(a) What is the unique property of carbon atom? How is this property helpful to us?(b) Explain why, diamond is hard while graphite is soft.

(a) Carbon can form long chains by combining atoms to atoms. Catenation is the most unique property of a carbon atom.

It gives a large number of carbon compounds or organic compounds.

(b) Diamond and graphite are made up of carbon. Diamond is hard and graphite is soft because the strong covalent bond between carbon forms a tetrahedron which is hard to break. Graphite is a flat hexagonal ring forming layers of carbon atoms, hence it is softer.