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. The arrow at the end of a row opens a page on that ion.

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 …a hydrogen has been added to the oxyanion, which takes one off the size of the negative charge (older books say bi-)sulfate SO₄²⁻ becomes hydrogen sulfate HSO₄⁻, sulfite SO₃²⁻ becomes hydrogen sulfite HSO₃⁻, 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

PO₄³⁻phosphate

HPO₄²⁻hydrogen phosphate

H₂PO₄⁻dihydrogen phosphate

The oxygen count does not carry across

The -ate ion is the usual oxygen carrying ion for that element, and only for that element. The number of oxygens that goes with -ate, -ite, per- or hypo- is not the same from one central atom to the next, so two -ate ions can hold different numbers of oxygens and carry different charges.

NO₃⁻nitrate, 3 oxygens

SO₄²⁻sulfate, 4 oxygens

Both end in -ate. So -ate on its own does not promise three oxygens and it does not promise a charge of one minus. Learn the -ate ion of each element separately, then count up and down from it for the rest of that element's family.

Azide is not nitride

N₃⁻azide

N³⁻nitride

Azide is three nitrogens sharing one negative charge. Nitride is one nitrogen carrying three, and being a single atom it is not a polyatomic ion, so it is not on the list above. The whole difference is a subscript against a superscript, and it disappears the moment either one is written flat as plain text. In the quiz a bare N3- is read as azide, and the space in N 3- is what turns the 3 into a charge.

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.