1. A covalent bond is formed by the overlap of half-filled atomic orbitals which have definite directions. Hence covalent bond is directional.
2. In ionic compounds, each ion is surrounded by a number of oppositely charged ions and hence there is no definite direction.Ions may be attracted to one another in any direction
3. Covalent bond are called-directional-bonds-whereas-ionic-bonds-are-called-non directional bonds.
4.The covalent bonds, being directional, limit the number of atoms that they can bond while ionic and metallic bonds being non-directional try to increase the number of nearest neighbors as much as possible (to lower the overall bonding energy of the system).
4. The increased number of nearest neighbors means the metals and ionic solids tend to be close packed, and higher density than covalent solids
Covalently bonded materials are less dense than metallic or ionically bonded ones because covalent bonds are directional in nature whereas metallic and ionic are not; when bonds are directional, the atoms cannot pack together in as dense a manner, yielding a lower mass density.