In diamond, each C -atom is covalently bonded to four other C-atoms to give a tetrahedral unit, so it shows sp3 hybridisation. Therefore, each C-atom forms four sigma bonds with neighbouring C-atoms. In diamond each C-atom utilizes its four unpaired electrons in bond formation. These bonding electrons are localized. Due to this reason diamond is a bad conductor of electricity. In graphite each C-atom is covalently bonded to three C-atoms to give trigonal geometry. Each C-atom in graphite is sp2 -hybridized. Three out of four valence electrons of each C-atom are used in bond formation while the fourth electron is free to move in the structure of graphite. Due to this reason graphite is a good conductor of electricity.