supports both neutral compounds and charged ions.
  • Handles complex formulas with proper charge notation.
  • Offers step-by-step reasoning and applied rules.
  • Validates input and result consistency.
  • Displays similar example compounds for deeper learning.

Why oxidation numbers matter for redox chemistry

Understanding oxidation numbers is essential for balancing redox equations, analysing electron transfer, and identifying oxidation-reduction reactions. This calculator:

  • Saves time in assigning oxidation states manually.
  • Enhances learning with visual breakdown and theory.
  • Improves accuracy in chemical problem-solving.
  • Supports chemistry education and lab preparation.

Examples You Can Try

  • H₂SO₄ – Sulfuric acid (S: +6, O: -2)
  • NH₄⁺ – Ammonium ion (N: -3, H: +1)
  • Cr₂O₇²⁻ – Dichromate ion (Cr: +6, O: -2)
  • KMnO₄ – Potassium permanganate (Mn: +7)
  • Fe(CN)₆³⁻ – Ferricyanide complex

Questions about assigning oxidation numbers

What is an oxidation number?

An oxidation number represents the hypothetical charge an atom would have if all bonds were considered ionic. It helps track electron transfer in reactions.

Can this tool balance chemical equations?

No, this calculator focuses solely on oxidation numbers. For equation balancing, consider using a chemical equation balancer or stoichiometry calculator.

Is the oxidation number always a whole number?

Usually, yes. However, exceptions exist, such as in superoxides (e.g., KO₂), where oxygen has an oxidation state of -½.

Can I calculate oxidation numbers for polyatomic ions?

Yes. Enter the formula with its charge (e.g., PO4^3- or NO3-) and select "Anion" to analyse it correctly.

What if I don’t know the charge of the ion?

Use the standard notation or select a neutral compound. The calculator can often infer the correct states if the formula is valid.

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Educational purpose and limitations

This calculator is intended for educational purposes. While it applies standard rules, advanced compounds may require manual verification or reference to academic sources.