Empirical Formulae & Formulae of Ionic Compounds (Cambridge (CIE) O Level Chemistry): Revision Note

Empirical Formulae

The molecular formula is the formula that shows the number and type of each atom in a molecule

• E.g. the molecular formula of ethanoic acid is C₂H₄O₂

• The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound

  • E.g. the empirical formula of ethanoic acid is CH₂O

• Organic molecules, such as ethanoic acid, often have different empirical and molecular formulae

• The formula of an ionic compound is always an empirical formula

Deducing Formulae of Ionic Compounds

• The formulae of these compounds can be calculated if you know the charge on the ions

• The Periodic Table can help work out the charge on many elements:

  • Group I elements form ions with a 1+ charge

  • Group II elements form ions with a 2+ charge

  • Group III elements form ions with a 3+ charge

  • Group V elements form ions with a 3- charge

  • Group VI elements form ions with a 2- charge

  • Group VII elements form ions with a 1- charge

• Below are some other common ions and their charges

• Note that a Roman numeral next to the element tells you the charge on the ion, e.g. copper(II) ions have a charge 2+

• There are several common compound ions included in the table 

  • Some chemists call these polyatomic ions

Common Ions & Their Charges Table

• The overall sum of the charges of an ionic compound should be 0 

• You therefore need to work out the ratio of the ions to ensure this is the case 

• When you write the formula of a compound ion it is necessary to use brackets around the compound ion where more than one of that ion is needed in the formula

  • For example copper(II) hydroxide is Cu(OH)₂