AP Chemistry Polyatomic Ions
Names, formulas, charges, and the naming patterns that make them easier to remember than a flat list.
Polyatomic ion names and charges aren't on the AP Chemistry reference sheet, so they have to be memorized separately, and most study resources hand you a flat list to memorize by brute force. There's a faster way: a handful of naming patterns account for most of the list, so once you know the patterns, you're really only memorizing a few anchor ions instead of twenty unrelated ones.
The Two Positive Ions
Start here, because there are only two common polyatomic cations, everything else on this page is negative:
- Ammonium, NH₄⁺
- Hydronium, H₃O⁺
Pattern 1: The -ate / -ite Pair
Within the same family, the -ate ion has one more oxygen atom than the -ite ion, and both carry the same charge. Learn one pair and you've effectively learned both:
- Sulfate SO₄²⁻ vs. sulfite SO₃²⁻ (one fewer O, same 2− charge)
- Nitrate NO₃⁻ vs. nitrite NO₂⁻ (one fewer O, same 1− charge)
Pattern 2: The Four-Member Halogen Series
Chlorine's oxyanion family is the clearest illustration of how far the oxygen-count pattern extends. All four carry the same 1− charge, only the oxygen count changes:
- Hypochlorite ClO⁻ (hypo-, one fewer O than -ite)
- Chlorite ClO₂⁻
- Chlorate ClO₃⁻
- Perchlorate ClO₄⁻ (per-, one more O than -ate)
Once this four-ion pattern clicks for chlorine, it transfers directly, the same hypo-/-ite/-ate/per- logic applies to any halogen's oxyanion family.
Pattern 3: Adding "Hydrogen" (or "Bi-")
Adding the word hydrogen (or the older prefix bi-) to an ion name adds one H to the formula and makes the charge exactly one less negative:
- Carbonate CO₃²⁻ → hydrogen carbonate (bicarbonate) HCO₃⁻
- Sulfate SO₄²⁻ → hydrogen sulfate (bisulfate) HSO₄⁻
Full Reference Table
| Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄ | 1+ |
| Hydronium | H₃O | 1+ |
| Hydroxide | OH | 1− |
| Cyanide | CN | 1− |
| Acetate | C₂H₃O₂ | 1− |
| Permanganate | MnO₄ | 1− |
| Nitrate | NO₃ | 1− |
| Nitrite | NO₂ | 1− |
| Hypochlorite | ClO | 1− |
| Chlorite | ClO₂ | 1− |
| Chlorate | ClO₃ | 1− |
| Perchlorate | ClO₄ | 1− |
| Hydrogen carbonate (bicarbonate) | HCO₃ | 1− |
| Hydrogen sulfate (bisulfate) | HSO₄ | 1− |
| Carbonate | CO₃ | 2− |
| Sulfate | SO₄ | 2− |
| Sulfite | SO₃ | 2− |
| Chromate | CrO₄ | 2− |
| Dichromate | Cr₂O₇ | 2− |
| Peroxide | O₂ | 2− |
| Oxalate | C₂O₄ | 2− |
| Phosphate | PO₄ | 3− |
Common Mistakes
- Forgetting polyatomic ions are one unit. When a formula needs more than one of a polyatomic ion, the whole ion goes in parentheses before the subscript, Ca(NO₃)₂, not CaNO₃₂ or CaN₂O₆.
- Mixing up -ate and -ite oxygen counts. -ate always has more oxygen than -ite within the same pair, never the reverse.
- Assuming every polyatomic ion is negative. Ammonium and hydronium are both common exceptions.
- Confusing the hydrogen-prefix charge change. Adding "hydrogen" adds one H and makes the charge one less negative, it doesn't flip the sign or change it by more than one.
Related Resources
- AP Chemistry Reference Sheet
- AP Chemistry Solubility Rules
- Unit 1 Review: Atomic Structure and Properties
- Unit 2 Review: Compound Structure and Properties
- AP Chemistry Study Guide
Frequently Asked Questions
What is a polyatomic ion?
A polyatomic ion is a charged particle made of two or more atoms bonded together, acting as a single unit with one overall charge. Sulfate (SO₄²⁻) is one ion made of one sulfur and four oxygen atoms, not four separate charges.
Are polyatomic ions on the AP Chemistry reference sheet?
No. College Board's reference sheet provides equations and constants, not a table of polyatomic ion names, formulas, or charges. This is one of the most common things students assume is on the reference sheet but isn't -- it has to be memorized separately.
Which polyatomic ions are positive?
Only two common ones: ammonium (NH₄⁺) and hydronium (H₃O⁺). Almost every other polyatomic ion you'll encounter is negatively charged.
What's the difference between the -ate and -ite suffixes?
Within the same family, the -ate ion has one more oxygen atom than the -ite ion, and both carry the same charge. Sulfate (SO₄²⁻) has one more oxygen than sulfite (SO₃²⁻); nitrate (NO₃⁻) has one more oxygen than nitrite (NO₂⁻).
What does adding "hydrogen" or "bi-" to an ion name do?
It adds one H to the formula and makes the charge one less negative. Carbonate (CO₃²⁻) becomes hydrogen carbonate, or bicarbonate (HCO₃⁻); sulfate (SO₄²⁻) becomes hydrogen sulfate, or bisulfate (HSO₄⁻). Both names for each ion are accepted.
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