Polyatomic Ions
Polyatomic ions are groups of atoms that travel together and carry one overall charge. You cannot reason your way to them mid-test — these are pure memorization, and they pay you back on naming, formula writing, solubility, and net ionic equations.
Key Facts to Memorize
Acetate
CH₃COO¹⁻ (also written C₂H₃O₂¹⁻) — vinegar's ion.
Sulfate
SO₄²⁻ — the workhorse of every solubility question.
Sulfite
SO₃²⁻ — one oxygen lighter than sulfate.
Permanganate
MnO₄¹⁻ — deep purple in solution.
Phosphate
PO₄³⁻ — the only common 3⁻ ion you'll be asked for.
Nitrite
NO₂¹⁻ — the '-ite' has one fewer oxygen.
Nitrate
NO₃¹⁻ — one nitrogen, three oxygens, 1⁻ charge.
Perchlorate
ClO₄¹⁻ — the most oxygen in the chlorine family.
Chlorate
ClO₃¹⁻ — the base name of the chlorine family.
Chlorite
ClO₂¹⁻ — one oxygen fewer than chlorate.
Hypochlorite
ClO¹⁻ — the fewest oxygens; found in bleach.
Hydroxide
OH¹⁻ — the ion that makes a base a base.
Cyanide
CN¹⁻ — carbon and nitrogen with a 1⁻ charge.
Hydrogen carbonate (bicarbonate)
HCO₃¹⁻ — carbonate plus an H, charge drops to 1⁻.
Dichromate
Cr₂O₇²⁻ — two chromiums, seven oxygens, 2⁻ charge.
Carbonate
CO₃²⁻ — fizzes with acid to make CO₂.
Ammonium
NH₄¹⁺ — the one common positive polyatomic ion.
Al says…
Learn them in families. Once you know sulfate is SO₄²⁻, sulfite is just one oxygen lighter. Same trick works for nitrate/nitrite and the chlorate family.
Practice This Topic
External Tools
- Blooket review game
Mrs. Cauthron's polyatomic-ion Blooket set — play it here.
- Quizlet: common polyatomic ions
Great for a 5-minute warm-up on the bus.