Polyatomic Ions 100: Formula, Charge and Naming Pattern
The charge is what you get wrong. It is on every card.
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| 表 | 裏 |
|---|---|
| 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 |
| 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 |
| 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 |
| 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 |
| 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 |
| 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 |
| nitrite | Formula: NO₂⁻ Charge: 1− Family: -ate and -ite families Naming: One oxygen fewer than nitrate, same charge. Seen in: sodium nitrite, NaNO₂ |
| 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 |
| 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 |
| 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 |
| perbromate | Formula: BrO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The per- member of the bromine series. Seen in: potassium perbromate, KBrO₄ |
| periodate | Formula: IO₄⁻ Charge: 1− Family: -ate and -ite families Naming: The per- member of the iodine series. Seen in: sodium periodate, NaIO₄ |
| 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 |
| oxalate | Formula: C₂O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Two linked carboxylate groups; forms insoluble calcium salts. Seen in: calcium oxalate, CaC₂O₄ |
| 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 |
| selenite | Formula: SeO₃²⁻ Charge: 2− Family: -ate and -ite families Naming: The selenium analogue of sulfite. Seen in: sodium selenite, Na₂SeO₃ |
| 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 |
| 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 |
| aluminate | Formula: AlO₂⁻ Charge: 1− Family: -ate and -ite families Naming: Formed when aluminium hydroxide dissolves in excess base. Seen in: sodium aluminate, NaAlO₂ |
| 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 |
| 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 |
| succinate | Formula: C₄H₄O₄²⁻ Charge: 2− Family: -ate and -ite families Naming: Another citric acid cycle intermediate. Seen in: sodium succinate, Na₂C₄H₄O₄ |
| 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 |
| 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 |
| dihydrogen arsenate | Formula: H₂AsO₄⁻ Charge: 1− Family: Hydrogen-containing ions Naming: Follows the dihydrogen phosphate pattern. Seen in: sodium dihydrogen arsenate, NaH₂AsO₄ |
| carbide | Formula: C₂²⁻ Charge: 2− Family: Common exceptions Naming: The acetylide ion; reacts with water to give ethyne. Seen in: calcium carbide, CaC₂ |
| hexafluorophosphate | Formula: PF₆⁻ Charge: 1− Family: Common exceptions Naming: Common in lithium-ion battery electrolytes. Seen in: lithium hexafluorophosphate, LiPF₆ |
| 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 |
| 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 |
| metaborate | Formula: BO₂⁻ Charge: 1− Family: -ate and -ite families Naming: The dehydrated form of borate. Seen in: sodium metaborate, NaBO₂ |
このデッキについて
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.
よくある質問
- Can I import the whole deck on the free plan?
- Yes. Importing a saved deck runs no new AI generation and spends no AI credits, so the free plan imports every card. You can study, edit and delete them afterwards.
- Will importing it twice create duplicates?
- No. Cards you already have are skipped and only cards added in a revision come through. Including re-imports after deleting it, one official deck can be imported three times per account.
- Does it work on the web and in the mobile app?
- Yes. The deck is added to your account, so the web app, the iOS app and the Android app all show the same cards and the same progress.
- Can I edit the cards after importing?
- Yes. Once imported they are your cards: edit either side, delete the ones you do not need, retag them, or move them into another deck.
- 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.
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.