Formulae of ionic compounds

are made up of oppositely charged ions joined together by . They almost always contain at least one element and at least one element.

The formula of an ionic compound can be deduced from the formulae of its ions. Remember that for the metal elements in groups 1, 2, and 3 the charge on the ion can be deduced by how many outer shell electrons there were in the neutral atom. Similarly, the non-metal elements in groups 6 (IUPAC group 16) and 7 (IUPAC group 17), the ionic charge can be deduced by working out how many electrons must be gained to fill the outer shell.

Positive ions	Negative ions
lithium Li⁺	chloride Cl^-
sodium Na⁺	bromide Br^-
potassium K⁺	iodide I^-
silver Ag⁺	oxide O^2-
barium Ba²⁺
calcium Ca²⁺
copper(II) Cu²⁺
iron(II) Fe²⁺
lead(II) Pb²⁺
magnesium Mg²⁺
zinc Zn²⁺
aluminium Al³⁺
iron(III) Fe³⁺

Positive ions	lithium Li⁺
Negative ions	chloride Cl^-

Positive ions	sodium Na⁺
Negative ions	bromide Br^-

Positive ions	potassium K⁺
Negative ions	iodide I^-

Positive ions	silver Ag⁺
Negative ions	oxide O^2-

Positive ions	barium Ba²⁺

Positive ions	calcium Ca²⁺

Positive ions	copper(II) Cu²⁺

Positive ions	iron(II) Fe²⁺

Positive ions	lead(II) Pb²⁺

Positive ions	magnesium Mg²⁺

Positive ions	zinc Zn²⁺

Positive ions	aluminium Al³⁺

Positive ions	iron(III) Fe³⁺

Deducing a formula

Although ionic compounds contain electrically charged ions, they are neutral overall. The formula for an ionic compound must give the same number of positive and negative . Here are some examples.

Example 1

Sodium chloride contains Na⁺ and Cl^- ions:

this is one positive charge and one negative charge
the number of charges are already the same
so the formula is NaCl

Example 2

Magnesium oxide contains Mg2⁺ and O2^- ions:

this is two positive charges and two negative charges
the number of charges are already the same
so the formula is MgO

Example 3

Aluminium oxide contains Al³⁺ and O^2- ions:

this is three positive charges and two negative charges
to make the number of charges the same, we need two Al³⁺ ions and three O^2- ions
so the formula is Al₂O₃

Compound ions

are formed from groups of two or more atoms.

Positive ions	Negative ions
ammonium NH₄⁺	hydroxide OH^-
	nitrate NO₃^-
	carbonate CO₃^2-
	sulfate SO₄^2-

Positive ions	ammonium NH₄⁺
Negative ions	hydroxide OH^-

Positive ions
Negative ions	nitrate NO₃^-

Positive ions
Negative ions	carbonate CO₃^2-

Positive ions
Negative ions	sulfate SO₄^2-

If the formula of an ionic compound needs more than one polyatomic ion, the formula of this ion is written inside brackets.

Example

Calcium hydroxide contains Ca²⁺ and OH^- ions:

this is two positive charges and one negative charge
to make the number of charges the same, we need one Ca²⁺ ion and two OH^- ions
so the formula is Ca(OH)₂

Question

Deduce the formula for lead(II) sulfate.

Question

Deduce the formula for ammonium carbonate.

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