Explain
a. The basic of sumilarities and differences between metallic and ionic crystals.
b. Ionic solids are hard and brittle.

Explanation

(i) Basis os similarities. The basis of similarities between the metallic and ionic crystals are the prsence of strong electrostatic forces of attraction. These are present among the ions in the ionic crystals and among the kernels and valence electons in the metallic crystals. That is why both metals and ionic compounds are good conductors of electricity and have high melting point.
Basis of differences. The basis of differences is the absence of mobility of ions in the ionic crystals while the same is present in the valence electrons and kernels in case of metallic crystals. As a consequence, the ionic compounds condduct electricity only in the molten state while the metals can do so even in the solid state.
(ii) The ionic solids are hard and brittle because of strong electrostatic forces of attraction which are present in oppositely charged ions.
(iii) The ionic solids are hard because of the presence of strong inter ionic forces of attraction in the oppositely charged ions. These ions are arranged in three dimensional space. The ionic solids are brittle because the ionic bond is non-directional.