Polyatomic Ions Reference
Every common polyatomic ion in one organized chart — name, formula and charge, grouped from 1+ cations through 3− anions — with one-tap links to our interactive molar mass, equation balancer and stoichiometry calculators so you can put the ion to work right away.
Positive ions (cations)
| Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | 1+ |
| Hydronium | H₃O⁺ | 1+ |
| Mercury(I) (dimercury) | Hg₂²⁺ | 2+ |
Charge 1− anions
| Name | Formula | Charge |
|---|---|---|
| Hydroxide | OH⁻ | 1− |
| Nitrate | NO₃⁻ | 1− |
| Nitrite | NO₂⁻ | 1− |
| Hydrogen carbonate (bicarbonate) | HCO₃⁻ | 1− |
| Acetate (ethanoate) | CH₃COO⁻ | 1− |
| Cyanide | CN⁻ | 1− |
| Permanganate | MnO₄⁻ | 1− |
| Hypochlorite | ClO⁻ | 1− |
| Chlorite | ClO₂⁻ | 1− |
| Chlorate | ClO₃⁻ | 1− |
| Perchlorate | ClO₄⁻ | 1− |
| Hydrogen sulfate (bisulfate) | HSO₄⁻ | 1− |
| Dihydrogen phosphate | H₂PO₄⁻ | 1− |
| Thiocyanate | SCN⁻ | 1− |
Charge 2− anions
| Name | Formula | Charge |
|---|---|---|
| Carbonate | CO₃²⁻ | 2− |
| Sulfate | SO₄²⁻ | 2− |
| Sulfite | SO₃²⁻ | 2− |
| Chromate | CrO₄²⁻ | 2− |
| Dichromate | Cr₂O₇²⁻ | 2− |
| Peroxide | O₂²⁻ | 2− |
| Oxalate | C₂O₄²⁻ | 2− |
| Silicate | SiO₃²⁻ | 2− |
| Hydrogen phosphate | HPO₄²⁻ | 2− |
| Thiosulfate | S₂O₃²⁻ | 2− |
Charge 3− anions
| Name | Formula | Charge |
|---|---|---|
| Phosphate | PO₄³⁻ | 3− |
| Phosphite | PO₃³⁻ | 3− |
| Borate | BO₃³⁻ | 3− |
| Arsenate | AsO₄³⁻ | 3− |
These are the standard polyatomic ions used in general chemistry. Each formula's charge is the sum of its atoms' formal charges — for example sulfate SO₄²⁻ carries a 2− charge because it has two more electrons than protons across the ion. ✓
Positive ions (cations)
| Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | 1+ |
| Hydronium | H₃O⁺ | 1+ |
| Mercury(I) (dimercury) | Hg₂²⁺ | 2+ |
Charge 1− anions
| Name | Formula | Charge |
|---|---|---|
| Hydroxide | OH⁻ | 1− |
| Nitrate | NO₃⁻ | 1− |
| Nitrite | NO₂⁻ | 1− |
| Hydrogen carbonate (bicarbonate) | HCO₃⁻ | 1− |
| Acetate (ethanoate) | CH₃COO⁻ | 1− |
| Cyanide | CN⁻ | 1− |
| Permanganate | MnO₄⁻ | 1− |
| Hypochlorite | ClO⁻ | 1− |
| Chlorite | ClO₂⁻ | 1− |
| Chlorate | ClO₃⁻ | 1− |
| Perchlorate | ClO₄⁻ | 1− |
| Hydrogen sulfate (bisulfate) | HSO₄⁻ | 1− |
| Dihydrogen phosphate | H₂PO₄⁻ | 1− |
| Thiocyanate | SCN⁻ | 1− |
Charge 2− anions
| Name | Formula | Charge |
|---|---|---|
| Carbonate | CO₃²⁻ | 2− |
| Sulfate | SO₄²⁻ | 2− |
| Sulfite | SO₃²⁻ | 2− |
| Chromate | CrO₄²⁻ | 2− |
| Dichromate | Cr₂O₇²⁻ | 2− |
| Peroxide | O₂²⁻ | 2− |
| Oxalate | C₂O₄²⁻ | 2− |
| Silicate | SiO₃²⁻ | 2− |
| Hydrogen phosphate | HPO₄²⁻ | 2− |
| Thiosulfate | S₂O₃²⁻ | 2− |
Charge 3− anions
| Name | Formula | Charge |
|---|---|---|
| Phosphate | PO₄³⁻ | 3− |
| Phosphite | PO₃³⁻ | 3− |
| Borate | BO₃³⁻ | 3− |
| Arsenate | AsO₄³⁻ | 3− |
About the Polyatomic Ions Reference
Polyatomic ions are groups of atoms covalently bonded together that carry an overall electric charge, and they behave as a single unit when they combine with other ions to form compounds — the sulfate in copper(II) sulfate, CuSO₄, moves and reacts as one SO₄²⁻ group rather than as separate sulfur and oxygen atoms. This chart collects the polyatomic ions students are expected to memorize, grouped by charge: the three common cations (ammonium NH₄⁺, hydronium H₃O⁺, and the mercury(I) dimer Hg₂²⁺), the fourteen most common 1− anions (hydroxide, nitrate, nitrite, bicarbonate, acetate, cyanide, permanganate, the four chlorine oxyanions hypochlorite through perchlorate, bisulfate, dihydrogen phosphate and thiocyanate), the ten common 2− anions (carbonate, sulfate, sulfite, chromate, dichromate, peroxide, oxalate, silicate, hydrogen phosphate and thiosulfate), and the four common 3− anions (phosphate, phosphite, borate and arsenate). Every formula is written in clean Unicode notation — no images — so it copies cleanly into your notes and is easy for assistive technology and AI to read. What sets this page apart from a static PDF is that it is wired into UnitConv's interactive calculators: once you have the formula, tap "Calculate molar mass" to find its mass, "Balance an equation" to write a reaction that uses it, or "Stoichiometry calculator" to work out reacting quantities. Sources for these ions include Wikipedia's Polyatomic ion article and LibreTexts Chemistry. Use it as a homework reference, an exam revision list, or a quick lookup while naming ionic compounds.
How to use this reference
- 1 Find the ion you need in the grouped cards — cations, then 1−, 2− and 3− anions.
- 2 Read off its formula and charge directly, or use them to write the formula of an ionic compound (the charges must balance to zero).
- 3 Tap "Calculate molar mass," "Balance an equation" or "Stoichiometry calculator" to open an interactive tool and use the ion in a real problem.
How polyatomic ion names and charges work
Most polyatomic ion families follow a naming pattern built around a central atom and a variable number of oxygens. Within a family, the "-ate" ion is the more common, reference form (nitrate NO₃⁻, sulfate SO₄²⁻, phosphate PO₄³⁻), and the "-ite" ion has one fewer oxygen at the same charge (nitrite NO₂⁻, sulfite SO₃²⁻, phosphite PO₃³⁻) — so memorizing the "-ate" ion and its charge lets you derive the "-ite" ion for free. Chlorine's oxyanions extend this pattern furthest, with four steps distinguished by prefixes and suffixes at a constant 1− charge: hypochlorite ClO⁻ (one oxygen), chlorite ClO₂⁻ (two), chlorate ClO₃⁻ (three, the "-ate" reference) and perchlorate ClO₄⁻ (four) — "hypo-" means less than "-ite", and "per-" means more than "-ate". Adding a hydrogen to an anion reduces its charge by exactly 1 and adds "hydrogen" or "bi-" to the name: carbonate CO₃²⁻ becomes hydrogen carbonate (bicarbonate) HCO₃⁻, and sulfate SO₄²⁻ becomes hydrogen sulfate (bisulfate) HSO₄⁻. The overall charge of a polyatomic ion is simply the sum of the formal charges (or oxidation states) of every atom in the group — for ammonium NH₄⁺, nitrogen's normal −3 oxidation state combines with four hydrogens at +1 each to leave a net +1, and for dichromate Cr₂O₇²⁻, two chromium(VI) centers bridged by seven oxygens work out to the same net 2− charge as a single chromate CrO₄²⁻.
Frequently asked questions
What is a polyatomic ion?
A polyatomic ion is a charged species made of two or more atoms covalently bonded together, such as sulfate (SO₄²⁻) or ammonium (NH₄⁺). Unlike a monatomic ion (a single charged atom, like Na⁺ or Cl⁻), a polyatomic ion keeps its group of atoms intact as it moves through a reaction and combines with other ions — it acts as one unit when forming an ionic compound.
What is the formula for sulfate?
Sulfate is SO₄²⁻ — one sulfur atom bonded to four oxygen atoms, carrying an overall charge of 2−. It is one of the most common polyatomic anions, found in compounds such as calcium sulfate (CaSO₄, the mineral gypsum) and copper(II) sulfate (CuSO₄).
What is the difference between sulfate and sulfite?
Sulfate is SO₄²⁻ (four oxygens) and sulfite is SO₃²⁻ (three oxygens) — sulfite has one fewer oxygen atom but the same 2− charge. This "-ate has one more oxygen than -ite, same charge" pattern holds across most polyatomic ion families, including nitrate NO₃⁻ versus nitrite NO₂⁻, and phosphate PO₄³⁻ versus phosphite PO₃³⁻.
How do you find the charge of a polyatomic ion?
The charge of a polyatomic ion is the sum of the oxidation states (or formal charges) of every atom in it. For a known ion, the fastest way is simply to memorize or look it up — this chart lists the charge for every common ion. To derive a compound's overall formula, remember that the positive and negative charges must balance to zero: calcium (Ca²⁺) with phosphate (PO₄³⁻) requires three calcium ions for every two phosphate ions, giving Ca₃(PO₄)₂, since 3×(+2) + 2×(−3) = 0.
Related tools
Turn any of these ions into a real chemistry calculation on UnitConv: use the molar mass calculator to find the mass of a compound built from one of these ions, the equation balancer to write and balance a reaction that includes it, or the stoichiometry calculator to work out how much product or reactant is involved.