Polyatomic Ions 100: Formula, Charge and Naming Pattern
The charge is what you get wrong. It is on every card.
100 cardsLast updated 2026-08-17
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Polyatomic ions are not hard to recognise — they are hard to write down. You remember that sulfate has four oxygens and then guess at the charge, and the formula for calcium phosphate collapses. This deck gives each of 100 ions its formula, its charge on a line of its own, the naming rule that separates it from its neighbours, and one compound you have actually seen it in. They are grouped the way they are confused: the -ate/-ite pairs together so the single oxygen difference is visible, the hydrogen-containing ions together so you can see what adding a proton does to the charge, the positive ions together because there are so few, and the exceptions that follow no rule in their own group. Import it and the deck joins your spaced-repetition schedule. Ions you can write stretch out and drop out of sight; the ones you keep mis-charging come back until you stop.
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What's inside
Showing 100 representative cards from the full 100-card deck.
| Front | Back |
|---|---|
| ammonium | Formula: NH₄⁺ Charge: 1+ Family: Positive ions Naming: The only common positive polyatomic ion you meet early; it behaves like an alkali metal cation in formulas. Seen in: ammonium chloride, NH₄Cl |
| hydronium | Formula: H₃O⁺ Charge: 1+ Family: Positive ions Naming: What a proton actually is in water; acids are written as producing it. Seen in: hydronium chloride in solution, H₃O⁺Cl⁻ |
| mercury(I) | Formula: Hg₂²⁺ Charge: 2+ Family: Positive ions Naming: Two mercury atoms bonded to each other share the charge, so the formula is never Hg⁺. Seen in: mercury(I) chloride, Hg₂Cl₂ |
| phosphonium | Formula: PH₄⁺ Charge: 1+ Family: Positive ions Naming: The phosphorus analogue of ammonium, formed the same way by adding a proton. Seen in: phosphonium iodide, PH₄I |
| nitronium | Formula: NO₂⁺ Charge: 1+ Family: Positive ions Naming: The attacking species in aromatic nitration; note the charge is positive unlike nitrite. Seen in: nitronium tetrafluoroborate, NO₂BF₄ |
| uranyl | Formula: UO₂²⁺ Charge: 2+ Family: Positive ions Naming: Uranium bound to two oxygens keeps a 2+ charge and stays intact in solution. Seen in: uranyl nitrate, UO₂(NO₃)₂ |
| hydroxide | Formula: OH⁻ Charge: 1− Family: Common exceptions Naming: Not an -ate or -ite; it is the ion that makes a solution basic. Seen in: sodium hydroxide, NaOH |
| peroxide | Formula: O₂²⁻ Charge: 2− Family: Common exceptions Naming: Two oxygens sharing a 2− charge, with oxygen in the unusual −1 state. Seen in: hydrogen peroxide, H₂O₂ |
| superoxide | Formula: O₂⁻ Charge: 1− Family: Common exceptions Naming: One less electron than peroxide; formed by the heavier alkali metals. Seen in: potassium superoxide, KO₂ |
| cyanide | Formula: CN⁻ Charge: 1− Family: Common exceptions Naming: Ends in -ide despite having two atoms, which breaks the usual naming rule. Seen in: sodium cyanide, NaCN |
| azide | Formula: N₃⁻ Charge: 1− Family: Common exceptions Naming: Three nitrogens in a row; decomposes to nitrogen gas, which is how airbags inflate. Seen in: sodium azide, NaN₃ |
| acetate | Formula: CH₃COO⁻ Charge: 1− Family: -ate and -ite families Naming: Also written C₂H₃O₂⁻; the first organic ion most courses introduce. Seen in: sodium acetate, CH₃COONa |
| carbonate | Formula: CO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: The 2− charge is why carbonates of group 2 metals need no subscript. Seen in: calcium carbonate, CaCO₃ |
| nitrate | Formula: NO₃⁻ Charge: 1− Family: -ate and -ite families Naming: -ate is the oxygen-rich member of the pair; nitrite is NO₂⁻. Seen in: silver nitrate, AgNO₃ |
| nitrite | Formula: NO₂⁻ Charge: 1− Family: -ate and -ite families Naming: One oxygen fewer than nitrate, same charge. Seen in: sodium nitrite, NaNO₂ |
| sulfate | Formula: SO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Sulfite is SO₃²⁻; only the oxygen count changes, not the charge. Seen in: copper(II) sulfate, CuSO₄ |
| sulfite | Formula: SO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: One oxygen fewer than sulfate, same 2− charge. Seen in: sodium sulfite, Na₂SO₃ |
| phosphate | Formula: PO₄³⁻ Charge: 3− Family: -ate and -ite families Naming: The 3− charge is unusual and drives most of the tricky formula work. Seen in: calcium phosphate, Ca₃(PO₄)₂ |
| phosphite | Formula: PO₃³⁻ Charge: 3− Family: -ate and -ite families Naming: One oxygen fewer than phosphate, same 3− charge. Seen in: sodium phosphite, Na₃PO₃ |
| chlorate | Formula: ClO₃⁻ Charge: 1− Family: -ate and -ite families Naming: The middle of a four-member series running from hypochlorite to perchlorate. Seen in: potassium chlorate, KClO₃ |
| chlorite | Formula: ClO₂⁻ Charge: 1− Family: -ate and -ite families Naming: One oxygen fewer than chlorate. Seen in: sodium chlorite, NaClO₂ |
| hypochlorite | Formula: ClO⁻ Charge: 1− Family: -ate and -ite families Naming: hypo- marks one oxygen fewer than the -ite; this is the bleach ion. Seen in: sodium hypochlorite, NaClO |
| perchlorate | Formula: ClO₄⁻ Charge: 1− Family: -ate and -ite families Naming: per- marks one oxygen more than the -ate. Seen in: potassium perchlorate, KClO₄ |
| bromate | Formula: BrO₃⁻ Charge: 1− Family: -ate and -ite families Naming: Same pattern as chlorate one row down the halogens. Seen in: potassium bromate, KBrO₃ |
| perbromate | Formula: BrO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The per- member of the bromine series. Seen in: potassium perbromate, KBrO₄ |
| iodate | Formula: IO₃⁻ Charge: 1− Family: -ate and -ite families Naming: Added to table salt as a source of iodine. Seen in: potassium iodate, KIO₃ |
| periodate | Formula: IO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The per- member of the iodine series. Seen in: sodium periodate, NaIO₄ |
| chromate | Formula: CrO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Yellow in solution; converts to orange dichromate in acid. Seen in: potassium chromate, K₂CrO₄ |
| dichromate | Formula: Cr₂O₇²⁻ Charge: 2− Family: -ate and -ite families Naming: Two chromiums but still 2−, which is the easy mistake. Seen in: potassium dichromate, K₂Cr₂O₇ |
| permanganate | Formula: MnO₄⁻ Charge: 1− Family: -ate and -ite families Naming: Deep purple; a standard oxidising agent in titrations. Seen in: potassium permanganate, KMnO₄ |
| manganate | Formula: MnO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Same formula as permanganate but 2−, and green rather than purple. Seen in: potassium manganate, K₂MnO₄ |
| oxalate | Formula: C₂O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Two linked carboxylate groups; forms insoluble calcium salts. Seen in: calcium oxalate, CaC₂O₄ |
| borate | Formula: BO₃³⁻ Charge: 3− Family: -ate and -ite families Naming: Usually met as the tetraborate in borax rather than on its own. Seen in: sodium borate, Na₃BO₃ |
| tetraborate | Formula: B₄O₇²⁻ Charge: 2− Family: -ate and -ite families Naming: The ion in borax; four borons share a 2− charge. Seen in: borax, Na₂B₄O₇ |
| silicate | Formula: SiO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: The metasilicate form; silicate minerals are chains of these units. Seen in: sodium silicate, Na₂SiO₃ |
| orthosilicate | Formula: SiO₄⁴⁻ Charge: 4− Family: -ate and -ite families Naming: The 4− charge appears in olivine and other rock-forming minerals. Seen in: magnesium orthosilicate, Mg₂SiO₄ |
| arsenate | Formula: AsO₄³⁻ Charge: 3− Family: -ate and -ite families Naming: Mimics phosphate closely enough to be a metabolic poison. Seen in: sodium arsenate, Na₃AsO₄ |
| arsenite | Formula: AsO₃³⁻ Charge: 3− Family: -ate and -ite families Naming: One oxygen fewer than arsenate. Seen in: sodium arsenite, Na₃AsO₃ |
| selenate | Formula: SeO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: The selenium analogue of sulfate, one row down. Seen in: sodium selenate, Na₂SeO₄ |
| selenite | Formula: SeO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: The selenium analogue of sulfite. Seen in: sodium selenite, Na₂SeO₃ |
| thiosulfate | Formula: S₂O₃²⁻ Charge: 2− Family: -ate and -ite families Naming: thio- means an oxygen has been replaced by sulfur; used to fix photographs. Seen in: sodium thiosulfate, Na₂S₂O₃ |
| thiocyanate | Formula: SCN⁻ Charge: 1− Family: -ate and -ite families Naming: Cyanide with a sulfur attached; gives a blood-red complex with iron(III). Seen in: potassium thiocyanate, KSCN |
| cyanate | Formula: OCN⁻ Charge: 1− Family: -ate and -ite families Naming: The oxygen version of thiocyanate; note the different first atom. Seen in: potassium cyanate, KOCN |
| persulfate | Formula: S₂O₈²⁻ Charge: 2− Family: -ate and -ite families Naming: Also called peroxydisulfate; contains an O–O bond, hence per-. Seen in: ammonium persulfate, (NH₄)₂S₂O₈ |
| dithionite | Formula: S₂O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: A strong reducing agent used to strip colour. Seen in: sodium dithionite, Na₂S₂O₄ |
| dithionate | Formula: S₂O₆²⁻ Charge: 2− Family: -ate and -ite families Naming: Two more oxygens than dithionite, same charge. Seen in: sodium dithionate, Na₂S₂O₆ |
| pyrophosphate | Formula: P₂O₇⁴⁻ Charge: 4− Family: -ate and -ite families Naming: Also called diphosphate; released when ATP is used. Seen in: sodium pyrophosphate, Na₄P₂O₇ |
| metaphosphate | Formula: PO₃⁻ Charge: 1− Family: -ate and -ite families Naming: Same atoms as phosphite but 1− rather than 3−; the charge is the tell. Seen in: sodium metaphosphate, NaPO₃ |
| hypophosphite | Formula: H₂PO₂⁻ Charge: 1− Family: -ate and -ite families Naming: Two hydrogens are bonded to phosphorus, not available as acid. Seen in: sodium hypophosphite, NaH₂PO₂ |
| molybdate | Formula: MoO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Follows the sulfate shape with a transition metal at the centre. Seen in: sodium molybdate, Na₂MoO₄ |
| tungstate | Formula: WO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: The tungsten analogue of molybdate. Seen in: sodium tungstate, Na₂WO₄ |
| vanadate | Formula: VO₃⁻ Charge: 1− Family: -ate and -ite families Naming: The metavanadate form, the one usually tabulated. Seen in: sodium vanadate, NaVO₃ |
| aluminate | Formula: AlO₂⁻ Charge: 1− Family: -ate and -ite families Naming: Formed when aluminium hydroxide dissolves in excess base. Seen in: sodium aluminate, NaAlO₂ |
| zincate | Formula: ZnO₂²⁻ Charge: 2− Family: -ate and -ite families Naming: Formed when zinc hydroxide dissolves in excess base. Seen in: sodium zincate, Na₂ZnO₂ |
| stannate | Formula: SnO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: Tin in its higher oxidation state, dissolved by base. Seen in: sodium stannate, Na₂SnO₃ |
| ferrate | Formula: FeO₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Iron in the +6 state; a strong oxidiser used in water treatment. Seen in: potassium ferrate, K₂FeO₄ |
| chromite | Formula: CrO₂⁻ Charge: 1− Family: -ate and -ite families Naming: Chromium(III) dissolved in base; the mineral chromite is FeCr₂O₄. Seen in: sodium chromite, NaCrO₂ |
| formate | Formula: HCOO⁻ Charge: 1− Family: -ate and -ite families Naming: The one-carbon carboxylate; also written CHO₂⁻. Seen in: sodium formate, HCOONa |
| propanoate | Formula: C₂H₅COO⁻ Charge: 1− Family: -ate and -ite families Naming: The three-carbon carboxylate, used as a preservative. Seen in: calcium propanoate, Ca(C₂H₅COO)₂ |
| benzoate | Formula: C₆H₅COO⁻ Charge: 1− Family: -ate and -ite families Naming: A ring carboxylate widely used as a food preservative. Seen in: sodium benzoate, C₆H₅COONa |
| citrate | Formula: C₆H₅O₇³⁻ Charge: 3− Family: -ate and -ite families Naming: Three carboxyl groups give the 3− charge; binds calcium. Seen in: sodium citrate, Na₃C₆H₅O₇ |
| tartrate | Formula: C₄H₄O₆²⁻ Charge: 2− Family: -ate and -ite families Naming: Two carboxyl groups; the ion in cream of tartar. Seen in: sodium tartrate, Na₂C₄H₄O₆ |
| lactate | Formula: CH₃CH(OH)COO⁻ Charge: 1− Family: -ate and -ite families Naming: Produced by muscles under low oxygen. Seen in: sodium lactate, CH₃CH(OH)COONa |
| salicylate | Formula: C₇H₅O₃⁻ Charge: 1− Family: -ate and -ite families Naming: The active ion of aspirin after hydrolysis. Seen in: sodium salicylate, NaC₇H₅O₃ |
| malate | Formula: C₄H₄O₅²⁻ Charge: 2− Family: -ate and -ite families Naming: A citric acid cycle intermediate. Seen in: sodium malate, Na₂C₄H₄O₅ |
| succinate | Formula: C₄H₄O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Another citric acid cycle intermediate. Seen in: sodium succinate, Na₂C₄H₄O₄ |
| fumarate | Formula: C₄H₂O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: The trans isomer of maleate; also in the citric acid cycle. Seen in: sodium fumarate, Na₂C₄H₂O₄ |
| glutamate | Formula: C₅H₈NO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The amino acid anion used as a flavour enhancer. Seen in: monosodium glutamate, NaC₅H₈NO₄ |
| gluconate | Formula: C₆H₁₁O₇⁻ Charge: 1− Family: -ate and -ite families Naming: Used to carry metal ions in supplements. Seen in: calcium gluconate, Ca(C₆H₁₁O₇)₂ |
| carbamate | Formula: NH₂COO⁻ Charge: 1− Family: -ate and -ite families Naming: A carboxylate with nitrogen in place of one carbon substituent. Seen in: ammonium carbamate, NH₄NH₂COO |
| hydrogen carbonate | Formula: HCO₃⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Also called bicarbonate; one hydrogen has taken one of carbonate's charges. Seen in: sodium hydrogen carbonate, NaHCO₃ |
| hydrogen sulfate | Formula: HSO₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Also called bisulfate; adding one H to sulfate leaves 1−. Seen in: sodium hydrogen sulfate, NaHSO₄ |
| hydrogen sulfite | Formula: HSO₃⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Also called bisulfite; used as a preservative. Seen in: sodium hydrogen sulfite, NaHSO₃ |
| hydrogen sulfide | Formula: HS⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Also called bisulfide; the singly protonated sulfide ion. Seen in: sodium hydrogen sulfide, NaHS |
| hydrogen phosphate | Formula: HPO₄²⁻ Charge: 2− Family: Hydrogen-containing ions Naming: One hydrogen on phosphate leaves 2−; a buffer inside cells. Seen in: sodium hydrogen phosphate, Na₂HPO₄ |
| dihydrogen phosphate | Formula: H₂PO₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Two hydrogens leave 1−; the other half of the phosphate buffer. Seen in: sodium dihydrogen phosphate, NaH₂PO₄ |
| hydrogen oxalate | Formula: HC₂O₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Also called binoxalate; one carboxyl still deprotonated. Seen in: potassium hydrogen oxalate, KHC₂O₄ |
| hydrogen tartrate | Formula: HC₄H₄O₆⁻ Charge: 1− Family: Hydrogen-containing ions Naming: The ion in cream of tartar, potassium hydrogen tartrate. Seen in: potassium hydrogen tartrate, KHC₄H₄O₆ |
| hydrogen phthalate | Formula: HC₈H₄O₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Its potassium salt is a primary standard for titrations. Seen in: potassium hydrogen phthalate, KHC₈H₄O₄ |
| hydrogen arsenate | Formula: HAsO₄²⁻ Charge: 2− Family: Hydrogen-containing ions Naming: Follows the hydrogen phosphate pattern one row down. Seen in: sodium hydrogen arsenate, Na₂HAsO₄ |
| dihydrogen arsenate | Formula: H₂AsO₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Follows the dihydrogen phosphate pattern. Seen in: sodium dihydrogen arsenate, NaH₂AsO₄ |
| hydrogen selenate | Formula: HSeO₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: The selenium analogue of hydrogen sulfate. Seen in: sodium hydrogen selenate, NaHSeO₄ |
| hydrogen selenite | Formula: HSeO₃⁻ Charge: 1− Family: Hydrogen-containing ions Naming: The selenium analogue of hydrogen sulfite. Seen in: sodium hydrogen selenite, NaHSeO₃ |
| amide | Formula: NH₂⁻ Charge: 1− Family: Common exceptions Naming: Ammonia less one proton; a very strong base. Seen in: sodium amide, NaNH₂ |
| triiodide | Formula: I₃⁻ Charge: 1− Family: Common exceptions Naming: Three iodines sharing one charge; the species in iodine solution. Seen in: potassium triiodide, KI₃ |
| carbide | Formula: C₂²⁻ Charge: 2− Family: Common exceptions Naming: The acetylide ion; reacts with water to give ethyne. Seen in: calcium carbide, CaC₂ |
| tetrahydroborate | Formula: BH₄⁻ Charge: 1− Family: Common exceptions Naming: Also called borohydride; a standard reducing agent. Seen in: sodium borohydride, NaBH₄ |
| tetrafluoroborate | Formula: BF₄⁻ Charge: 1− Family: Common exceptions Naming: A weakly coordinating ion used to isolate reactive cations. Seen in: sodium tetrafluoroborate, NaBF₄ |
| hexafluorophosphate | Formula: PF₆⁻ Charge: 1− Family: Common exceptions Naming: Common in lithium-ion battery electrolytes. Seen in: lithium hexafluorophosphate, LiPF₆ |
| hexafluorosilicate | Formula: SiF₆²⁻ Charge: 2− Family: Common exceptions Naming: Added to water supplies as a fluoride source. Seen in: sodium hexafluorosilicate, Na₂SiF₆ |
| hexacyanoferrate(II) | Formula: Fe(CN)₆⁴⁻ Charge: 4− Family: Common exceptions Naming: Also called ferrocyanide; iron is +2 inside the complex. Seen in: potassium ferrocyanide, K₄Fe(CN)₆ |
| hexacyanoferrate(III) | Formula: Fe(CN)₆³⁻ Charge: 3− Family: Common exceptions Naming: Also called ferricyanide; iron is +3 inside the complex. Seen in: potassium ferricyanide, K₃Fe(CN)₆ |
| tetrachloroaurate | Formula: AuCl₄⁻ Charge: 1− Family: Common exceptions Naming: The gold species formed when gold dissolves in aqua regia. Seen in: sodium tetrachloroaurate, NaAuCl₄ |
| tetrahydroxozincate | Formula: Zn(OH)₄²⁻ Charge: 2− Family: Common exceptions Naming: The hydrated form of zincate, written with explicit hydroxides. Seen in: sodium tetrahydroxozincate, Na₂Zn(OH)₄ |
| tetrahydroxoaluminate | Formula: Al(OH)₄⁻ Charge: 1− Family: Common exceptions Naming: The hydrated form of aluminate; shows why aluminium dissolves in base. Seen in: sodium tetrahydroxoaluminate, NaAl(OH)₄ |
| diamminesilver(I) | Formula: Ag(NH₃)₂⁺ Charge: 1+ Family: Positive ions Naming: The complex that keeps silver dissolved in Tollens' reagent. Seen in: Tollens' reagent, [Ag(NH₃)₂]OH |
| tetraamminecopper(II) | Formula: Cu(NH₃)₄²⁺ Charge: 2+ Family: Positive ions Naming: The deep blue complex formed when excess ammonia meets copper(II). Seen in: tetraamminecopper(II) sulfate, [Cu(NH₃)₄]SO₄ |
| perrhenate | Formula: ReO₄⁻ Charge: 1− Family: -ate and -ite families Naming: Follows the permanganate shape with rhenium at the centre. Seen in: ammonium perrhenate, NH₄ReO₄ |
| pertechnetate | Formula: TcO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The form technetium takes in medical imaging. Seen in: sodium pertechnetate, NaTcO₄ |
| metaborate | Formula: BO₂⁻ Charge: 1− Family: -ate and -ite families Naming: The dehydrated form of borate. Seen in: sodium metaborate, NaBO₂ |
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How are the 100 ions grouped?
62 in the -ate and -ite families, 13 hydrogen-containing ions, 8 positive ions and 17 common exceptions. Every card is also tagged with its charge, so you can drill all the 3− ions together — which is where most formula mistakes come from.
Why is the charge on its own line?
Because that is the part that gets dropped. Most people can recall that phosphate is PO₄ and then write Ca₂PO₄ instead of Ca₃(PO₄)₂. Putting the charge on its own line means you are asked for it every single time the card comes up, rather than reading past it inside a formula.
Editorial method and sources
Memly's editors use the primary sources below to check scope and terminology, then independently write and review the cards.
Polyatomic Ions 100: Formula, Charge and Naming Pattern
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Compiled 2026-08-17. Every card is written by Memly, with formulas and charges checked against standard chemistry references. Several of these ions are toxic or strongly oxidising; nothing here is handling guidance or a laboratory procedure.