(i) Similarities : Both ionic and metallic crystals have electrostatic forces of attraction. In ionic crystals, these are between the oppositely charged ions. In metals, these are among the valence electrons and the kernels. That is why both have non-directional bonds.
Differences : (a) In ionic crystals, the ions are not free to move. Hence, they cannot conduct electricity in the solid state. They can do so only in the molten state or in aqueous solution. In metals, the valence electrons are free to move. Hence, they can conduct electricity in the solid state.
(b) Ionic bond is strong due to electrostatic forces of attraction. Metallic bond may be weak or strong depending upon the number of valence electrons and the size of the kernels.
(ii) Ionic crystals are hard because they have strong electrostatic forcesof attraction among the oppositely charged ions. They are brittle becauseionic bond is non-directional.