Guides
Common Valences and Ion Charges (reference table)
Valence — the charge for ions — underlies how chemical formulas are written and what compounds contain. Below are the element valences and polyatomic-ion charges used most often in school chemistry, with quick memory aids.
Quick rules
K, Na, H, Ag and the ammonium ion are +1; Ca, Mg, Ba, Zn, Cu (and iron, often +2) are +2; Al is +3; oxygen and sulfur are usually −2; the halogens Cl, Br, I, F are −1. Iron can be +2 or +3 and copper +1 or +2 — read it from the compound.
Common element valences
| Charge | Element / ion |
|---|---|
| +1 | K⁺, Na⁺, H⁺, Ag⁺, Li⁺ |
| +2 | Ca²⁺, Mg²⁺, Ba²⁺, Zn²⁺, Cu²⁺, Fe²⁺ |
| +3 | Al³⁺, Fe³⁺ |
| −1 | Cl⁻, Br⁻, I⁻, F⁻ |
| −2 | O²⁻, S²⁻ |
Common polyatomic ions
| Charge | Ion |
|---|---|
| −1 | OH⁻ (hydroxide), NO₃⁻ (nitrate), HCO₃⁻ (bicarbonate), ClO₃⁻, MnO₄⁻ |
| −2 | SO₄²⁻ (sulfate), CO₃²⁻ (carbonate), SO₃²⁻, SiO₃²⁻ |
| −3 | PO₄³⁻ (phosphate) |
| +1 | NH₄⁺ (ammonium) |
How to use it
Formulas follow the rule that total positive charge equals total negative charge. Use the crossover method: swap the charges to become subscripts. For example Al³⁺ with SO₄²⁻ gives Al₂(SO₄)₃. See how to write chemical formulas; practice ions in learning mode.