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A polyatomic ion is a group of atoms, bonded covalently, that carries an overall charge and behaves as a single unit in ionic compounds. Sulfate, SO₄²⁻, stays together whether it’s in copper sulfate, sulfuric acid or a bag of Epsom salt.
Most chemistry courses expect you to know a couple of dozen of them. That sounds like a lot of memorizing, but a few naming patterns do most of the work.
The list
Positive ions
| Name | Formula |
|---|---|
| Ammonium | NH₄⁺ |
| Hydronium | H₃O⁺ |
| Mercury(I) | Hg₂²⁺ |
Charge −1
| Name | Formula |
|---|---|
| Hydroxide | OH⁻ |
| Nitrate | NO₃⁻ |
| Nitrite | NO₂⁻ |
| Hydrogen carbonate (bicarbonate) | HCO₃⁻ |
| Hydrogen sulfate (bisulfate) | HSO₄⁻ |
| Dihydrogen phosphate | H₂PO₄⁻ |
| Acetate (ethanoate) | CH₃COO⁻ |
| Cyanide | CN⁻ |
| Thiocyanate | SCN⁻ |
| Permanganate | MnO₄⁻ |
| Perchlorate | ClO₄⁻ |
| Chlorate | ClO₃⁻ |
| Chlorite | ClO₂⁻ |
| Hypochlorite | ClO⁻ |
Charge −2
| Name | Formula |
|---|---|
| Sulfate | SO₄²⁻ |
| Sulfite | SO₃²⁻ |
| Carbonate | CO₃²⁻ |
| Hydrogen phosphate | HPO₄²⁻ |
| Chromate | CrO₄²⁻ |
| Dichromate | Cr₂O₇²⁻ |
| Oxalate | C₂O₄²⁻ |
| Thiosulfate | S₂O₃²⁻ |
| Peroxide | O₂²⁻ |
Charge −3
| Name | Formula |
|---|---|
| Phosphate | PO₄³⁻ |
| Phosphite | PO₃³⁻ |
(Phosphite is listed as PO₃³⁻ in most introductory courses, following the -ate/-ite pattern below. In real solutions, the ion from phosphorous acid keeps one hydrogen bonded directly to phosphorus and is usually HPO₃²⁻.)
Pattern 1: -ate and -ite
-ate and -ite ions both contain oxygen. The -ite ion has one fewer oxygen than the -ate ion, with the same charge.
- Nitrate NO₃⁻ → nitrite NO₂⁻
- Sulfate SO₄²⁻ → sulfite SO₃²⁻
- Phosphate PO₄³⁻ → phosphite PO₃³⁻
So you only need to memorize the -ate version; the -ite follows.
Pattern 2: per- and hypo- (the halogen series)
Chlorine forms four oxyanions, and the prefixes extend the pattern:
| Prefix/suffix | Oxygens | Formula |
|---|---|---|
| per…ate | one more than -ate | ClO₄⁻ perchlorate |
| …ate | the reference | ClO₃⁻ chlorate |
| …ite | one fewer than -ate | ClO₂⁻ chlorite |
| hypo…ite | one fewer than -ite | ClO⁻ hypochlorite |
Bromine and iodine follow the same scheme: bromate BrO₃⁻, iodate IO₃⁻, periodate IO₄⁻, hypobromite BrO⁻. All of them carry a −1 charge. (Hypochlorite is the active ingredient in household bleach.)
Pattern 3: adding hydrogen
Adding an H⁺ to an ion raises its charge by 1 and adds “hydrogen” to the name:
- Carbonate CO₃²⁻ → hydrogen carbonate HCO₃⁻
- Sulfate SO₄²⁻ → hydrogen sulfate HSO₄⁻
- Phosphate PO₄³⁻ → hydrogen phosphate HPO₄²⁻ → dihydrogen phosphate H₂PO₄⁻
The older prefix bi- means the same thing: bicarbonate is hydrogen carbonate (as in bicarbonate of soda, NaHCO₃).
Pattern 4: “thio” means sulfur replaces oxygen
- Sulfate SO₄²⁻ → thiosulfate S₂O₃²⁻ (one O swapped for S)
- Cyanate OCN⁻ → thiocyanate SCN⁻
Using polyatomic ions in formulas
Balance the charges exactly as for simple ions, and put the polyatomic ion in brackets whenever you need more than one:
- Calcium nitrate: Ca²⁺ + 2 NO₃⁻ → Ca(NO₃)₂
- Ammonium sulfate: 2 NH₄⁺ + SO₄²⁻ → (NH₄)₂SO₄
- Aluminium sulfate: 2 Al³⁺ + 3 SO₄²⁻ → Al₂(SO₄)₃
- Iron(III) hydroxide: Fe³⁺ + 3 OH⁻ → Fe(OH)₃
If only one polyatomic ion is needed, leave the brackets off: NaNO₃, not Na(NO₃).
Don’t change the subscripts inside a polyatomic ion — Ca(NO₃)₂ is correct, CaN₂O₆ describes the same atoms but hides the nitrate units and is never written that way.
Why they stay together
The atoms inside a polyatomic ion are held by strong covalent bonds, often with resonance spreading the charge over several atoms. When an ionic compound dissolves, it separates into its cation and polyatomic anion, but the polyatomic ion itself doesn’t fall apart.
Quick answers
Which polyatomic ions are positive? Ammonium, NH₄⁺, is by far the most common. Hydronium, H₃O⁺, is important in acid chemistry.
Is hydroxide an -ide ion like chloride? It ends in -ide but it’s polyatomic (O and H). Cyanide, CN⁻, is the same kind of exception.
Do I need to know every ion’s charge? Learn the -ate forms and the patterns above; nearly everything else follows.
Try it
The molar mass calculator accepts brackets, so you can check formulas like Al₂(SO₄)₃. The oxidation number calculator works with ions directly: try Cr2O7^2- or ClO4-.
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