Polyatomic Ions 100: Formula and Naming Pattern Set

The charge is what you get wrong. It is in every answer.

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 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.

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What's inside

Showing 100 representative cards from the full 100-card deck.

Front	Back
ammonium — what is its formula?	NH₄⁺ The only common positive polyatomic ion you meet early; it behaves like an alkali metal cation in formulas.
hydronium — what is its formula?	H₃O⁺ What a proton actually is in water; acids are written as producing it.
mercury(I) — what is its formula?	Hg₂²⁺ Two mercury atoms bonded to each other share the charge, so the formula is never Hg⁺.
phosphonium — what is its formula?	PH₄⁺ The phosphorus analogue of ammonium, formed the same way by adding a proton.
nitronium — what is its formula?	NO₂⁺ The attacking species in aromatic nitration; note the charge is positive unlike nitrite.
uranyl — what is its formula?	UO₂²⁺ Uranium bound to two oxygens keeps a 2+ charge and stays intact in solution.
hydroxide — what is its formula?	OH⁻ Not an -ate or -ite; it is the ion that makes a solution basic.
peroxide — what is its formula?	O₂²⁻ Two oxygens sharing a 2− charge, with oxygen in the unusual −1 state.
superoxide — what is its formula?	O₂⁻ One less electron than peroxide; formed by the heavier alkali metals.
cyanide — what is its formula?	CN⁻ Ends in -ide despite having two atoms, which breaks the usual naming rule.
azide — what is its formula?	N₃⁻ Three nitrogens in a row; decomposes to nitrogen gas, which is how airbags inflate.
acetate — what is its formula?	CH₃COO⁻ Also written C₂H₃O₂⁻; the first organic ion most courses introduce.
carbonate — what is its formula?	CO₃²⁻ The 2− charge is why carbonates of group 2 metals need no subscript.
nitrate — what is its formula?	NO₃⁻ -ate is the oxygen-rich member of the pair; nitrite is NO₂⁻.
nitrite — what is its formula?	NO₂⁻ One oxygen fewer than nitrate, same charge.
sulfate — what is its formula?	SO₄²⁻ Sulfite is SO₃²⁻; only the oxygen count changes, not the charge.
sulfite — what is its formula?	SO₃²⁻ One oxygen fewer than sulfate, same 2− charge.
phosphate — what is its formula?	PO₄³⁻ The 3− charge is unusual and drives most of the tricky formula work.
phosphite — what is its formula?	PO₃³⁻ One oxygen fewer than phosphate, same 3− charge.
chlorate — what is its formula?	ClO₃⁻ The middle of a four-member series running from hypochlorite to perchlorate.
chlorite — what is its formula?	ClO₂⁻ One oxygen fewer than chlorate.
hypochlorite — what is its formula?	ClO⁻ hypo- marks one oxygen fewer than the -ite; this is the bleach ion.
perchlorate — what is its formula?	ClO₄⁻ per- marks one oxygen more than the -ate.
bromate — what is its formula?	BrO₃⁻ Same pattern as chlorate one row down the halogens.
perbromate — what is its formula?	BrO₄⁻ The per- member of the bromine series.
iodate — what is its formula?	IO₃⁻ Added to table salt as a source of iodine.
periodate — what is its formula?	IO₄⁻ The per- member of the iodine series.
chromate — what is its formula?	CrO₄²⁻ Yellow in solution; converts to orange dichromate in acid.
dichromate — what is its formula?	Cr₂O₇²⁻ Two chromiums but still 2−, which is the easy mistake.
permanganate — what is its formula?	MnO₄⁻ Deep purple; a standard oxidising agent in titrations.
manganate — what is its formula?	MnO₄²⁻ Same formula as permanganate but 2−, and green rather than purple.
oxalate — what is its formula?	C₂O₄²⁻ Two linked carboxylate groups; forms insoluble calcium salts.
borate — what is its formula?	BO₃³⁻ Usually met as the tetraborate in borax rather than on its own.
tetraborate — what is its formula?	B₄O₇²⁻ The ion in borax; four borons share a 2− charge.
silicate — what is its formula?	SiO₃²⁻ The metasilicate form; silicate minerals are chains of these units.
orthosilicate — what is its formula?	SiO₄⁴⁻ The 4− charge appears in olivine and other rock-forming minerals.
arsenate — what is its formula?	AsO₄³⁻ Mimics phosphate closely enough to be a metabolic poison.
arsenite — what is its formula?	AsO₃³⁻ One oxygen fewer than arsenate.
selenate — what is its formula?	SeO₄²⁻ The selenium analogue of sulfate, one row down.
selenite — what is its formula?	SeO₃²⁻ The selenium analogue of sulfite.
thiosulfate — what is its formula?	S₂O₃²⁻ thio- means an oxygen has been replaced by sulfur; used to fix photographs.
thiocyanate — what is its formula?	SCN⁻ Cyanide with a sulfur attached; gives a blood-red complex with iron(III).
cyanate — what is its formula?	OCN⁻ The oxygen version of thiocyanate; note the different first atom.
persulfate — what is its formula?	S₂O₈²⁻ Also called peroxydisulfate; contains an O–O bond, hence per-.
dithionite — what is its formula?	S₂O₄²⁻ A strong reducing agent used to strip colour.
dithionate — what is its formula?	S₂O₆²⁻ Two more oxygens than dithionite, same charge.
pyrophosphate — what is its formula?	P₂O₇⁴⁻ Also called diphosphate; released when ATP is used.
metaphosphate — what is its formula?	PO₃⁻ Same atoms as phosphite but 1− rather than 3−; the charge is the tell.
hypophosphite — what is its formula?	H₂PO₂⁻ Two hydrogens are bonded to phosphorus, not available as acid.
molybdate — what is its formula?	MoO₄²⁻ Follows the sulfate shape with a transition metal at the centre.
tungstate — what is its formula?	WO₄²⁻ The tungsten analogue of molybdate.
vanadate — what is its formula?	VO₃⁻ The metavanadate form, the one usually tabulated.
aluminate — what is its formula?	AlO₂⁻ Formed when aluminium hydroxide dissolves in excess base.
zincate — what is its formula?	ZnO₂²⁻ Formed when zinc hydroxide dissolves in excess base.
stannate — what is its formula?	SnO₃²⁻ Tin in its higher oxidation state, dissolved by base.
ferrate — what is its formula?	FeO₄²⁻ Iron in the +6 state; a strong oxidiser used in water treatment.
chromite — what is its formula?	CrO₂⁻ Chromium(III) dissolved in base; the mineral chromite is FeCr₂O₄.
formate — what is its formula?	HCOO⁻ The one-carbon carboxylate; also written CHO₂⁻.
propanoate — what is its formula?	C₂H₅COO⁻ The three-carbon carboxylate, used as a preservative.
benzoate — what is its formula?	C₆H₅COO⁻ A ring carboxylate widely used as a food preservative.
citrate — what is its formula?	C₆H₅O₇³⁻ Three carboxyl groups give the 3− charge; binds calcium.
tartrate — what is its formula?	C₄H₄O₆²⁻ Two carboxyl groups; the ion in cream of tartar.
lactate — what is its formula?	CH₃CH(OH)COO⁻ Produced by muscles under low oxygen.
salicylate — what is its formula?	C₇H₅O₃⁻ The active ion of aspirin after hydrolysis.
malate — what is its formula?	C₄H₄O₅²⁻ A citric acid cycle intermediate.
succinate — what is its formula?	C₄H₄O₄²⁻ Another citric acid cycle intermediate.
fumarate — what is its formula?	C₄H₂O₄²⁻ The trans isomer of maleate; also in the citric acid cycle.
glutamate — what is its formula?	C₅H₈NO₄⁻ The amino acid anion used as a flavour enhancer.
gluconate — what is its formula?	C₆H₁₁O₇⁻ Used to carry metal ions in supplements.
carbamate — what is its formula?	NH₂COO⁻ A carboxylate with nitrogen in place of one carbon substituent.
hydrogen carbonate — what is its formula?	HCO₃⁻ Also called bicarbonate; one hydrogen has taken one of carbonate's charges.
hydrogen sulfate — what is its formula?	HSO₄⁻ Also called bisulfate; adding one H to sulfate leaves 1−.
hydrogen sulfite — what is its formula?	HSO₃⁻ Also called bisulfite; used as a preservative.
hydrogen sulfide — what is its formula?	HS⁻ Also called bisulfide; the singly protonated sulfide ion.
hydrogen phosphate — what is its formula?	HPO₄²⁻ One hydrogen on phosphate leaves 2−; a buffer inside cells.
dihydrogen phosphate — what is its formula?	H₂PO₄⁻ Two hydrogens leave 1−; the other half of the phosphate buffer.
hydrogen oxalate — what is its formula?	HC₂O₄⁻ Also called binoxalate; one carboxyl still deprotonated.
hydrogen tartrate — what is its formula?	HC₄H₄O₆⁻ The ion in cream of tartar, potassium hydrogen tartrate.
hydrogen phthalate — what is its formula?	HC₈H₄O₄⁻ Its potassium salt is a primary standard for titrations.
hydrogen arsenate — what is its formula?	HAsO₄²⁻ Follows the hydrogen phosphate pattern one row down.
dihydrogen arsenate — what is its formula?	H₂AsO₄⁻ Follows the dihydrogen phosphate pattern.
hydrogen selenate — what is its formula?	HSeO₄⁻ The selenium analogue of hydrogen sulfate.
hydrogen selenite — what is its formula?	HSeO₃⁻ The selenium analogue of hydrogen sulfite.
amide — what is its formula?	NH₂⁻ Ammonia less one proton; a very strong base.
triiodide — what is its formula?	I₃⁻ Three iodines sharing one charge; the species in iodine solution.
carbide — what is its formula?	C₂²⁻ The acetylide ion; reacts with water to give ethyne.
tetrahydroborate — what is its formula?	BH₄⁻ Also called borohydride; a standard reducing agent.
tetrafluoroborate — what is its formula?	BF₄⁻ A weakly coordinating ion used to isolate reactive cations.
hexafluorophosphate — what is its formula?	PF₆⁻ Common in lithium-ion battery electrolytes.
hexafluorosilicate — what is its formula?	SiF₆²⁻ Added to water supplies as a fluoride source.
hexacyanoferrate(II) — what is its formula?	Fe(CN)₆⁴⁻ Also called ferrocyanide; iron is +2 inside the complex.
hexacyanoferrate(III) — what is its formula?	Fe(CN)₆³⁻ Also called ferricyanide; iron is +3 inside the complex.
tetrachloroaurate — what is its formula?	AuCl₄⁻ The gold species formed when gold dissolves in aqua regia.
tetrahydroxozincate — what is its formula?	Zn(OH)₄²⁻ The hydrated form of zincate, written with explicit hydroxides.
tetrahydroxoaluminate — what is its formula?	Al(OH)₄⁻ The hydrated form of aluminate; shows why aluminium dissolves in base.
diamminesilver(I) — what is its formula?	Ag(NH₃)₂⁺ The complex that keeps silver dissolved in Tollens' reagent.
tetraamminecopper(II) — what is its formula?	Cu(NH₃)₄²⁺ The deep blue complex formed when excess ammonia meets copper(II).
perrhenate — what is its formula?	ReO₄⁻ Follows the permanganate shape with rhenium at the centre.
pertechnetate — what is its formula?	TcO₄⁻ The form technetium takes in medical imaging.
metaborate — what is its formula?	BO₂⁻ The dehydrated form of borate.

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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 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.

Editorial method and sources

Memly's editors use the primary sources below to check scope and terminology, then independently write and review the cards.

IUPAC: Nomenclature of Inorganic Chemistry (Red Book)