How to memorize the polyatomic ions (without memorizing every one)

The common polyatomic ions

A polyatomic ion is a group of atoms bonded together that carries an overall charge and acts as a single unit in a compound. These are the 25 that OpenStax’s chemistry textbook lists as common, grouped by charge:

Your teacher’s list might add a few, such as oxalate (C₂O₄²⁻), thiosulfate (S₂O₃²⁻), thiocyanate (SCN⁻) or iodate (IO₃⁻). The deck at the end of this page includes those four too, and every ion in it runs both ways: name to formula, and formula to name.

Four rules that do half the work

1. -ite has one fewer oxygen than -ate, and the same charge. Sulfate SO₄²⁻ becomes sulfite SO₃²⁻; nitrate NO₃⁻ becomes nitrite NO₂⁻. Learn the -ate form and you get the -ite form free.

2. per- adds an oxygen; hypo- takes one more away. The chlorine family runs in order: perchlorate ClO₄⁻, chlorate ClO₃⁻, chlorite ClO₂⁻, hypochlorite ClO⁻. The charge stays −1 all the way down. Bromine and iodine follow the same pattern (bromate BrO₃⁻, iodate IO₃⁻).

3. Each hydrogen you add cancels one negative charge. Phosphate PO₄³⁻ becomes hydrogen phosphate HPO₄²⁻, then dihydrogen phosphate H₂PO₄⁻. Carbonate CO₃²⁻ becomes hydrogen carbonate HCO₃⁻. An older name for “hydrogen” in these ions is bi-: bicarbonate is hydrogen carbonate.

4. thio- swaps an oxygen for a sulfur. Sulfate SO₄²⁻ becomes thiosulfate S₂O₃²⁻: same charge, one oxygen replaced by sulfur.

A mnemonic for the -ate anchors

The rules need a few -ate ions to start from. A mnemonic from Missouri University of Science and Technology gives you six at once:

Nick the Camel ate a Clam Supper and Crepes (for dessert) in Phoenix

Only the capitalized words count. Each one is an ion: the number of consonants is the number of oxygens, and the number of vowels is the negative charge:

Add the four rules and those six anchors give you nitrite, sulfite, hydrogen sulfate, hydrogen sulfite, hydrogen carbonate, hydrogen and dihydrogen phosphate, perchlorate, chlorite and hypochlorite: sixteen ions from six words. “Clam” works for bromate (BrO₃⁻) and iodate (IO₃⁻) too, since they follow chlorine’s pattern.

The ones you just have to learn

The rules don’t reach these, so put them on cards first:

• ammonium NH₄⁺ and hydronium H₃O⁺, the two positive ones
• hydroxide OH⁻, cyanide CN⁻, peroxide O₂²⁻ and azide N₃⁻
• acetate CH₃COO⁻ (also written C₂H₃O₂⁻)
• permanganate MnO₄⁻ and dichromate Cr₂O₇²⁻

The mix-ups that cost points

• Nitrate versus nitrite, sulfate versus sulfite: count the oxygens. -ate has more.
• Sulfate, sulfite and sulfide: sulfide, S²⁻, isn’t polyatomic at all. The -ide ending usually means a single atom; hydroxide, cyanide, peroxide and azide are the exceptions to know.
• Chlorate versus chloride: chloride is just Cl⁻.
• Ammonium versus ammonia: ammonium NH₄⁺ is the ion; ammonia NH₃ is a neutral molecule.
• Hydroxide versus hydronium: OH⁻ is negative, H₃O⁺ is positive.
• Chromate versus dichromate: two chromium atoms and seven oxygens in dichromate, and the charge stays 2−.

A week that makes it stick

Spreading practice over several days is one of only two study techniques rated high utility in a major review (Dunlosky et al., 2013), and the longer you need to remember something, the wider your review gaps should be (Cepeda et al., 2006). For a quiz next week:

More lists worth memorizing

The same approach works for any fixed list your class tests: group it, find the pattern, then let spaced flashcards do the rest. See how to memorize the 12 cranial nerves, the 20 amino acids and the unit circle, each with a free deck.