Comparing Ionic & Covalent Compounds (Edexcel GCSE Chemistry)

• Ionic compounds have high melting and boiling points
• This is because the oppositely charged ions in the lattice structure are attracted to each other by strong electrostatic forces which hold them firmly in place
• Large amounts of energy are needed to overcome these forces so the m.p. and b.p. are high
• Ionic substances can conduct electricity when in either the molten state or when dissolved in solution. In both cases the ions must be able to move and carry the charge
• In solid ionic substances the ions are in fixed positions and cannot move, hence they do not conduct electricity

Particles in solution/molten form can move and conduct electricity but in solid form they are in fixed positions and are unable to conduct

• Simple covalent substances, such as carbon dioxide and methane, have very strong covalent bonds between the atoms in each molecule, but much weaker intermolecular forces between individual molecules
• When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds
• Less energy is needed to break the molecules apart, so they have lower m.p. and b.p. than ionic compounds
• They are poor conductors of electricity as there are no free ions or electrons to move and carry charge
• Most covalent compounds do not conduct at all in the solid state and are thus insulators
• Common insulators include the plastic coating around household electrical wiring, rubber and wood

Diagram showing the plastic coating surrounding the conducting metal wires in an electric cable