Polyatomic ions

Some ions are formed from groups of atoms. The table shows the names and formulae of some polyatomic ions.

Cations are positively charged ions, and anions are negatively charged ions.
CationsAnions
Ammonium, NH4+Hydroxide, OH-
Calcium, Ca2+Nitrate, NO3-
Sodium, Na+Carbonate, CO32-
Lead, Pb2+Sulfate, SO42-

The formulae of compounds containing polyatomic ions are worked out in a similar way to single atom ions. Except when there is more than one polyatomic ion, then its formula is written inside brackets.

Example

Calcium hydroxide contains Ca2+ and OH- ions:

  • this is two positive charges and one negative charge
  • to balance it will need, one Ca2+ ions and two OH- ions
  • so the formula is Ca(OH)2
Question

Deduce the formula for sodium carbonate.

Sodium carbonate contains Na+ and CO32- ions:

  • this is one positive charge and two negative charges
  • to balance, two Na+ ions and one CO32- ions are needed
  • so the formula is Na2CO3
Question

Deduce the formula for lead nitrate.

Lead nitrate contains Pb2+ and NO3- ions:

  • this is two positive charges and one negative charge
  • to balance it will need one Pb2+ ion and two NO3- ions
  • so the formula is Pb(NO3)2