Why is diamond hard and graphite soft if both are made of carbon?
Diamond has three dimensional network of strong covalent bonds. It is very difficult to break extended covalent bonding and therefore, diamond is the hardest substance on earth. On the other hand, graphite has layer structure and the layers are held together by weak Vander Waals forces. Therefore, graphite cleaves easily between the layers and, therefore, it is very soft and slippery.