Constructing ionic formulae

• Ionic compound


  An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron)
• Constructing ionic formulae


  1. Identify the ions

  2. Cancel out the charges

  3. Construct the formula
• The details of how ions are formed are not required for this topic
• Sodium chloride


  Contains sodium ions (Na+) and chloride ions (Cl-)
• Ionic compound


  • Overall charge must be zero
  • Same number of positive charges as negative charges
• Constructing formula for sodium chloride


  1. Identify the ions

  2. Cancel out the charges

  3. Construct the formula as NaCl
• Magnesium hydroxide


  Contains magnesium ions (Mg2+) and hydroxide ions (OH-)
• Constructing formula for magnesium hydroxide


  1. Identify the ions

  2. Cancel out the charges

  3. Construct the formula as Mg(OH)2
• Compound ions


  • carbonate (CO3 2-)
  • sulfate (SO4 2-)
  • nitrate (NO3 -)
  • ammonium (NH4 +)
• Iron(III) oxide


  Contains iron(III) ions (Fe3+) and oxide ions (O2-)
• Constructing formula for iron(III) oxide


  1. Identify the ions

  2. Cancel out the charges

  3. Construct the formula as Fe2O3
• Subscript


  A letter or number written below the line