Graphite is soft primarily because of its layered crystal structure and the nature of the bonds between layers. Key points:

• Layered structure: Graphite consists of planes of carbon atoms arranged in hexagonal lattices (graphene layers) stacked on top of each other.
• Intra-layer bonds: Within each layer, carbon atoms are held together by strong covalent bonds, making each layer hard and rigid.
• Interlayer forces: The layers are held together by weak van der Waals forces between adjacent layers, not by strong bonds. These weak forces allow the layers to slide past one another easily.
• Slippery and soft result: The easy interlayer sliding reduces friction and gives graphite its lubricating, soft, and greasy feel.

Additional context:

• The delocalized electrons within each graphene layer contribute to electrical conductivity but do not significantly strengthen the interlayer interactions, so the layers still slide readily.
• This combination of strong in-plane bonding and weak interlayer bonding explains why graphite is soft and an excellent dry lubricant, in contrast to diamond, which has strong covalent bonds in a three-dimensional network and is very hard.