Why diamond is hard while graphite is soft ?

Explanation

In diamond, each carbon atom is `sp^(3)` hybridised. Each carbon atom is tetrahedrally attached to other carbon atoms by covalent bonds. It has three dimensional network structure. All the carbon atoms in diamond are closely packed. Thus, diamond is very hard and has high melting point. In graphite, each carbon atom is `sp^(3)` hybridised. Each carbon atom is bonded to other three C-atoms by covalent bonds and forms hexagonal planar rings. Fourth valence electron of each carbon is free to move. This makes the graphite soft. In graphite, two successive layers are held together by weak vander Waal.s forces. Therefore, one layer can slip over another layer. Hence, graphite is soft as well as lubricative in nature.