What are the most common polyatomic ions to memorize?

Most general chemistry courses ask for ammonium, hydroxide, nitrate, nitrite, carbonate, hydrogen carbonate, sulfate, sulfite, hydrogen sulfate, phosphate, hydrogen phosphate, acetate, cyanide, permanganate, chromate, dichromate, oxalate, and the chlorine oxyanions. Turn on the intro-course filter above to see that shorter list.

How do I remember the -ate and -ite endings?

The -ate ion is the base ion. The -ite ion has one fewer oxygen and the same charge. Nitrate is NO₃ with a 1− charge and nitrite is NO₂ with a 1− charge. Add per- for one more oxygen than -ate, or hypo- for one fewer than -ite.

Is bicarbonate the same as hydrogen carbonate?

Yes. Both names describe HCO₃ with a 1− charge. Hydrogen carbonate is the systematic name and bicarbonate is the traditional one. Hydrogen sulfate (bisulfate) works the same way.

Which polyatomic ions are positive?

Ammonium, NH₄ with a 1+ charge, and hydronium, H₃O with a 1+ charge, are the common positive polyatomic ions. Mercury(I), Hg₂ with a 2+ charge, is also written as a polyatomic cation because it is two mercury atoms bonded together.

When do I use parentheses around a polyatomic ion?

Use parentheses when the formula needs more than one copy of the ion, for example Mg(OH)₂ or Ca₃(PO₄)₂. If you need only one copy, as in NaOH or CaCO₃, no parentheses are needed.

Can I print the polyatomic ions chart?

Yes. Use the Print button above the chart, or download a PNG or SVG. You can also print blank practice sheets with the names or the formulas hidden, plus an answer key. Everything is free and needs no account.

Chemistry Reference

Polyatomic Ions Chart Printable List & Flashcards

Free polyatomic ions chart with 38 common ions, charges and naming rules. Search, filter, drill with flashcards, then download or print.

38 common ions with verified formulas and chargesFilter by charge, search by name or formulaFlashcards and blank practice sheetsPNG, SVG and print, no sign-up

ChargeIntro-course list only (22)

Showing 38 of 38 ions

Charge +1 (cations, 2)

NH₄⁺
ammonium
H₃O⁺
hydronium

Charge +2 (cations, 1)

Hg₂²⁺
mercury(I)
also: mercurous, dimercury
Two Hg atoms bonded together

Charge −1 (anions, 21)

CH₃COO⁻
acetate
also: ethanoate
Also written C2H3O2
OH⁻
hydroxide
CN⁻
cyanide
OCN⁻
cyanate
SCN⁻
thiocyanate
NO₃⁻
nitrate
NO₂⁻
nitrite
MnO₄⁻
permanganate
HCO₃⁻
hydrogen carbonate
also: bicarbonate
HSO₄⁻
hydrogen sulfate
also: bisulfate
H₂PO₄⁻
dihydrogen phosphate
ClO⁻
hypochlorite
ClO₂⁻
chlorite
ClO₃⁻
chlorate
ClO₄⁻
perchlorate
BrO⁻
hypobromite
BrO₂⁻
bromite
BrO₃⁻
bromate
BrO₄⁻
perbromate
IO₃⁻
iodate
IO₄⁻
periodate

Charge −2 (anions, 9)

CO₃²⁻
carbonate
SO₄²⁻
sulfate
SO₃²⁻
sulfite
S₂O₃²⁻
thiosulfate
HPO₄²⁻
hydrogen phosphate
also: monohydrogen phosphate
CrO₄²⁻
chromate
Cr₂O₇²⁻
dichromate
C₂O₄²⁻
oxalate
O₂²⁻
peroxide
Two O atoms bonded together

Charge −3 (anions, 4)

PO₄³⁻
phosphate
PO₃³⁻
phosphite
Textbook pattern form; the real ion in water is HPO3 2−
AsO₄³⁻
arsenate
BO₃³⁻
borate

Charge −4 (anions, 1)

SiO₄⁴⁻
silicate
also: orthosilicate
Metasilicate is SiO3 2−

Free Polyatomic Ions Charts

Every chart below is drawn from the same verified data as the tool above. View, download and print them free, no sign-up.

Full chart

Polyatomic Ions Chart by Charge

All 38 ions grouped by charge, from +1 cations to the -4 silicate ion.

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Reference charts

Naming patterns

Practice sheets

What is a polyatomic ion?

A polyatomic ion is a group of two or more atoms that are bonded together and carry one overall electric charge. The group behaves as a single unit in chemical reactions, so the formula, the charge and the name all travel together. Sulfate is SO₄ with a 2− charge, ammonium is NH₄ with a 1+ charge, and neither can be changed without turning it into a different ion. Almost all common polyatomic ions are anions. The ammonium and hydronium ions are the main cations students meet, and mercury(I), Hg₂ with a 2+ charge, is the one that surprises people because it is a pair of mercury atoms bonded to each other.

How the naming pattern works

The -ate ending is the base name for an oxyanion, an ion that contains oxygen. Sulfate is SO₄²⁻, nitrate is NO₃⁻, phosphate is PO₄³⁻.
The -ite ending means one oxygen fewer than the -ate ion while the charge stays the same. Sulfite is SO₃²⁻, nitrite is NO₂⁻, phosphite is PO₃³⁻ (the textbook pattern form; see the note further down).
The per- prefix adds one oxygen to the -ate ion, and hypo- removes one from the -ite ion. For chlorine the four steps are perchlorate ClO₄⁻, chlorate ClO₃⁻, chlorite ClO₂⁻, and hypochlorite ClO⁻. Bromine follows the same pattern, and iodine is usually taught with iodate and periodate only.
The hydrogen prefix adds H+ and lowers the charge by one. Carbonate CO₃²⁻ becomes hydrogen carbonate HCO₃⁻ (also called bicarbonate), and phosphate PO₄³⁻ becomes hydrogen phosphate HPO₄²⁻ and then dihydrogen phosphate H₂PO₄⁻.

Writing formulas with polyatomic ions

Balance the charges so the compound is neutral. Calcium is 2+ and nitrate is 1−, so you need two nitrates for each calcium.
Put parentheses around the polyatomic ion when you need more than one of it: Ca(NO₃)₂, Mg(OH)₂, (NH₄)₂SO₄. Leave them off when you need only one, as in CaSO₄ or NaOH.
Remember that a subscript outside the parentheses multiplies every atom inside. Al₂(SO₄)₃ contains two aluminum atoms, three sulfur atoms and twelve oxygen atoms.
Name the compound by naming the cation first and then the polyatomic anion, without changing the anion ending: ammonium phosphate, (NH₄)₃PO₄.

Common mistakes to avoid

Mixing up sulfate and sulfite, or nitrate and nitrite. The -ate ion always has one more oxygen, and both share the same charge.
Treating the subscript as the charge. In CO₃²⁻ the 3 counts oxygen atoms and the 2− is the charge.
Writing chromate and dichromate as the same thing. Chromate is CrO₄²⁻, dichromate is Cr₂O₇²⁻, and both carry 2−.
Forgetting that hydrogen carbonate and bicarbonate are two names for HCO₃⁻, so a test may use either.
Dropping the parentheses in formulas like Ca(OH)₂, which would otherwise read as Ca with a single O and an H₂.

How to memorize the list

Start with the patterns instead of the whole table. Learn the four chlorine steps, then apply them to the other halogens. Learn that -ite is one oxygen fewer than -ate and that the charge does not move. Then add the few ions that follow no pattern: ammonium, hydroxide, cyanide, acetate, permanganate and the chromates. Use the flashcards above in both directions, filter by one charge at a time, and print the blank practice sheets to test yourself on paper. Formulas and charges were cross-checked against published references such as university help sheets and open chemistry textbooks. Some courses use slightly different names, for example silicate is SiO₄⁴⁻ here and SiO₃²⁻ is called metasilicate, and phosphite is listed as PO₃³⁻ to show the -ite pattern even though the ion that actually forms from phosphorous acid is HPO₃²⁻, so follow your own textbook when it differs.