# Polyatomic ion
A polyatomic ion is an electrically charged molecule, a group of two or more atoms joined by covalent bonds that carries a net charge and acts as a single unit in chemical reactions and in salts.[1](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds) Familiar examples include nitrate (NO₃⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), phosphate (PO₄³⁻), and the cations ammonium (NH₄⁺) and hydronium (H₃O⁺).[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
| Key fact | Detail |
|---|---|
| Definition | Electrically charged molecules, groups of bonded atoms acting as discrete units, distinct from monatomic ions formed from a single atom[1](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds) |
| IUPAC framing | An anion is "a monoatomic or polyatomic species having one or more elementary charges of the electron"; the charged-molecule case is covered by the molecular ion entry, whose synonyms are molecular anion and molecular cation[3](https://goldbook.iupac.org/terms/view/M03988) • [4](https://goldbook.iupac.org/terms/view/A00358) |
| Oxyanion charge rule | For the listed ions, charge matches the valence electrons of the central atom minus twice the oxygen count: PO₄³⁻ = 5 − 8, SO₄²⁻ = 6 − 8, CO₃²⁻ = 4 − 6, NO₃⁻ = 5 − 6[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
| Sulfate geometry | Tetrahedral SO₄²⁻, molar mass 96.06 Da, S–O bond length 149 pm against 157 pm for S–OH in sulfuric acid[5](https://www.chemeurope.com/en/encyclopedia/Sulfate.html) |
| Charge limits in water | Ions with charges greater than 2− generally do not exist in solution unless protonated; oxide O²⁻ has not been observed in solution[6](https://www.chemeurope.com/en/encyclopedia/Polyatomic_ion.html) |
| Sulfite regulation | US wine labeling requires a sulfite declaration at 10 ppm or more total sulfur dioxide; total SO₂ in wine should not exceed 350 ppm[7](https://www.ttb.gov/regulated-commodities/beverage-alcohol/wine/wine-labeling-declaration-of-sulfites) • [8](https://www.ttb.gov/system/files/images/pdfs/ssd/tm500.pdf) |
| Interstellar anions | C₁₀H⁻ was reported toward the TMC-1 molecular cloud at greater than 9σ confidence, with the highest anion-to-neutral column density ratio measured to date[9](https://iopscience.iop.org/article/10.3847/2041-8213/acb648) |
## Definition and scope
Three distinctions fix the concept. A monatomic ion is a single charged atom, such as Na⁺ or Cl⁻; a polyatomic ion is a charged group of bonded atoms; and a neutral molecule has bonds but no net charge.[1](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds) Textbook treatments define polyatomic ions as molecular ions, covalently bonded species of two or more atoms that carry a net charge and behave as one unit.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
IUPAC's formal vocabulary approaches the same idea from two sides. Its Gold Book defines a molecular ion as an ion formed by removing one or more electrons from, or adding electrons to, a molecule without fragmenting the molecular structure, and lists molecular anion and molecular cation as synonyms.[3](https://goldbook.iupac.org/terms/view/M03988) Its anion entry covers both cases at once: an anion is a monoatomic or polyatomic species having one or more elementary charges of the electron.[4](https://goldbook.iupac.org/terms/view/A00358) Boundary species such as NH₄⁺ and H₃O⁺ are charged molecules in every operational sense, and standard references list them among the common polyatomic cations.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
## Bonding and structure
The internal glue is covalent. Ionic bonds are electrostatic attraction between oppositely charged ions, while covalent bonds share electron pairs; a polyatomic ion is covalently bonded inside and ionically bonded to its counterions in a salt.[1](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds)
**Resonance** is the standard description of that distribution. A set of two or more Lewis structures collectively describes the electronic bonding of a single polyatomic species, with fractional bonds and fractional charges; their net sum, the resonance hybrid, represents the delocalization of electrons.[10](https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/10%3A_Chemical_Bonding_I%3A_Basic_Concepts/10.5%3A_Resonance) [Resonance](https://www.edgepedia.org/resonance) structures are particularly common in oxoanions of the p-block elements, such as sulfate and phosphate.[10](https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/10%3A_Chemical_Bonding_I%3A_Basic_Concepts/10.5%3A_Resonance)
Sulfate shows how the descriptions have evolved. Gilbert Lewis's 1916 paper described sulfate bonding with no double bonds and a formal charge of 2+ on sulfur; [Linus Pauling](https://www.edgepedia.org/linus-pauling) later proposed two π bonds involving d orbitals to account for the observed 149 pm S–O bond length, which is shorter than the 157 pm S–OH bond in sulfuric acid.[5](https://www.chemeurope.com/en/encyclopedia/Sulfate.html) Polyatomic ions also serve as ligands: sulfate can bind a metal through one or two of its oxygens, as in the neutral complex PtSO₄(P(C₆H₅)₃)₂, where it acts as a bidentate ligand.[5](https://www.chemeurope.com/en/encyclopedia/Sulfate.html)
## Common ions and nomenclature
The most-taught families are the oxoanions, polyatomic ions containing one or more oxygen atoms.[1](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds) For these four common ions, the charge can be calculated as the number of valence electrons on the central non-oxygen atom minus twice the number of oxygen atoms, giving phosphate PO₄³⁻ (5 − 8), sulfate SO₄²⁻ (6 − 8), carbonate CO₃²⁻ (4 − 6), and nitrate NO₃⁻ (5 − 6).[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) Second-row carbon and nitrogen form oxyanions with three oxygens, while third-row phosphorus and sulfur form oxyanions with four.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
| Ion | Formula | Name pattern |
|---|---|---|
| Nitrate / nitrite | NO₃⁻ / NO₂⁻ | one fewer oxygen, same charge, takes -ite[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
| Sulfate / sulfite | SO₄²⁻ / SO₃²⁻ | same pattern[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
| Phosphate / phosphite | PO₄³⁻ / PO₃³⁻ | same pattern[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
| Hydrogen sulfite | HSO₃⁻ | partially protonated oxyanions take hydrogen or dihydrogen prefixes (HSO₄⁻ hydrogen sulfate, H₂PO₄⁻ dihydrogen phosphate)[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
| Hydroxide, cyanide | OH⁻, CN⁻ | common non-oxoanions[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) |
The corresponding acids replace -ate with -ic acid and -ite with -ous acid: HNO₃ nitric acid, H₂SO₄ sulfuric acid, and H₂SO₃ sulfurous acid.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) These traditional rules work for the most common ions, sulfate, phosphate, nitrate, and chlorate, but not for all polyatomic ions.[6](https://www.chemeurope.com/en/encyclopedia/Polyatomic_ion.html) IUPAC's systematic scheme instead uses additive names with charge numbers in parentheses immediately after the name, such as (1+), (3+), or (2−), with unity always indicated; the endings are -ide for monoatomic, homopolyatomic, and parent-hydride-derived species, -ate for additively named heteropolyatomic species, and -ite for a few still-allowed non-systematic names.[11](https://iupac.org/cms/wp-content/uploads/2018/05/Inorganic-Brief-Guide-V1-3.pdf) • [12](https://moureu.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap5-3.04.pdf) IUPAC prefers the charge number to the oxidation number because determining the oxidation number is sometimes ambiguous and subjective.[12](https://moureu.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap5-3.04.pdf) In compound formulas the ion is enclosed in parentheses with the subscript outside, as in Fe(NO₃)₃, (NH₄)₃PO₄, and Ca₃(PO₄)₂.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
## Sulfur(IV) oxyanions: sulfite and relatives
Sulfite, SO₃²⁻, is the one-oxygen-lower member of the sulfate pair. IUPAC's additive name for it is trioxidosulfate(2−), with sulfite retained as an allowed non-systematic name.[12](https://moureu.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap5-3.04.pdf) Sulfites are less oxidized than sulfates and can be oxidized to sulfate; sulfates are typically more stable and less reactive than sulfites, which is why sulfite serves as a preservative and reducing agent.[13](https://33science.com/2026/01/08/polyatomic-ions/) Partial protonation gives hydrogen sulfite, HSO₃⁻.[2](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds) A further sibling is thiosulfate, S₂O₃²⁻, in which one oxygen of sulfate is replaced by sulfur; its sulfur-sulfur bonding underlies its use in photographic fixing and as a cyanide antidote, along with its ability to reduce halogens and complex metals.[13](https://33science.com/2026/01/08/polyatomic-ions/)
## Behavior in salts and solution
Charge magnitude governs what can exist dissolved in water. Small ions with high charges are very rare: oxide, O²⁻, has not been observed in solution and is not treated as a component in reaction mechanisms, and ions with charges greater than 2− are generally rare in solution; orthosilicate, SiO₄⁴⁻, enjoys no status as an ion in aqueous solution except perhaps under extreme temperatures.[6](https://www.chemeurope.com/en/encyclopedia/Polyatomic_ion.html)
Solubility follows the counterion as much as the anion. Most sulfates dissolve readily, with calcium, strontium, and barium sulfate as the poorly soluble exceptions; the near-total insolubility of barium sulfate makes its precipitation a standard gravimetric method for measuring sulfate.[5](https://www.chemeurope.com/en/encyclopedia/Sulfate.html)
## By the numbers
- Sulfate: empirical formula SO₄²⁻, molecular mass 96.06 Da, tetrahedral geometry, S–O bond length 149 pm.[5](https://www.chemeurope.com/en/encyclopedia/Sulfate.html)
- Wine labeling: a sulfite declaration is required when sulfur dioxide or a sulfiting agent is detected at 10 or more ppm, measured as total sulfur dioxide.[7](https://www.ttb.gov/regulated-commodities/beverage-alcohol/wine/wine-labeling-declaration-of-sulfites)
- Wine limits: total sulfur dioxide should not exceed 350 ppm under 27 CFR 4.22(b)(1), and wine from organically grown grapes must contain less than 100 ppm SO₂.[8](https://www.ttb.gov/system/files/images/pdfs/ssd/tm500.pdf)
- [Winemaking](https://www.edgepedia.org/winemaking) additions: approximately 20–200 mg/L of SO₂ may be added during winemaking, and approximately 10–50 mg/L is formed by yeast during fermentation; sulfur dioxide and its salts have been added during winemaking since the 17th century.[14](https://www.oiv.int/public/medias/7840/oiv-collective-expertise-document-so2-and-wine-a-review.pdf)
- Membrane selectivity: a 2025 monovalent-anion-selective membrane achieved a Cl⁻/SO₄²⁻ selectivity of approximately 244 with a flux of 2.1 mol m⁻² h⁻¹.[15](https://www.nature.com/articles/s41467-025-64196-2)
## Detection, uses, and recent developments
**Regulated analysis of sulfite.** Sulfite quantification is a mature regulatory field. The codified Monier-Williams method computes sulfite as ppm SO₂ using ppm = (32.03 × V_B × N × 1000) ÷ Wt, where 32.03 is the milliequivalent weight of sulfur dioxide, V_B the NaOH titrant volume, N its normality, and Wt the sample weight in grams.[16](https://www.govinfo.gov/content/pkg/CFR-2013-title21-vol2/pdf/CFR-2013-title21-vol2-part101-appA.pdf) The TTB aeration-oxidation method passes air through an acidified wine sample, carrying SO₂ into a hydrogen peroxide trapping solution that is then titrated.[8](https://www.ttb.gov/system/files/images/pdfs/ssd/tm500.pdf) The OIV maintains titrimetric and iodometric reference methods for free and total sulfur dioxide in wine.[14](https://www.oiv.int/public/medias/7840/oiv-collective-expertise-document-so2-and-wine-a-review.pdf) A January 18, 2022 [Federal Register](https://www.edgepedia.org/federal-register) rule amended 21 CFR §§ 101.100(a)(4) and 130.9(a) to incorporate AOAC Official Method 990.28 and to add an LC-MS/MS method, with mg/kg (ppm) as the unit of measure; the older distillation-based methodology had a detection limit of 10 ppm and limited throughput to three samples per apparatus in an 8-hour day, while the LC-MS/MS method allows a single analyst to complete as many as 30 samples in a day.[17](https://www.federalregister.gov/documents/2022/01/18/2022-00816/new-method-for-the-analysis-of-sulfites-in-foods) In the FDA's validated LC-MS/MS procedure, the sample is mixed with 0.2% formaldehyde solution to convert free sulfite to the stable adduct hydroxymethylsulfonate (HMS), the extract is heated at 80 °C for 30 minutes to convert sulfite-carbonyl adducts to HMS, cleaned on a C18 SPE cartridge, and quantified against an internal standard, Na₂³⁴SO₃; results are converted from Na₂SO₃ to SO₂ using the molar ratio of 126 to 64.[18](https://www.fda.gov/media/114411/download)
**Interstellar molecular anions.** Polyatomic ions exist far from any solvent. The first astronomical detection of the C₁₀H⁻ anion was made toward the TMC-1 molecular cloud using the GOTHAM survey on the [Green Bank Telescope](https://www.edgepedia.org/green-bank-telescope), at greater than 9σ confidence; its column density ratio to neutral C₁₀H is the highest measured between an anion and neutral species to date and is at odds with current theories for interstellar anion abundances.[9](https://iopscience.iop.org/article/10.3847/2041-8213/acb648) Earlier surveys reported the first detections of C₃N⁻ and C₅N⁻ in Lupus-1A and of C₄H⁻ and C₆H⁻ in L483, plus new lines of C₆H⁻ toward six targeted sources including TMC-1 CP.[19](https://www.aanda.org/articles/aa/pdf/2023/09/aa47077-23.pdf) A 2024 ab initio study computed a full 5D reactive potential energy surface for the HCN + H⁻ reaction as a formation route to CN⁻, the smallest cyanopolyyne anion observed in several interstellar environments.[20](https://iopscience.iop.org/article/10.3847/1538-4357/ad630c) The TMC-1 detection itself is contested: a later survey of TMC-1 did not detect C₁₀H⁻ or C₁₀H in its data, while naming C₉N⁻ and C₁₀H⁻ as the best candidates for the B300 component in IRC+10216.[21](https://www.aanda.org/articles/aa/full_html/2026/09/aa61780-26/aa61780-26.html)
**Polysulfides and new sulfate chemistry.** Polysulfide anions Sₙ²⁻ are being quantified and characterized. A novel HPLC–ICP-MS method enables speciation and quantification of polysulfide anions and molecular sulfur in lithium polysulfide solutions without single-component standards, detecting the ³²S main isotope despite ¹⁶O₂ interference.[22](https://pubs.rsc.org/ja/article/39/10/2480/852726/Determination-of-polysulfide-anions-and-molecular) A 2025 DFT study calculated structures and UV-Vis-NIR, ESR, and Raman spectra of polysulfide Sₙ²⁻ anions, Sₙ•⁻ radical anions, and Sₙ neutral molecules (n = 2–6) in sodalite-cage minerals, clarifying previously ambiguous spectroscopic interpretations.[23](https://www.mdpi.com/2075-163X/15/11/1207) In 2025 chemists isolated and structurally characterized the first (inter-)halogen polysulfates, including [I₃]₄[S₄O₁₃]₂(SO₃), [IBr₂]₂[S₄O₁₃], [ICl₂][HS₂O₇], and [ICl₂]₂[S₂O₇], all accessible by reaction of I₂, Br₂, or ICl₃ with neat SO₃ in sealed glass ampoules.[24](https://pubs.rsc.org/en/content/articlehtml/2025/dt/d5dt02088c)
## References
1. [2.6 Ionic and Molecular Compounds, Chemistry 2e, OpenStax](https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds)
2. [3.3: Polyatomic ions and their compounds, Chemistry LibreTexts (Malik)](https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introduction_to_General_Chemistry_(Malik)/03%3A_Compounds/3.03%3A_Polyatomic_ions_and_their_compounds)
3. [IUPAC Gold Book, molecular ion (M03988)](https://goldbook.iupac.org/terms/view/M03988)
4. [IUPAC Gold Book, anion (A00358)](https://goldbook.iupac.org/terms/view/A00358)
5. [Sulfate, Chemeurope Encyclopedia](https://www.chemeurope.com/en/encyclopedia/Sulfate.html)
6. [Polyatomic ion, Chemeurope Encyclopedia](https://www.chemeurope.com/en/encyclopedia/Polyatomic_ion.html)
7. [Wine Labeling: Declaration of Sulfites, TTB](https://www.ttb.gov/regulated-commodities/beverage-alcohol/wine/wine-labeling-declaration-of-sulfites)
8. [SSD-TM-500 AO Sulfite, TTB](https://www.ttb.gov/system/files/images/pdfs/ssd/tm500.pdf)
9. [Astronomical Detection of the Interstellar Anion C10H− toward TMC-1, ApJL (GOTHAM)](https://iopscience.iop.org/article/10.3847/2041-8213/acb648)
10. [10.5: Resonance, Chemistry LibreTexts (Petrucci et al.)](https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/10%3A_Chemical_Bonding_I%3A_Basic_Concepts/10.5%3A_Resonance)
11. [IUPAC Brief Guide to the Nomenclature of Inorganic Chemistry](https://iupac.org/cms/wp-content/uploads/2018/05/Inorganic-Brief-Guide-V1-3.pdf)
12. [IUPAC Red Book (provisional), anion nomenclature chapter](https://moureu.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap5-3.04.pdf)
13. [The Complete List of Polyatomic Ions, 33Science](https://33science.com/2026/01/08/polyatomic-ions/)
14. [SO2 and Wine: A Review, OIV Collective Expertise Document](https://www.oiv.int/public/medias/7840/oiv-collective-expertise-document-so2-and-wine-a-review.pdf)
15. [Monovalent anion-selective membranes fabricated via in situ interfacial polymerization, Nature Communications (2025)](https://www.nature.com/articles/s41467-025-64196-2)
16. [21 CFR Part 101, Appendix A, Monier-Williams calculation](https://www.govinfo.gov/content/pkg/CFR-2013-title21-vol2/pdf/CFR-2013-title21-vol2-part101-appA.pdf)
17. [New Method for the Analysis of Sulfites in Foods, Federal Register (2022)](https://www.federalregister.gov/documents/2022/01/18/2022-00816/new-method-for-the-analysis-of-sulfites-in-foods)
18. [Determination of Sulfites in Food using LC-MS/MS, FDA CAM 004.04](https://www.fda.gov/media/114411/download)
19. [Abundance and excitation of molecular anions in interstellar clouds, A&A (2023)](https://www.aanda.org/articles/aa/pdf/2023/09/aa47077-23.pdf)
20. [Revisiting a Chemical Route to the Formation of CN− in the Interstellar Medium, ApJ (2024)](https://iopscience.iop.org/article/10.3847/1538-4357/ad630c)
21. [The spatial distribution of anions, neutrals, and cations in TMC-1, A&A (2026)](https://www.aanda.org/articles/aa/full_html/2026/09/aa61780-26/aa61780-26.html)
22. [Determination of polysulfide anions and molecular sulfur via HPLC–ICP-MS, J. Anal. At. Spectrom.](https://pubs.rsc.org/ja/article/39/10/2480/852726/Determination-of-polysulfide-anions-and-molecular)
23. [Spectroscopic Properties of Polysulfide Anions, Radical Anions, and Molecules, Minerals (2025)](https://www.mdpi.com/2075-163X/15/11/1207)
24. [Introducing (inter-)halogen polysulfates, Dalton Transactions (2025)](https://pubs.rsc.org/en/content/articlehtml/2025/dt/d5dt02088c)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Sulfites and sulfur(IV) oxyanions*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license