Diamond is extremely hard but graphite is soft and slippery -explain with reason.

Explanation

In diamond crystal, each carbon atom being `sp^3`- hybridised is linked to four other carbon atoms tetrahedrally by covalent bonds. Innumerable number of such tetrahedral units are linked together to form a three-dimensional giant array of molecules containing very strong bonds extended in all directions. Because of such three-dimensional network of strong covalent bonds, diamond is extremely hard. On the other hand, in graphite crystal, each carbon atom being `sp^2`- hybridised is linked to three other carbon atoms forming a network of planar hexagons and these two dimensional layers exist in different parallel planes, lying horizontally one above another a distance of at 3.35Å. Since these layers are held together by weak van der Waals forces, one layer easily slides over the other at the slightest application of pressure. This explains why graphite is soft and slippery.