Giant Ionic Structures (WJEC GCSE Chemistry: Combined Science)

The Structure of an Ionic Compound

• Ionic compounds have a giant lattice structure
• The lattice consists of a regular arrangement of alternating positive and negative ions in which the ions are tightly packed together
• Strong electrostatic forces of attraction are present between oppositely charged ions, holding the lattice together
• Electrostatic forces are strong, acting in all directions - they form the basis of ionic bonding

Models to represent ionic lattices

The ball and stick model is on the left and the 3D space-filling model on the right

Linking the Bonding & Properties

• Ionic compounds have high melting and boiling points
  • They have giant structures
  • There are strong electrostatic forces of attraction between oppositely charged ions in all directions
  • The forces need lots of energy to overcome them 
• Ionic compounds are poor conductors in the solid state
  • The ions are in fixed positions in the lattice
  • They are therefore unable to move and carry a charge 
• Ionic compounds are good conductors of electricity in the molten state or in solution 
  • When the ionic compound is melted or dissolved in water, the ions are able to move and carry a charge

Diagram to show the electrical conductivity of ionic compounds

The ions can move when the compound is molten or in solution but can't when the compound is solid

Higher Tier

• The melting point of ionic compounds depends on the charges of the ions 
• The greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point 
  • For example, magnesium oxide consists of Mg²⁺ and O^2- so will have a higher melting point than sodium chloride which contains the ions, Na⁺ and Cl^-