1. In ionic crystalline solids, constituent particles are positively charged cations and negatively charged anions placed at alternate lattice points. 

2. The ions are held by strong coulombic electrostatic forces of attraction compensating opposite forces. Hence they are hard. 

3. Since there are no free electrons, they are not malleable and on applying a shearing force, ionic crystals break into small units. Hence they are brittle.

In ionic solids the formation of solid compound is due to the arrangement of cations and anions bound by strong coloumbic force i.e., electro static force . So ionic solids are hard and brittle in nature . These have high melting and boilig point .