Both diamond and graphite are composed purely of carbon atoms explain why diamond is so much harder than graphite?

Answer 1

Without going into too many details, it's because of the way the carbon atoms are arranged and bonded to each other in the two substances. In diamond, they're bound in a three-dimensional lattice. In graphite, they're bound in two-dimensional sheets with only weak intermolecular forces between the neighboring layers.

Answer 2

The hardness of minerals is compared using the Mohs Hardness Scale, a relative scale numbered 1 (softest) to 10 (hardest). Graphite is very soft and has a hardness of 1 to 2 on this scale. Diamonds are the hardest known natural substance and have a hardness of 10. No other naturally occurring substance has a hardness of 10. The crystal structure of graphite yields physical properties that permit the use of graphite as a lubricant and as pencil lead. The gem and industrial properties of diamond, physical properties that we cherish and exploit, are also a result of diamond's crystal structure.

The reason for the differences in hardness and other physical properties can be explained with the molecular models below. In graphite, the individual carbon atoms link up to form sheets of carbon atoms. Each sheet of carbon atoms is translated (offset) by one-half of a unit such that alternate sheets are in the same position. Within each sheet every carbon atom is bonded to three adjacent carbon atoms that lie at the apices of equilateral triangles. This produces hexagonal rings of carbon atoms. Each carbon atom has four valence electrons available to participate in the formation of chemical bonds. Three of these electrons are used in forming strong covalent bonds with the adjacent atoms in the sheet. Covalent bonds are a type of chemical bond in which electrons are shared between atoms. The fourth electron is free to wander over the surface of the sheet making graphite an electrical conductor. The spacing between the sheets of carbon atoms is greater than the diameter of the individual atoms. Weak bonding forces called van der Waals forces hold the sheets together. Because these forces are weak, the sheets can easily slide past each other. The sliding of these sheets gives graphite its softness for writing and its lubricating properties.