Common Polyatomic Ions

Organized by family and naming pattern, color-coded by charge, with the classic mnemonic.

Why it matters

These charged atom-groups show up in a huge fraction of compounds and reactions. Knowing them cold means you never re-derive the same twice, and you recognize it instantly inside a formula.

When you use it

Naming compounds, writing formulas, balancing equations, and drawing of salts and acids.

Where it leads

Ionic compound formulas, acid naming, and much faster Lewis structures (the whole ion keeps one ).

The naming pattern (the real cheat code)

Learn the pattern once and you can name a whole family. The base ion ends in -ate. Swap to -ite and you keep the same charge but drop one oxygen. Reach for the extremes with hypo- (one fewer than -ite) and per- (one more than -ate).

ClO⁻Hypochlorite1 oxygen

ClO₂⁻Chlorite2 oxygens

ClO₃⁻Chlorate3 oxygens

ClO₄⁻Perchlorate4 oxygens

The whole chlorine family sits at −1: only the oxygen count changes as you climb the ladder from hypochlorite to perchlorate.

-ate vs -ite

Nitrate is NO₃⁻ and nitrite is NO₂⁻: the -ite always has one fewer oxygen than the -ate, while the charge stays the same. The same holds for sulfate (SO₄²⁻) and sulfite (SO₃²⁻).

Adding a hydrogen

Carbonate is CO₃²⁻; add an H⁺ and you get bicarbonate, HCO₃⁻. The extra H⁺ raises the charge by one, so a −2 ion becomes −1.

The ions, by family and charge

+ charge− charge2− charge3− charge

Family	Name	Formula	Charge	Oxidation state	Tip
Nitrogen	Nitrate	NO₃⁻	−	N +5	The "standard" -ate
	Nitrite	NO₂⁻	−	N +3	-ite means one less oxygen
Sulfur	Sulfate	SO₄²⁻	2−	S +6	Heavy −2 charge
	Sulfite	SO₃²⁻	2−	S +4	Same charge as sulfate
Chlorine	Hypochlorite	ClO⁻	−	Cl +1	hypo- = fewest oxygens
	Chlorite	ClO₂⁻	−	Cl +3	-ite, two oxygens
	Chlorate	ClO₃⁻	−	Cl +5	-ate, three oxygens
	Perchlorate	ClO₄⁻	−	Cl +7	per- = one more/over
Carbon	Carbonate	CO₃²⁻	2−	C +4	The −2 carbon oxyanion
	Bicarbonate	HCO₃⁻	−	C +4	Carbonate plus an H⁺
	Acetate	CH₃COO⁻	−	n/a	Everywhere in intro labs
	Cyanide	CN⁻	−	n/a	Carbon triple-bonded to N
Phosphorus	Phosphate	PO₄³⁻	3−	P +5	Rare −3 charge
Oxygen/H	Hydroxide	OH⁻	−	n/a	The base-maker
Cations	Ammonium	NH₄⁺	+	N −3	One of the few positives
	Hydronium	H₃O⁺	+	n/a	Water plus an H⁺

The oxidation-state column is a secondary detail for later: it is the charge the central atom would carry if every bond went to the more electronegative oxygen.

Mnemonic: "Nick the Camel ate a Clam for Supper in Phoenix"

NickNitrateNO₃⁻N, charge 1−, 3 oxygens

CamelCarbonateCO₃²⁻C, charge 2−, 3 oxygens

ClamChlorateClO₃⁻Cl, charge 1−, 3 oxygensinferred

SupperSulfateSO₄²⁻S, charge 2−, 4 oxygensinferred

PhoenixPhosphatePO₄³⁻P, charge 3−, 4 oxygensinferred

Each word gives the central atom, its charge, and its oxygen count. Nick and Camel are the textbook pairs; Clam, Supper, and Phoenix follow the same pattern and are marked inferred rather than presented as canonical.

Common mistake

The charge belongs to the whole ion, not to any one atom. Sulfate is 2− as a single unit, so do not try to pin that charge onto a particular oxygen. The charge also stays the same when the ion appears more than once: the three sulfates in Al₂(SO₄)₃ are each still 2−.

Practice

Bromine follows chlorine's pattern exactly. If bromate is BrO₃⁻, what are the formula and charge of hypobromite?

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