Polyatomic ions — the ~30 you have to know
A polyatomic ion is a covalently bonded group of atoms carrying a net charge that travels through reactions as a single unit. Every general-chemistry course expects you to recognize the same short list on sight — formula, charge, and name — and to spell the -ate / -ite series correctly. This card is that list.
The core list
| Ion | Formula | Charge | Note or common source |
|---|---|---|---|
| Ammonium | NH₄⁺ | +1 | Only common polyatomic cation; NH₄Cl, (NH₄)₂SO₄ |
| Hydronium | H₃O⁺ | +1 | Aqueous proton — what “H⁺” really is in water |
| Hydroxide | OH⁻ | -1 | NaOH, KOH; the base anion |
| Nitrate | NO₃⁻ | -1 | HNO₃, KNO₃; always soluble |
| Nitrite | NO₂⁻ | -1 | NaNO₂ (cured-meat preservative) |
| Sulfate | SO₄²⁻ | -2 | H₂SO₄, Na₂SO₄, BaSO₄ |
| Sulfite | SO₃²⁻ | -2 | Na₂SO₃ (food preservative) |
| Hydrogen sulfate | HSO₄⁻ | -1 | Also called bisulfate |
| Phosphate | PO₄³⁻ | -3 | H₃PO₄, Na₃PO₄; bone mineral |
| Hydrogen phosphate | HPO₄²⁻ | -2 | Buffer species at physiological pH |
| Dihydrogen phosphate | H₂PO₄⁻ | -1 | Paired with HPO₄²⁻ in phosphate buffer |
| Phosphite | PO₃³⁻ | -3 | Rare in bio; H₃PO₃ is actually diprotic |
| Carbonate | CO₃²⁻ | -2 | Na₂CO₃, CaCO₃ (limestone, marble) |
| Bicarbonate | HCO₃⁻ | -1 | NaHCO₃ (baking soda); blood buffer |
| Acetate | CH₃COO⁻ (C₂H₃O₂⁻) | -1 | Conjugate base of acetic acid |
| Perchlorate | ClO₄⁻ | -1 | KClO₄; conjugate base of a strong acid |
| Chlorate | ClO₃⁻ | -1 | KClO₃ (oxidizer) |
| Chlorite | ClO₂⁻ | -1 | NaClO₂ (bleaching) |
| Hypochlorite | ClO⁻ | -1 | NaClO (household bleach) |
| Permanganate | MnO₄⁻ | -1 | KMnO₄ (deep-purple oxidant) |
| Chromate | CrO₄²⁻ | -2 | K₂CrO₄ (yellow) |
| Dichromate | Cr₂O₇²⁻ | -2 | K₂Cr₂O₇ (orange, strong oxidizer) |
| Cyanide | CN⁻ | -1 | NaCN, KCN |
| Thiocyanate | SCN⁻ | -1 | Fe(SCN)³ complex is blood-red |
| Cyanate | OCN⁻ | -1 | Constitutional isomer of fulminate CNO⁻ |
| Oxalate | C₂O₄²⁻ | -2 | Kidney-stone anion; parent acid H₂C₂O₄ |
| Thiosulfate | S₂O₃²⁻ | -2 | Na₂S₂O₃ (photography, iodometric titration) |
| Peroxide | O₂²⁻ | -2 | Na₂O₂; H₂O₂ is the protonated form |
| Azide | N₃⁻ | -1 | NaN₃ (airbag propellant, biology preservative) |
| Arsenate | AsO₄³⁻ | -3 | Toxic phosphate analog |
| Borate | BO₃³⁻ | -3 | Na₂B₄O₇·10H₂O (borax) is a related polyanion |
| Silicate | SiO₃²⁻ | -2 | Chain / framework silicates dominate crustal rocks |
Formulas and charges pulled from Zumdahl 10e Table 2.5 and cross-checked against Silberberg 9e Table 2.5.
The -ate / -ite pattern
For a given central atom, the “-ate” ion has one more oxygen than the “-ite” ion. The charge stays the same across the pair — only the oxygen count changes.
- SO₄²⁻ sulfate vs SO₃²⁻ sulfite — same -2 charge, sulfite has one less O.
- NO₃⁻ nitrate vs NO₂⁻ nitrite — same -1 charge.
- PO₄³⁻ phosphate vs PO₃³⁻ phosphite — same -3 charge.
The -ate is the one you meet first (nitrate, sulfate, phosphate, carbonate). The -ite is the “one less O” cousin.
The per- / hypo- prefixes (chlorine oxoanions)
Chlorine and the other halogens form a four-member series. Charge stays -1 the whole way; the oxygen count steps down by one at each row.
| Prefix pattern | Formula | Name | Oxygens |
|---|---|---|---|
| per- + -ate | ClO₄⁻ | perchlorate | 4 |
| -ate | ClO₃⁻ | chlorate | 3 |
| -ite | ClO₂⁻ | chlorite | 2 |
| hypo- + -ite | ClO⁻ | hypochlorite | 1 |
Memory hook: “per = one more O than -ate; hypo = one less O than -ite.” The same four-name pattern works for bromine (perbromate, bromate, bromite, hypobromite) and iodine (periodate, iodate, iodite, hypoiodite).
Corresponding acids
Drop the -ate / -ite ending and add H⁺‘s to charge-balance to get the parent acid:
- -ate → -ic acid — SO₄²⁻ → H₂SO₄ (sulfuric), NO₃⁻ → HNO₃ (nitric), ClO₃⁻ → HClO₃ (chloric).
- -ite → -ous acid — SO₃²⁻ → H₂SO₃ (sulfurous), NO₂⁻ → HNO₂ (nitrous), ClO₂⁻ → HClO₂ (chlorous).
- per- + -ate → per- + -ic acid — ClO₄⁻ → HClO₄ (perchloric).
- hypo- + -ite → hypo- + -ous acid — ClO⁻ → HClO (hypochlorous).
Mnemonics — useful, but limited
You’ll see “Nick the Camel ate a Clam for supper in Phoenix” quoted as a memory hook for common charges: Nitrate / nitrite (-1), Carbonate (-2), acetate (-1), Chlorate (-1), sulfate (-2), phosphate (-3). It’s a real study aid, but the encoding is loose — it gives you the charge but not which member of the -ate / -ite pair. Long term, learning the family patterns above is more robust: any single-atom slip is then recoverable from the pattern.
Common student mistakes
- Writing NH₄⁻ instead of NH₄⁺. Ammonium is the odd one out — a polyatomic cation with charge +1.
- Mixing up -ate and -ite charges. Members of a pair share a charge; only the oxygen count differs.
- Losing the charge across the chlorine series. Perchlorate, chlorate, chlorite, and hypochlorite all carry -1 — the O count drops from 4 to 1 but the charge does not.
- Confusing sulfate with sulfide. SO₄²⁻ is sulfate (polyatomic oxoanion); S²⁻ is sulfide (monatomic).
- Reading “bi-” as “two”. Bicarbonate is HCO₃⁻ — one hydrogen on one carbonate, not two carbonates. “Bi-” is legacy nomenclature for “hydrogen”.
Sources
- Zumdahl, Chemistry, 10th ed. (2018), Table 2.5 — common polyatomic ions and their charges
- Silberberg, Chemistry: The Molecular Nature of Matter and Change, 9th ed. (2021), Table 2.5 — polyatomic ions
- IUPAC Nomenclature of Inorganic Chemistry (Red Book, 2005), Ch. IR-5 — oxoacid and oxoanion names
- CRC Handbook of Chemistry and Physics, 97th ed. (2016) — inorganic species reference
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