Polyatomic Ions 100: Formula and Naming Pattern Set
The charge is what you get wrong. It is in every answer.
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| 表 | 裏 |
|---|---|
| ammonium — what is its formula? | NH₄⁺ The only common positive polyatomic ion you meet early; it behaves like an alkali metal cation in formulas. |
| mercury(I) — what is its formula? | Hg₂²⁺ Two mercury atoms bonded to each other share the charge, so the formula is never Hg⁺. |
| nitronium — what is its formula? | NO₂⁺ The attacking species in aromatic nitration; note the charge is positive unlike nitrite. |
| peroxide — what is its formula? | O₂²⁻ Two oxygens sharing a 2− charge, with oxygen in the unusual −1 state. |
| azide — what is its formula? | N₃⁻ Three nitrogens in a row; decomposes to nitrogen gas, which is how airbags inflate. |
| carbonate — what is its formula? | CO₃²⁻ The 2− charge is why carbonates of group 2 metals need no subscript. |
| nitrite — what is its formula? | NO₂⁻ One oxygen fewer than nitrate, same charge. |
| phosphate — what is its formula? | PO₄³⁻ The 3− charge is unusual and drives most of the tricky formula work. |
| chlorate — what is its formula? | ClO₃⁻ The middle of a four-member series running from hypochlorite to perchlorate. |
| hypochlorite — what is its formula? | ClO⁻ hypo- marks one oxygen fewer than the -ite; this is the bleach ion. |
| perbromate — what is its formula? | BrO₄⁻ The per- member of the bromine series. |
| periodate — what is its formula? | IO₄⁻ The per- member of the iodine series. |
| dichromate — what is its formula? | Cr₂O₇²⁻ Two chromiums but still 2−, which is the easy mistake. |
| oxalate — what is its formula? | C₂O₄²⁻ Two linked carboxylate groups; forms insoluble calcium salts. |
| orthosilicate — what is its formula? | SiO₄⁴⁻ The 4− charge appears in olivine and other rock-forming minerals. |
| selenite — what is its formula? | SeO₃²⁻ The selenium analogue of sulfite. |
| persulfate — what is its formula? | S₂O₈²⁻ Also called peroxydisulfate; contains an O–O bond, hence per-. |
| metaphosphate — what is its formula? | PO₃⁻ Same atoms as phosphite but 1− rather than 3−; the charge is the tell. |
| aluminate — what is its formula? | AlO₂⁻ Formed when aluminium hydroxide dissolves in excess base. |
| chromite — what is its formula? | CrO₂⁻ Chromium(III) dissolved in base; the mineral chromite is FeCr₂O₄. |
| tartrate — what is its formula? | C₄H₄O₆²⁻ Two carboxyl groups; the ion in cream of tartar. |
| succinate — what is its formula? | C₄H₄O₄²⁻ Another citric acid cycle intermediate. |
| hydrogen carbonate — what is its formula? | HCO₃⁻ Also called bicarbonate; one hydrogen has taken one of carbonate's charges. |
| dihydrogen phosphate — what is its formula? | H₂PO₄⁻ Two hydrogens leave 1−; the other half of the phosphate buffer. |
| dihydrogen arsenate — what is its formula? | H₂AsO₄⁻ Follows the dihydrogen phosphate pattern. |
| carbide — what is its formula? | C₂²⁻ The acetylide ion; reacts with water to give ethyne. |
| hexafluorophosphate — what is its formula? | PF₆⁻ Common in lithium-ion battery electrolytes. |
| hexacyanoferrate(III) — what is its formula? | Fe(CN)₆³⁻ Also called ferricyanide; iron is +3 inside the complex. |
| diamminesilver(I) — what is its formula? | Ag(NH₃)₂⁺ The complex that keeps silver dissolved in Tollens' reagent. |
| metaborate — what is its formula? | BO₂⁻ The dehydrated form of borate. |
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 and the naming rule that separates it from its neighbours. 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 does not use your AI allowance, 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 does the answer always carry the charge?
- 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. The answer is the complete formula, charge included, so you are asked for it every single time the card comes up.
Compiled 2026-08-17. Every card is written by Memly. Several of these ions are toxic or strongly oxidising; nothing here is handling guidance or a laboratory procedure.