Common Polyatomic Ions — Names, Formulas, and Charges
| Ion Name | Formula | Charge | Molar Mass (g/mol) | Category |
|---|---|---|---|---|
| Ammonium | NH₄⁺ | +1 | 18.039 | Cation |
| Hydronium | H₃O⁺ | +1 | 19.023 | Cation |
| Mercury(I) (mercurous) | Hg₂²⁺ | +2 | 401.18 | Cation |
| Hydroxide | OH⁻ | −1 | 17.007 | Anion |
| Cyanide | CN⁻ | −1 | 26.018 | Anion |
| Thiocyanate | SCN⁻ | −1 | 58.082 | Anion |
| Acetate | C₂H₃O₂⁻ | −1 | 59.044 | Anion |
| Formate | CHO₂⁻ | −1 | 45.018 | Anion |
| Permanganate | MnO₄⁻ | −1 | 118.934 | Anion |
| Nitrite | NO₂⁻ | −1 | 46.006 | Anion |
| Nitrate | NO₃⁻ | −1 | 62.005 | Anion |
| Hypochlorite | ClO⁻ | −1 | 51.452 | Anion (Chlorine oxyacid) |
| Chlorite | ClO₂⁻ | −1 | 67.452 | Anion (Chlorine oxyacid) |
| Chlorate | ClO₃⁻ | −1 | 83.451 | Anion (Chlorine oxyacid) |
| Perchlorate | ClO₄⁻ | −1 | 99.451 | Anion (Chlorine oxyacid) |
| Hypobromite | BrO⁻ | −1 | 95.904 | Anion (Bromine oxyacid) |
| Bromate | BrO₃⁻ | −1 | 127.903 | Anion (Bromine oxyacid) |
| Iodate | IO₃⁻ | −1 | 174.902 | Anion (Iodine oxyacid) |
| Periodate | IO₄⁻ | −1 | 190.901 | Anion (Iodine oxyacid) |
| Bicarbonate (hydrogen carbonate) | HCO₃⁻ | −1 | 61.017 | Anion |
| Bisulfate (hydrogen sulfate) | HSO₄⁻ | −1 | 97.071 | Anion |
| Bisulfite (hydrogen sulfite) | HSO₃⁻ | −1 | 81.072 | Anion |
| Dihydrogen phosphate | H₂PO₄⁻ | −1 | 96.987 | Anion |
| Carbonate | CO₃²⁻ | −2 | 60.009 | Anion |
| Sulfite | SO₃²⁻ | −2 | 80.064 | Anion |
| Sulfate | SO₄²⁻ | −2 | 96.063 | Anion |
| Thiosulfate | S₂O₃²⁻ | −2 | 112.13 | Anion |
| Chromate | CrO₄²⁻ | −2 | 115.994 | Anion |
| Dichromate | Cr₂O₇²⁻ | −2 | 215.988 | Anion |
| Oxalate | C₂O₄²⁻ | −2 | 88.019 | Anion |
| Hydrogen phosphate | HPO₄²⁻ | −2 | 95.979 | Anion |
| Peroxide | O₂²⁻ | −2 | 31.998 | Anion |
| Silicate | SiO₃²⁻ | −2 | 76.084 | Anion |
| Manganate | MnO₄²⁻ | −2 | 118.934 | Anion |
| Phosphate | PO₄³⁻ | −3 | 94.971 | Anion |
| Arsenate | AsO₄³⁻ | −3 | 138.919 | Anion |
| Borate | BO₃³⁻ | −3 | 58.81 | Anion |
| Citrate | C₆H₅O₇³⁻ | −3 | 189.1 | Anion (Organic) |
| Tartrate | C₄H₄O₆²⁻ | −2 | 148.071 | Anion (Organic) |
| Benzoate | C₇H₅O₂⁻ | −1 | 121.113 | Anion (Organic) |
A polyatomic ion is a covalently bonded cluster of atoms that carries a net charge and travels together through reactions — treat it as a single unit. When you need more than one in a formula, parenthesize: Ca(NO3)2, not CaN2O6. The bicarbonate/bisulfate -bi prefix is older nomenclature; modern IUPAC names them hydrogen carbonate and hydrogen sulfate, but both forms are still in everyday use. Molar masses are computed from IUPAC 2021 standard atomic weights and rounded to three decimal places; for ions involving elements with no stable isotope or with conventional weights given as ranges, the conventional value is used.
Frequently Asked Questions
How do you memorize polyatomic ions?
Anchor the -ate forms first: nitrate NO3-, sulfate SO4 2-, phosphate PO4 3-, carbonate CO3 2-, chlorate ClO3-. Then derive the rest from prefixes and suffixes within the same family — and the charge stays constant across the family. -ite has one fewer oxygen than -ate (sulfite SO3 2-), hypo- ... -ite has two fewer than -ate (hypochlorite ClO-), and per- ... -ate has one more (perchlorate ClO4-). Memorize roughly a dozen anchors and you can derive maybe forty ions, including the bromine and iodine series by analogy with chlorine.
What is the difference between a monatomic ion and a polyatomic ion?
A monatomic ion is a single charged atom — Na+, Cl-, Ca2+, S2-. A polyatomic ion is a covalently bonded cluster that carries a net charge as a unit — SO4 2-, NH4+, NO3-. The practical consequence: in a chemical reaction the polyatomic ion typically stays intact, so when you balance the equation you treat the whole cluster as a single species. That's why nitrates and acetates are spectator ions in most aqueous reactions — they show up unchanged on both sides and cancel out of the net ionic equation.
Why does ammonium have a positive charge while most polyatomic ions are negative?
Ammonium forms when NH3 picks up a proton through a coordinate covalent bond using nitrogen's lone pair — the donated proton brings no electrons, so the cluster ends up +1 overall. Most polyatomic ions are negative because they're conjugate bases of oxyacids: H2SO4 loses two protons to give SO4 2-, HNO3 loses one to give NO3-, and so on. The asymmetry just reflects how these ions are made — proton donors leave behind anions, while proton acceptors are the (rarer) cations. Hydronium (H3O+) and mercury(I) (Hg2 2+) round out the short list of common polyatomic cations.