Polyatomic ions are covalently bonded groups of atoms that carry a net charge and behave as single ions in ionic compounds. General chemistry nomenclature often requires the ability to determine the name or formula for each polyatomic ion, with its correct charge and (for oxyanions) its oxygen count pattern.
Polyatomic ions and formula notation
A polyatomic ion formula lists element symbols with subscripts for atom counts and a charge written as a superscript. Parentheses are used in compound formulas when more than one copy of the polyatomic ion is present (example: Ca(NO3)2).
The charge is an essential part of the identity. Two ions can share the same elements but differ in oxygen count and still carry the same charge (for example, sulfate SO42− and sulfite SO32−).
Oxyanion families and naming patterns
Many common polyatomic ions are oxyanions, containing a central element bonded to oxygen. Within a family, suffixes and prefixes correlate with oxygen count while the family charge typically remains fixed.
Suffix pattern for oxygen count
The pair “-ate” and “-ite” distinguishes two closely related oxyanions of the same element: the “-ate” ion has one more oxygen than the “-ite” ion, with the same charge in the common families.
Prefix pattern for extremes
The prefixes “per-” and “hypo-” mark the highest-oxygen and lowest-oxygen members of a family, respectively, with the same suffix set. A standard example appears in the chlorine oxyanion set: perchlorate ClO4−, chlorate ClO3−, chlorite ClO2−, hypochlorite ClO−.
Hydrogen-containing polyatomic ions
Several important ions are formed by adding hydrogen to a base oxyanion. The naming includes “hydrogen” (or “dihydrogen”) and the charge becomes less negative as hydrogen is added.
Carbonate and phosphate families illustrate the pattern clearly: CO32− becomes HCO3− (hydrogen carbonate), and PO43− becomes HPO42− (hydrogen phosphate) and H2PO4− (dihydrogen phosphate).
Reference table of common polyatomic ions
The table lists frequently encountered ions in introductory general chemistry, with names, formulas, and charges. Memorization targets in nomenclature courses commonly overlap with this set.
| Ion name | Formula | Charge | Family notes |
|---|---|---|---|
| ammonium | NH4+ | +1 | Common polyatomic cation |
| hydroxide | OH− | −1 | Base in acid–base chemistry |
| nitrate | NO3− | −1 | “-ate/ite” pair with nitrite |
| nitrite | NO2− | −1 | One fewer O than nitrate |
| sulfate | SO42− | −2 | “-ate/ite” pair with sulfite |
| sulfite | SO32− | −2 | One fewer O than sulfate |
| carbonate | CO32− | −2 | Hydrogen carbonate as H-added form |
| hydrogen carbonate (bicarbonate) | HCO3− | −1 | One H reduces charge magnitude by 1 |
| phosphate | PO43− | −3 | Hydrogen phosphate and dihydrogen phosphate derivatives |
| hydrogen phosphate | HPO42− | −2 | One H added to phosphate |
| dihydrogen phosphate | H2PO4− | −1 | Two H added to phosphate |
| acetate | C2H3O2− | −1 | Common organic polyatomic ion (also written as CH3COO−) |
| cyanide | CN− | −1 | Binary polyatomic ion |
| permanganate | MnO4− | −1 | High-oxygen manganese oxyanion |
| chromate | CrO42− | −2 | Related to dichromate |
| dichromate | Cr2O72− | −2 | “di-” indicates two central atoms |
| chlorate | ClO3− | −1 | Part of per-/hypo- chlorine series |
| perchlorate | ClO4− | −1 | Oxygen-rich extreme of chlorine series |
| chlorite | ClO2− | −1 | One fewer O than chlorate |
| hypochlorite | ClO− | −1 | Oxygen-poor extreme of chlorine series |
Representative examples connecting ions to compound formulas
Polyatomic ions behave as units when forming ionic compounds, and neutrality requires total positive charge to equal total negative charge. For sodium sulfate, the ion charges are Na+ and SO42−, so two sodium ions balance one sulfate ion: \[ 2(+1) + (-2) = 0 \quad \Rightarrow \quad \text{Na}_2\text{SO}_4 \]
For calcium nitrate, Ca2+ pairs with NO3−, and two nitrates balance one calcium: \[ (+2) + 2(-1) = 0 \quad \Rightarrow \quad \text{Ca(NO}_3\text{)}_2 \]
Common pitfalls
Oxygen count confusion is a frequent source of errors, especially between “-ate” and “-ite” pairs and within per-/hypo- families. Charge omission is equally disruptive, because a correct name with an incorrect charge cannot produce correct compound formulas.
Hydrogen-containing ions require special attention: the base ion name remains recognizable, while the word “hydrogen” signals a less negative charge (examples: HCO3− and H2PO4−).