Oxidation numbers by group
| Group | Family | Common oxidation numbers | Example |
|---|---|---|---|
| 1 | Alkali metals | +1 | Na is +1 in NaCl |
| 2 | Alkaline earth metals | +2 | Mg is +2 in MgO |
| 3-12 | Transition metals | Variable, often +2 or +3 | Fe is +3 in Fe2O3 |
| 13 | Boron group | +3 | Al is +3 in Al2O3 |
| 14 | Carbon group | +4, +2, −4 | C is +4 in CO2 and −4 in CH4 |
| 15 | Nitrogen group | −3, +3, +5 | N is −3 in NH3 |
| 16 | Oxygen group | −2, +4, +6 | O is −2 in H2O |
| 17 | Halogens | −1, +1, +5, +7 | Cl is −1 in NaCl |
| 18 | Noble gases | 0 | Ne is 0 |
Oxidation number rules
- An atom in a pure element has an oxidation number of 0, as in O2, Fe, and Br2.
- A monatomic ion has an oxidation number equal to its charge: Na+ is +1 and Cl− is −1.
- Fluorine is −1 in all of its compounds.
- Oxygen is usually −2. It is −1 in peroxides such as H2O2.
- Hydrogen is usually +1. It is −1 in metal hydrides such as NaH.
- The oxidation numbers in a neutral compound add up to 0. In a polyatomic ion, they add up to the ion charge.
Oxidation number examples
Example 1
What is the oxidation number of Mg in MgCl2?
- Cl is −1, so two Cl atoms contribute −2.
- The compound is neutral: Mg + (−2) = 0.
Answer: +2
Example 2
What is the oxidation number of C in CH4?
- H is +1, so four H atoms contribute +4.
- C + 4 = 0.
Answer: −4
Example 3
What is the oxidation number of O in O2?
- O2 is a pure element, so each atom has an oxidation number of 0.
Answer: 0
Example 4
What is the oxidation number of Cu in CuSO4?
- Sulfate, SO4, has a charge of −2. S is +6 and each O is −2.
- Cu + (−2) = 0.
Answer: +2
Example 5
What is the oxidation number of N in NO2?
- O is −2, so two O atoms contribute −4.
- N + (−4) = 0.
Answer: +4
Example 6
What is the oxidation number of Mn in KMnO4?
- K is +1, and four O atoms contribute −8.
- +1 + Mn + (−8) = 0.
Answer: +7
How to use this calculator
- Find the element on the periodic table to see its common oxidation numbers.
- To solve a compound, enter its formula and the target element.
- Enter the oxidation numbers you know, such as O:-2 or H:+1.
- Calculate and check whether the result is unique or an average.
Formula and variables
sum(n_i x_i) = net charge
| Symbol | Meaning | Unit |
|---|---|---|
| n_i | atom count for element i | atoms |
| x_i | oxidation state for element i | dimensionless |
Worked example
For Fe3O4 with oxygen explicitly assigned -2, the three iron atoms must contribute +8 in total, so iron has average oxidation state +8/3.
Interpretation
A unique result is justified only for one target site or an explicit target assignment. Multiple target atoms produce an average or possible assignment because a formula alone does not reveal individual sites.
Common mistakes
- Do not assume oxygen is always -2 or hydrogen is always +1; enter the assignment appropriate to the compound.
- Do not describe an average oxidation state as proof that every atom has that state.
Frequently asked questions
How do you find the oxidation number on the periodic table?
Use the group number. Group 1 is +1, group 2 is +2, group 13 is +3, group 17 is usually −1, and group 18 is 0.
Why do transition metals have more than one oxidation number?
Their d electrons are close in energy to their outer s electrons. They can lose different numbers of electrons in different compounds.
What is the oxidation number of an element in its pure form?
It is 0. Atoms in O2, Fe, and Br2 have an oxidation number of 0.
Is oxidation number the same as charge?
Only for monatomic ions. In other compounds, the oxidation number is a bookkeeping value, not a real charge.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.