Polyatomic ions

Groups of atoms that carry a charge and move as one piece. Learn the names, the formulas and the charges, then run a round of ten.

A polyatomic ion is a group of atoms bonded together that carries a charge as a whole. The charge belongs to the group, not to any one atom, and the group stays together through reactions. There is no shortcut: the names, formulas and charges have to be learned. The patterns below cut the list down to a few families.

The list

Tap a covered entry to peek at it. Entries marked less common are left out of the quiz unless you switch them on.

Patterns in the names

-atethe usual oxygen-containing ion of an elementnitrate NO₃⁻, sulfate SO₄²⁻, chlorate ClO₃⁻, phosphate PO₄³⁻

-iteone fewer oxygen than the -ate, same chargenitrite NO₂⁻, sulfite SO₃²⁻, chlorite ClO₂⁻

per- … -ateone more oxygen than the -ateperchlorate ClO₄⁻

hypo- … -iteone fewer oxygen than the -itehypochlorite ClO⁻

hydrogen …an H⁺ has been added, so the charge is one less negative (older books say bi-)carbonate CO₃²⁻ becomes hydrogen carbonate HCO₃⁻

thio-a sulfur sits where an oxygen would besulfate SO₄²⁻ becomes thiosulfate S₂O₃²⁻

ClO⁻hypochlorite

ClO₂⁻chlorite

ClO₃⁻chlorate

ClO₄⁻perchlorate

Inside a compound the ion stays in one piece. When you need more than one of it, wrap it in parentheses: Ca(NO₃)₂, (NH₄)₂SO₄. One copy needs no parentheses: Na₂SO₄, KMnO₄.