(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 (though both are made of carbon atoms).
(a) The most unique property of carbon atom is its ability to combine itself, atom to atom to form long chains. This property of self combination is useful to us because it gives rise to an extremely large number of carbon compounds (or organic compounds).
(b) A diamond crystal is a giant molecule of carbon atoms. Each carbon atom in the diamond crystal is linked to four other carbon atoms by strong covalent bonds. The four surrounding atoms are at the four vertices of a regular tetrahedron. This rigid structure of diamond makes it a very hard substance.
The structure of graphite is very different from that of diamond. A graphite crystal consists of layers of carbon atoms or sheets of carbon atoms and these layers are held together by weak Van der Waals forces. Due to this sheet like structure, graphite is a comparatively soft substance.