List of polyatomic ions with names, formulas and charges

This chart lists 39 common polyatomic ions with their formulas and charges. The naming patterns below help you remember them as families instead of one at a time.
Ammonium NH₄⁺ +1
Hydronium H₃O⁺ +1
Acetate CH₃COO⁻ −1
Hydroxide OH⁻ −1
Nitrate NO₃⁻ −1
Nitrite NO₂⁻ −1
Hydrogen carbonate (bicarbonate) HCO₃⁻ −1
Hydrogen sulfate HSO₄⁻ −1
Dihydrogen phosphate H₂PO₄⁻ −1
Permanganate MnO₄⁻ −1
Cyanide CN⁻ −1
Thiocyanate SCN⁻ −1
Hypochlorite ClO⁻ −1
Chlorite ClO₂⁻ −1
Chlorate ClO₃⁻ −1
Perchlorate ClO₄⁻ −1
Carbonate CO₃²⁻ −2
Sulfate SO₄²⁻ −2
Sulfite SO₃²⁻ −2
Hydrogen phosphate HPO₄²⁻ −2
Chromate CrO₄²⁻ −2
Dichromate Cr₂O₇²⁻ −2
Oxalate C₂O₄²⁻ −2
Peroxide O₂²⁻ −2
Phosphate PO₄³⁻ −3
Bromate BrO₃⁻ −1
Iodate IO₃⁻ −1
Periodate IO₄⁻ −1
Hydrogen sulfite (bisulfite) HSO₃⁻ −1
Cyanate OCN⁻ −1
Azide N₃⁻ −1
Formate HCOO⁻ −1
Amide NH₂⁻ −1
Thiosulfate S₂O₃²⁻ −2
Silicate SiO₃²⁻ −2
Manganate MnO₄²⁻ −2
Arsenate AsO₄³⁻ −3
Borate BO₃³⁻ −3
Mercury(I) Hg₂²⁺ +2

What the chart shows

A polyatomic ion is a group of atoms held together by covalent bonds that carries an overall charge. Each row gives the ion's name, formula and charge, and the charge belongs to the whole group: sulfate, SO₄²⁻, is one sulfur and four oxygen atoms sharing a 2− charge. In compounds the group stays together, so sodium nitrate, NaNO₃, contains the nitrate ion NO₃⁻.

Most polyatomic ions are negative. The only positive ones on this list are ammonium (NH₄⁺), hydronium (H₃O⁺) and mercury(I) (Hg₂²⁺).

Naming patterns: -ate, -ite, per- and hypo-

An -ate ion has one more oxygen atom than the matching -ite ion, and both have the same charge: nitrate NO₃⁻ and nitrite NO₂⁻, sulfate SO₄²⁻ and sulfite SO₃²⁻. Chlorine has four: perchlorate ClO₄⁻, chlorate ClO₃⁻, chlorite ClO₂⁻ and hypochlorite ClO⁻. Per- means one more oxygen than -ate, and hypo- one fewer than -ite. Bromate BrO₃⁻ and iodate IO₃⁻ follow the chlorate pattern.

Adding a hydrogen ion raises the charge by one: carbonate CO₃²⁻ becomes hydrogen carbonate HCO₃⁻, and phosphate PO₄³⁻ becomes hydrogen phosphate HPO₄²⁻, then dihydrogen phosphate H₂PO₄⁻. The prefix thio- means one oxygen is replaced by sulfur, as in cyanate OCN⁻ and thiocyanate SCN⁻.

Worked example: writing a formula

To write calcium phosphate, start with Ca²⁺ and PO₄³⁻. The smallest charge both can reach is 6, so you need three calcium ions (+6) and two phosphate ions (−6). Put the polyatomic ion in parentheses when there is more than one: Ca₃(PO₄)₂. Ammonium sulfate works the same way: two NH₄⁺ ions balance one SO₄²⁻, giving (NH₄)₂SO₄.

FAQ

Which polyatomic ion has a 3− charge?

The most common is phosphate, PO₄³⁻. The chart also lists arsenate (AsO₄³⁻) and borate (BO₃³⁻) with a 3− charge.

What is the difference between sulfate and sulfite?

Sulfate is SO₄²⁻ and sulfite is SO₃²⁻. Both have a 2− charge; the -ate ion simply has one more oxygen atom.

Which polyatomic ions are positive?

Ammonium, NH₄⁺, is by far the most common positive polyatomic ion. Hydronium, H₃O⁺, forms when acids dissolve in water, and mercury(I) exists as the paired ion Hg₂²⁺.

What is the charge of nitrate?

Nitrate, NO₃⁻, has a 1− charge. Nitrite, NO₂⁻, has the same charge but one fewer oxygen atom.