Explain:
$(i)$ The basis of similarities and differences between metallic and ionic crystals.
$(ii)$ Why are ionic solids hard and brittle?

(N/A) $(i)$ The basis of similarities between metallic and ionic crystals is that both these crystal types are held by electrostatic forces of attraction. In metallic crystals,the electrostatic force acts between positive metal ions and delocalized electrons. In ionic crystals,it acts between oppositely charged ions. Consequently,both types generally possess high melting points.
The basis of differences is that in metallic crystals,electrons are free to move,allowing them to conduct electricity in the solid state. In ionic crystals,ions are held in fixed positions and are not free to move; therefore,they are insulators in the solid state but conduct electricity in the molten state or in aqueous solution.
$(ii)$ The constituent particles of ionic crystals are ions held together in a three-dimensional lattice by strong electrostatic forces of attraction. Because these forces are very strong and directional,the ions are locked in fixed positions,making the crystals hard. They are brittle because the application of an external force can shift the layers of ions,causing like-charged ions to come close to each other,resulting in strong electrostatic repulsion that causes the crystal to fracture.