Common Polyatomic Ions — Names, Formulas, and Charges
| Name | Formula | Charge | Category |
|---|---|---|---|
| Ammonium | NH₄⁺ | +1 | Cation |
| Hydronium | H₃O⁺ | +1 | Cation |
| Mercury(I) (mercurous) | Hg₂²⁺ | +2 | Cation |
| Acetate | CH₃COO⁻ | –1 | Carboxylate |
| Formate | HCOO⁻ | –1 | Carboxylate |
| Oxalate | C₂O₄²⁻ | –2 | Carboxylate |
| Cyanide | CN⁻ | –1 | Other anion |
| Hydroxide | OH⁻ | –1 | Other anion |
| Peroxide | O₂²⁻ | –2 | Other anion |
| Thiocyanate | SCN⁻ | –1 | Other anion |
| Nitrate | NO₃⁻ | –1 | Nitrogen oxyanion |
| Nitrite | NO₂⁻ | –1 | Nitrogen oxyanion |
| Sulfate | SO₄²⁻ | –2 | Sulfur oxyanion |
| Sulfite | SO₃²⁻ | –2 | Sulfur oxyanion |
| Hydrogen sulfate (bisulfate) | HSO₄⁻ | –1 | Sulfur oxyanion |
| Hydrogen sulfite (bisulfite) | HSO₃⁻ | –1 | Sulfur oxyanion |
| Thiosulfate | S₂O₃²⁻ | –2 | Sulfur oxyanion |
| Phosphate | PO₄³⁻ | –3 | Phosphorus oxyanion |
| Hydrogen phosphate | HPO₄²⁻ | –2 | Phosphorus oxyanion |
| Dihydrogen phosphate | H₂PO₄⁻ | –1 | Phosphorus oxyanion |
| Phosphite | PO₃³⁻ | –3 | Phosphorus oxyanion |
| Carbonate | CO₃²⁻ | –2 | Carbon oxyanion |
| Hydrogen carbonate (bicarbonate) | HCO₃⁻ | –1 | Carbon oxyanion |
| Chlorate | ClO₃⁻ | –1 | Halogen oxyanion |
| Chlorite | ClO₂⁻ | –1 | Halogen oxyanion |
| Hypochlorite | ClO⁻ | –1 | Halogen oxyanion |
| Perchlorate | ClO₄⁻ | –1 | Halogen oxyanion |
| Bromate | BrO₃⁻ | –1 | Halogen oxyanion |
| Iodate | IO₃⁻ | –1 | Halogen oxyanion |
| Periodate | IO₄⁻ | –1 | Halogen oxyanion |
| Chromate | CrO₄²⁻ | –2 | Transition metal oxyanion |
| Dichromate | Cr₂O₇²⁻ | –2 | Transition metal oxyanion |
| Permanganate | MnO₄⁻ | –1 | Transition metal oxyanion |
| Silicate | SiO₃²⁻ | –2 | Other oxyanion |
| Borate | BO₃³⁻ | –3 | Other oxyanion |
| Arsenate | AsO₄³⁻ | –3 | Other oxyanion |
A polyatomic ion is a covalently bonded cluster that carries a net charge and travels as a unit. The oxyanion naming convention compresses what would otherwise be a memorization slog: -ate is the reference form, -ite has one fewer oxygen at the same charge, hypo-...-ite has two fewer than -ate, and per-...-ate has one more than -ate. Chlorine spans the full range (ClO-, ClO2-, ClO3-, ClO4-); other halogens and main-group nonmetals fill in by analogy. The hydrogen prefix (or older bi- prefix, as in bicarbonate) marks a protonated form with one fewer negative charge.
Frequently Asked Questions
What is the naming pattern for oxyanions?
Number of oxygens varies, charge stays the same. Chlorine gives the cleanest example: hypochlorite ClO- (fewest), chlorite ClO2-, chlorate ClO3- (the -ate anchor), perchlorate ClO4- (one more than -ate). Hypo- means 'under' the -ite form by one O; per- means 'above' the -ate form by one O. The pattern carries through bromine and iodine (BrO-, BrO3-, IO3-, IO4-), through sulfur (SO3 2- vs SO4 2-) and nitrogen (NO2- vs NO3-). Learn the chlorine series and most of the table follows by analogy.
What does the 'bi-' or 'hydrogen' prefix mean?
It marks a protonated form: the parent anion has picked up an H+, reducing its negative charge by one. Carbonate (CO3 2-) becomes bicarbonate or hydrogen carbonate (HCO3-); sulfate (SO4 2-) becomes bisulfate or hydrogen sulfate (HSO4-); phosphate goes through two such steps to dihydrogen phosphate. These half-protonated species are the workhorses of physiological and lab buffer systems — bicarbonate buffers blood near pH 7.4, the H2PO4-/HPO4 2- pair buffers around pH 7.2. Modern IUPAC prefers 'hydrogen', but 'bi-' is still everywhere in older texts.
Which polyatomic ions should every chemistry student memorize?
Lock in this short list and the rest follow from the naming pattern: ammonium NH4+, hydroxide OH-, nitrate NO3-, nitrite NO2-, sulfate SO4 2-, sulfite SO3 2-, phosphate PO4 3-, carbonate CO3 2-, bicarbonate HCO3-, acetate CH3COO-, cyanide CN-, permanganate MnO4-, chromate CrO4 2-, dichromate Cr2O7 2-, and the full chlorine series ClO- through ClO4-. From those anchors you derive bromate, iodate, periodate, and the protonated/deprotonated variants. Maybe fifteen ions in, you can name and write formulas for the rest by analogy.