These are crystalline solids in which the constituent particles are non-metal atoms linked to the adjacent atoms by covalent bonds throughout the crystal forming a giant three dimensional structure. Covalent network that is why these solids are called network solids. Hence covalent solids are called giant solids and the constituting molecules are called giant molecules. Since covalent bonds are strong and dimensional, atoms are held strongly at their lattice positions.
The covalent network solids are very hard or brittle depending on the event of bonding. These covalent network solids have high melting points and they may even decompose before melting. Network solid act as good conductors of electricity and they are also good insulators. It is depends upon the availability of free electrons. The examples of covalent solids are diamond, graphite, carborundum, fullerene, boron nitride, etc.
Crystalline solid Diamond and silicon carbide are typical examples of covalent solids or network solids. Diamond has a big giant network structure consisting of very large number of carbon atoms bonded to each other by covalent bonds. Each carbon atom in diamond is sp3 hybridised and lies at the centre of a regular tetrahedron while other four carbon atoms are present at the corners of the tetrahedron. It is the hardest known substance with a very high melting point of 39300 degree Celsius. The unit cell of the crystalline diamond is cubic.
Properties of covalent bonds graphite is a covalent crystalline solid and which is an allotrope of ca
rbon. Ionic solids all carbon atoms are sp2 hybridised. Each carbon atom is covalently bonded to other three sp2 hybridised carbon atoms forming sigma bonds and the fourth electron in 2pz orbital of each carbon atom is used in the formation of a pie bond. This results in the formation of hexagonal rings in two dimensions. In graphite, the layers consisting of hexagonal carbon network are held together by weak van der Waal’s forces imparting softness. Thus, each layer can slide over the other easily. It is because of this that graphite is soft, and can act as a lubricant.