What are polyatomic ions?

A charged group of atoms

Inside a polyatomic ion, the atoms are held together by covalent bonds, the same kind of bond that holds a water molecule together. The group as a whole has more or fewer electrons than protons, so it carries a net charge. The charge belongs to the whole ion, not to one atom, which is why it is written at the upper right of the complete formula, as in SO₄²⁻. Ions made of a single atom, such as Na⁺ and Cl⁻, are called monatomic ions.

In a compound, a polyatomic ion behaves like a single ion. In sodium nitrate, NaNO₃, the sodium ion (Na⁺) and the nitrate ion (NO₃⁻) form an ionic compound, and when the salt dissolves in water, each nitrate ion stays together as one unit.

Common polyatomic ions and their charges

Ions with a −1 charge include hydroxide (OH⁻), nitrate (NO₃⁻), nitrite (NO₂⁻), hydrogen carbonate or bicarbonate (HCO₃⁻), acetate (CH₃COO⁻), permanganate (MnO₄⁻), cyanide (CN⁻), chlorate (ClO₃⁻) and perchlorate (ClO₄⁻).

Ions with a −2 charge include carbonate (CO₃²⁻), sulfate (SO₄²⁻), sulfite (SO₃²⁻), chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻) and peroxide (O₂²⁻). Phosphate (PO₄³⁻) is the most common −3 ion. The two positive ions you will meet most often are ammonium (NH₄⁺) and hydronium (H₃O⁺), both +1.

Notice that hydroxide, cyanide and peroxide end in -ide even though they contain more than one atom. Most -ide names belong to single-atom ions such as chloride (Cl⁻) and oxide (O²⁻), so these are worth memorizing as exceptions.

The -ate and -ite pattern

Many polyatomic ions are oxyanions: one element combined with oxygen. When an element forms two such ions with the same charge, the one with more oxygen atoms ends in -ate and the one with one fewer oxygen atom ends in -ite. Nitrate is NO₃⁻ and nitrite is NO₂⁻; sulfate is SO₄²⁻ and sulfite is SO₃²⁻. The charge stays the same; only the number of oxygen atoms changes.

Chlorine forms a series of four, which adds two prefixes. Per- means one more oxygen than the -ate ion, and hypo- means one fewer than the -ite ion: perchlorate (ClO₄⁻), chlorate (ClO₃⁻), chlorite (ClO₂⁻) and hypochlorite (ClO⁻), the ion in household bleach. Bromine and iodine follow the same pattern, as in bromate (BrO₃⁻), iodate (IO₃⁻) and periodate (IO₄⁻).

Adding hydrogen

Adding a hydrogen ion (H⁺) to a negative polyatomic ion makes a new ion whose charge is one less negative. Carbonate (CO₃²⁻) becomes hydrogen carbonate (HCO₃⁻), also called bicarbonate, and sulfate (SO₄²⁻) becomes hydrogen sulfate (HSO₄⁻). Phosphate can take one or two: PO₄³⁻ becomes hydrogen phosphate (HPO₄²⁻) and then dihydrogen phosphate (H₂PO₄⁻).

Baking soda is sodium hydrogen carbonate, NaHCO₃: one Na⁺ balances one HCO₃⁻.

Writing formulas with polyatomic ions

To write the formula of an ionic compound, combine the ions so the total charge is zero and write the positive ion first. When you need more than one of a polyatomic ion, put it in parentheses and write the number after it. Never change the subscripts inside the ion, because that would make a different ion.

Sodium hydroxide: Na⁺ and OH⁻ balance 1:1, so the formula is NaOH. Calcium nitrate: Ca²⁺ needs two NO₃⁻ ions, so it is Ca(NO₃)₂. Ammonium sulfate: two NH₄⁺ ions balance one SO₄²⁻, giving (NH₄)₂SO₄. Aluminum sulfate: two Al³⁺ ions (total +6) balance three SO₄²⁻ ions (total −6), giving Al₂(SO₄)₃.

To name a compound, name the positive ion and then the negative ion. CaCO₃ is calcium carbonate, the main compound in chalk and limestone, and KMnO₄ is potassium permanganate.