why ionic solids are hard and brittle
They are hard because of strong electrostatic forces between atoms. They are brittle because of low mobility of defects called dislocations formed in ionic solids. We all know that metals are ductile and they deform plastically because of movement of dislocations on certain crystallographic planes. In contrast, ionic solids donât deform plastically because of low mobility of dislocations. The low mobility comes from the fact that it is difficult to break ionic bond for a dislocation to move.