PeriodicTable.world · Chemistry reference
Common ions & charges
Print common monatomic and polyatomic ion formulas, charge-balancing examples, and the distinction between ionic charge and oxidation state.
| Positive ions | Charge | Negative ions | Charge |
|---|---|---|---|
| Li⁺, Na⁺, K⁺ | +1 | F⁻, Cl⁻, Br⁻, I⁻ | −1 |
| Mg²⁺, Ca²⁺, Ba²⁺ | +2 | O²⁻, S²⁻ | −2 |
| Al³⁺ | +3 | N³⁻ (nitride) | −3 |
| Fe²⁺ / Fe³⁺ | +2 / +3 | — | — |
| Cu⁺ / Cu²⁺ | +1 / +2 | — | — |
| Name | Formula | Name | Formula |
|---|---|---|---|
| Ammonium | NH₄⁺ | Hydroxide | OH⁻ |
| Nitrate | NO₃⁻ | Nitrite | NO₂⁻ |
| Sulfate | SO₄²⁻ | Sulfite | SO₃²⁻ |
| Carbonate | CO₃²⁻ | Hydrogen carbonate | HCO₃⁻ |
| Phosphate | PO₄³⁻ | Acetate | CH₃COO⁻ |
How to use it
- For a neutral ionic compound, positive and negative charges must add up to zero. Use the smallest whole-number ratio of positive ions (cations) to negative ions (anions).
- Keep a polyatomic ion intact when balancing charges. Use parentheses when more than one copy is needed: Ca(NO₃)₂ contains two nitrate ions.
- Roman numerals specify oxidation state: iron(III) is +3. For a monatomic ion this equals its ionic charge.
Worked example
Al³⁺ + SO₄²⁻: two aluminium ions give +6 and three sulfate ions give −6, so the neutral formula is Al₂(SO₄)₃.
Remember the exceptions
- These are selected common ions, not every possible charge or oxidation state.
- Oxidation state is a number assigned using electron-counting rules. Oxygen has oxidation state −2 in H₂O, but water contains covalent bonds, not free O²⁻ ions.
- Subscripts count atoms or ion groups; superscripts describe net charge. Never change the formula of an ion to balance a salt.