Common Polyatomic Ions: Names, Formulas, and Charges

Why Polyatomic Ions Are Worth Memorizing Once

A polyatomic ion is a group of atoms that carries a net electrical charge and behaves as a single unit in chemical reactions. You cannot name an ionic compound, balance a precipitation reaction, or write a correct formula without being able to recognize these groups on sight. Unlike monatomic ions (Na⁺, Cl⁻, Mg²⁺), whose charges you can read off the periodic table, polyatomic ions have fixed formulas and charges that simply have to be memorized.

The good news: there are only about twenty common ones, and once you see the patterns, they are easier to remember than a phone list of twenty unrelated numbers.

The Essential List

These are the polyatomic ions you will encounter constantly.

Anions (Negatively Charged)

| Name | Formula | Charge | |---|---|---| | Hydroxide | OH⁻ | -1 | | Nitrate | NO₃⁻ | -1 | | Nitrite | NO₂⁻ | -1 | | Hydrogen carbonate (bicarbonate) | HCO₃⁻ | -1 | | Acetate | C₂H₃O₂⁻ (or CH₃COO⁻) | -1 | | Cyanide | CN⁻ | -1 | | Permanganate | MnO₄⁻ | -1 | | Hypochlorite | ClO⁻ | -1 | | Chlorite | ClO₂⁻ | -1 | | Chlorate | ClO₃⁻ | -1 | | Perchlorate | ClO₄⁻ | -1 | | Carbonate | CO₃²⁻ | -2 | | Sulfate | SO₄²⁻ | -2 | | Sulfite | SO₃²⁻ | -2 | | Chromate | CrO₄²⁻ | -2 | | Dichromate | Cr₂O₇²⁻ | -2 | | Peroxide | O₂²⁻ | -2 | | Phosphate | PO₄³⁻ | -3 | | Phosphite | PO₃³⁻ | -3 |

Cations (Positively Charged)

| Name | Formula | Charge | |---|---|---| | Ammonium | NH₄⁺ | +1 | | Hydronium | H₃O⁺ | +1 |

Ammonium is by far the most common polyatomic cation. Hydronium is the species that actually carries the positive charge in aqueous acid solutions (H⁺ in water is really H₃O⁺).

Patterns That Cut the Memorization in Half

The -ate / -ite pattern

When two polyatomic ions differ only in their oxygen count, the one with more oxygens ends in -ate and the one with fewer ends in -ite. The charge stays the same.

• Sulfate (SO₄²⁻) and sulfite (SO₃²⁻)
• Nitrate (NO₃⁻) and nitrite (NO₂⁻)
• Phosphate (PO₄³⁻) and phosphite (PO₃³⁻)
• Chlorate (ClO₃⁻) and chlorite (ClO₂⁻)

The per- / hypo- extensions

For chlorine, the pattern extends to four ions with identical -1 charges:

• Perchlorate (ClO₄⁻) — per- = one more oxygen than -ate
• Chlorate (ClO₃⁻)
• Chlorite (ClO₂⁻)
• Hypochlorite (ClO⁻) — hypo- = one less oxygen than -ite

Bromine and iodine form analogous series (perbromate, bromate, bromite, hypobromite and perchlorate analogs for iodine), though you will see the chlorine ones far more often.

The hydrogen-prefix pattern

Adding an H to a polyatomic ion raises its charge by +1 (makes it less negative) because the H contributes a +1:

• Carbonate (CO₃²⁻) → hydrogen carbonate / bicarbonate (HCO₃⁻)
• Phosphate (PO₄³⁻) → hydrogen phosphate (HPO₄²⁻) → dihydrogen phosphate (H₂PO₄⁻)
• Sulfate (SO₄²⁻) → hydrogen sulfate / bisulfate (HSO₄⁻)

This is why baking soda (sodium hydrogen carbonate / sodium bicarbonate, NaHCO₃) is less basic than washing soda (sodium carbonate, Na₂CO₃) — it still has a proton attached.

Using Polyatomic Ions in Formulas

When you build the formula of an ionic compound, balance the positive and negative charges. If you need more than one polyatomic ion, wrap it in parentheses and add a subscript.

• Calcium hydroxide: Ca²⁺ needs two OH⁻ → Ca(OH)₂
• Ammonium sulfate: two NH₄⁺ balance one SO₄²⁻ → (NH₄)₂SO₄
• Aluminum phosphate: Al³⁺ balances PO₄³⁻ → AlPO₄ (no parentheses needed when only one polyatomic is used)
• Calcium phosphate: three Ca²⁺ balance two PO₄³⁻ → Ca₃(PO₄)₂

The "criss-cross" rule helps: take the absolute value of one ion's charge and use it as the subscript for the other. Then reduce to the simplest ratio if possible.

A Memorization Strategy

Trying to memorize all 20 in one sitting is painful and ineffective. A better approach:

1. Day 1 — learn the 10 most common: ammonium, hydroxide, nitrate, carbonate, bicarbonate, sulfate, phosphate, acetate, cyanide, permanganate. Practice writing each formula and charge from the name, and vice versa.
2. Day 2 — add the -ite partners: nitrite, sulfite, phosphite, chlorite.
3. Day 3 — add the chlorine family and hydrogen-prefixed ions: hypochlorite, chlorate, perchlorate, hydrogen sulfate, hydrogen phosphate.
4. Day 4+ — review daily for a week or two until recall is effortless.

Flashcards work well here, either paper or a spaced-repetition app. The list is short enough to master in a week and long enough that you will not be guessing on a test.

Why This Pays Off

Nearly every ionic-compound problem in intro chemistry assumes you can identify these ions. Net ionic equations, precipitation reactions, acid-base neutralization, and redox balancing all get much faster once recognition is automatic. Invest the hour or two it takes to memorize this list and it pays off on every chemistry assignment for the rest of the course.