Polyatomic Ions
Every common polyatomic ion with its formula and charge, searchable and grouped by charge.
Showing 36 of 36.
| Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | 1+ |
| Hydronium | H₃O⁺ | 1+ |
| Mercury(I) | Hg₂²⁺ | 2+ |
| Acetate | C₂H₃O₂⁻ | 1− |
| Amide | NH₂⁻ | 1− |
| Bromate | BrO₃⁻ | 1− |
| Chlorate | ClO₃⁻ | 1− |
| Chlorite | ClO₂⁻ | 1− |
| Cyanate | OCN⁻ | 1− |
| Cyanide | CN⁻ | 1− |
| Dihydrogen phosphate | H₂PO₄⁻ | 1− |
| Hydrogen carbonate (bicarbonate) | HCO₃⁻ | 1− |
| Hydrogen sulfate (bisulfate) | HSO₄⁻ | 1− |
| Hydrogen sulfite (bisulfite) | HSO₃⁻ | 1− |
| Hydroxide | OH⁻ | 1− |
| Hypochlorite | ClO⁻ | 1− |
| Iodate | IO₃⁻ | 1− |
| Nitrate | NO₃⁻ | 1− |
| Nitrite | NO₂⁻ | 1− |
| Perchlorate | ClO₄⁻ | 1− |
| Permanganate | MnO₄⁻ | 1− |
| Thiocyanate | SCN⁻ | 1− |
| Carbonate | CO₃²⁻ | 2− |
| Chromate | CrO₄²⁻ | 2− |
| Dichromate | Cr₂O₇²⁻ | 2− |
| Hydrogen phosphate | HPO₄²⁻ | 2− |
| Oxalate | C₂O₄²⁻ | 2− |
| Peroxide | O₂²⁻ | 2− |
| Silicate | SiO₃²⁻ | 2− |
| Sulfate | SO₄²⁻ | 2− |
| Sulfite | SO₃²⁻ | 2− |
| Thiosulfate | S₂O₃²⁻ | 2− |
| Arsenate | AsO₄³⁻ | 3− |
| Borate | BO₃³⁻ | 3− |
| Phosphate | PO₄³⁻ | 3− |
| Phosphite | PO₃³⁻ | 3− |
The charge belongs to the whole ion, not to one atom in it. In a compound, the subscript outside the brackets counts the ions: Al₂(SO₄)₃ holds two aluminium ions and three sulfate ions, and the charges cancel as 2(3+) + 3(2−) = 0.
A polyatomic ion is a group of atoms carrying one overall charge, such as sulfate SO₄²⁻ or ammonium NH₄⁺. The charge belongs to the whole group, so it stays together through a reaction and is written as one unit inside brackets when a formula needs more than one of them.
Why these have to be memorised
Monatomic ion charges can be read off the periodic table — sodium is in group 1 so it forms Na⁺, oxygen is in group 16 so it forms O²⁻. Polyatomic ions offer no such shortcut. There is nothing in the periodic table that tells you sulfate is SO₄²⁻ rather than SO₃²⁻, so this is one of the few genuine memorisation tasks in an introductory chemistry course.
The compensation is that the list is short and it repeats. Once you know these three dozen, naming ionic compounds, balancing equations and predicting precipitates all become mechanical, because the ions travel through reactions intact rather than breaking apart.
The naming pattern does most of the work
Within a family the suffixes count oxygens. Take chlorate ClO₃⁻ as the reference point: one oxygen fewer is chlorite ClO₂⁻, two fewer is hypochlorite ClO⁻, and one more is perchlorate ClO₄⁻. The charge never changes across the family — only the oxygen count does. Learn one member of each family and the prefixes and suffixes give you the rest.
- 1 Write each ion with its charge. Aluminium is Al³⁺ and sulfate is SO₄²⁻.
- 2 Find how many of each make the charges cancel. Two Al³⁺ gives 6+ and three SO₄²⁻ gives 6−, so the ratio is 2 to 3.
- 3 Put brackets round the polyatomic ion before its subscript. Three sulfates is written (SO₄)₃, never SO₄3 or SO₁₂.
- 4 Combine them. The formula is Al₂(SO₄)₃, and 2(3+) + 3(2−) = 0 confirms it is neutral.
- 5 Name it with the ion names unchanged. Aluminium sulfate — the polyatomic ion keeps its own name in the compound.
The −ate / −ite pattern
Chlorine shows the full set. The charge stays 1− throughout; only the oxygen count moves.
| Prefix / suffix | Oxygens relative to −ate | Chlorine example |
|---|---|---|
| per…−ate | One more | Perchlorate ClO₄⁻ |
| −ate | Reference form | Chlorate ClO₃⁻ |
| −ite | One fewer | Chlorite ClO₂⁻ |
| hypo…−ite | Two fewer | Hypochlorite ClO⁻ |
Two more patterns worth knowing
Both save memorising several ions separately.
| Pattern | What it does | Example |
|---|---|---|
| Adding H⁺ | Raises the charge by one | PO₄³⁻ → HPO₄²⁻ → H₂PO₄⁻ |
| bi− prefix | The older name for adding one H⁺ | Carbonate CO₃²⁻ → bicarbonate HCO₃⁻ |
| thio− prefix | One oxygen replaced by sulfur | Sulfate SO₄²⁻ → thiosulfate S₂O₃²⁻ |
The mistakes that show up in marking
The commonest is dropping the brackets. Mg(NO₃)₂ has two nitrate ions; written MgNO₃₂ or MgN₂O₆ it no longer shows the ion that is actually present, and the second form suggests a compound that does not exist as written. Brackets go round any polyatomic ion that appears more than once.
The second is changing the ion to balance a charge. Sulfate is SO₄²⁻ and nothing else; if the charges do not cancel, change how many ions you have, never the formula of the ion. And keep the similar pairs apart — nitrate NO₃⁻ against nitrite NO₂⁻, sulfate SO₄²⁻ against sulfite SO₃²⁻ — because a single oxygen is the whole difference between two different compounds.