Oxidation Number Calculator
Instantly determine the oxidation states of atoms in any compound or ion using our smart chemical parser.
Quick Rules
- F is always -1
- Group 1 is always +1
- Group 2 is always +2
- O is usually -2
- H is usually +1
- Sum of states = Charge
This tool assumes standard states for common ligands. For complex coordination compounds, specific exceptions may apply.
The Ultimate Oxidation Number Calculator
Oxidation numbers are the hidden currency of chemistry. Whether you are balancing a tough Redox equation for an exam or analyzing a corrosion problem in engineering, knowing the charge distribution is critical. Our Oxidation Number Calculator makes this process instant, turning complex algebra into a single click.
What is an Oxidation State?
Think of oxidation state as a "theoretical charge." We imagine that every bond in a molecule is 100% ionic—meaning the more electronegative atom "steals" both electrons. The resulting charge on the atom is its oxidation number.
This helps chemists track electron flow. If an atom's number increases during a reaction, it has lost electrons (Oxidation). If it decreases, it gained electrons (Reduction).
How to Use This Tool
- 1Enter Formula: Type any chemical formula. It works for simple stuff like
CO2effectively. - 2Add Charges (Optional): For ions, add the charge at the end, like
SO4^2-or justSO4-2. - 3Get Breakdown: The tool will apply the hierarchy of rules and solve algebraically for the unknown atom step-by-step.
The 6 Golden Rules of Assignment
Finding oxidation numbers manually requires you to follow a strict hierarchy. Rules higher on the list always beat rules lower down.
1. Free Elements are Zero
Rule #0Any pure element (Fe, O₂, P₄, S₈) has a state of 0.
2. The "Ion" Rule
Rule #1For a monoatomic ion, the state equals the charge (e.g., Na⁺ is +1, Cl⁻ is -1).
3. Fluorine is King
Rule #2Fluorine is always -1 in commands. No exceptions.
4. Oxygen is Usually -2
Rule #3Oxygen is -2, unless bonded to F (becomes +) or in a peroxide (-1).
5. Hydrogen is +1
Rule #4H is +1 with non-metals. However, with metals (Hydrides), it becomes -1.
6. The Sum Rule
Rule #5Sum of all states = Net Charge. This allows you to "Solve for X" for everything else.
Why This Matters (Real World)
Oxidation states aren't just for chemistry exams. They explain fundamental processes in our world:
- 🔋 BatteriesLithium-ion batteries work by shuttling Li⁺ ions, changing the oxidation state of the cathode material (like Cobalt) to store energy.
- 🦀 CorrosionRust is simply Iron ($Fe^0$) oxidizing to Iron Oxide ($Fe^3$). Understanding this helps engineers prevent bridges from collapsing.
- 🌿 BiologyCellular respiration is a giant Redox chain. Your body "oxidizes" glucose ($C^0$ to $C^4$) to release energy for you to live.
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Frequently Asked Questions
Common questions about oxidation states, exceptions, and calculator usage.
What is an oxidation number?
An oxidation number (or state) is a theoretical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. It tracks electron ownership.
How do I calculate oxidation numbers manually?
Use the 'Sum Rule': The sum of all oxidation numbers in a neutral molecule is 0, while for an ion, it equals the charge. Assign knowns (like O=-2, H=+1) and solve for the unknown variable algebraically.
What is the oxidation number of Oxygen in H2O2?
In hydrogen peroxide (H2O2), Oxygen behaves as a peroxide with an oxidation state of -1, instead of its usual -2. This is a common exception.
Can an element have a fractional oxidation number?
Yes! While atoms don't split electrons, partial charges occur in resonance. For example, in the superoxide ion (KO2), oxygen has an average state of -1/2.
Why is Fluorine always -1?
Fluorine is the most electronegative element (4.0 on Pauling scale). It is the 'bully' of the periodic table and always attracts electrons more strongly than any partner.
How does this help with Redox reactions?
In a Redox reaction, one species is oxidized (number increases) and one is reduced (number decreases). Finding these numbers is the critical first step to balancing these equations.
What is the difference between Valency and Oxidation State?
Valency tells you 'how many' bonds an atom can form (capacity) and is usually a fixed whole number. Oxidation state tells you the 'electronic status' in a specific compound and can vary widely.
Is this tool free to use?
Yes, standard to all FreeTools Pro utilities, this calculator is 100% free, runs in your browser client-side, and requires no login or installation.
Does it calculate transition metals?
Absolutely. Transition metals (like Fe, Mn) are 'variable' elements. Our tool uses the fixed rules of ligands (like O, Cl) to isolate and solve for the metal's state algebraically.
What are the rules for Group 1 and 2 metals?
Alkali metals (Group 1 like Li, Na, K) are always +1. Alkaline Earth metals (Group 2 like Mg, Ca) are always +2. They are excellent 'anchors' for solving unknown states.
What happens with metal hydrides?
When Hydrogen bonds with a metal (like NaH), the metal is less electronegative and gives its electron to Hydrogen. This forces Hydrogen to take a -1 state (Hydride ion).
How to handle polyatomic ions?
You can treat the ion as a single block if you know its charge, or break it down. Our tool allows you to input the ion formula (e.g., 'PO4 3-'). Our tool will break it down to P(+5) and O(-2).