Working out the charges of ions

The charge of many ions can be worked out using patterns in the periodic table. Elements in the same group form ions with the same charge.

The charge of many ions can be worked out from using patterns in the Periodic Table.

Transition metals (in the central block between groups 2 and 3) can have ions with different charges. The number in the name of the compound shows the charge of the metal ions in that compound.

For example, copper(II) sulfate contains Cu2+ ions.

The table shows the formulae of some common transition metal ions.

Metal ionFormula
Silver(I)Ag+
Copper(II)Cu2+
Lead(II)Pb2+
Iron(II)Fe2+
Iron(III)Fe3+

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

Cations (positively charged)Anions (negatively charged)
Ammonium NH4+Hydroxide OH-
Nitrate NO3-
Carbonate CO32-
Sulfate SO42-

The formulae of compounds containing polyatomic ions are worked out in a similar way to single atom ions. If more than one polyatomic ion is needed, 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+ ion and two OH- ions,
  • so its 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(II) 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

Naming ionic compounds with -ide and -ate

The name of an ionic compound ends in:

  • -ide if it contains just two elements
  • -ate if it contains three or more elements, one of which is oxygen
Name (-ide)FormulaName (-ate)Formula
Calcium carbideCaC2Calcium carbonateCaCO3
Potassium iodideKIPotassium iodateKIO3
Iron(II) sulfideFeSIron(II) sulfateFeSO4