9.2.1 Catenation:

        The remarkable property of carbon atom to form bonds with itself and give rise to a single large structure or chain is called catenation. The carbon chains can be straight or balanced forming large molecules.

        Catenation is also defined as “The property of direct bonding between atoms of same element to form a chain.”

Allotropes of Carbon:

        The phenomenon of existence of a substance in various physical forms but same chemical form is known as allotropy by carbon atoms. Diamond is a hard, beautiful crystalline substance while graphite is a soft, grayish black crystalline substance. In diamond, each carbon atom is linked to four other neighbouring carbon atoms held at the corner of a regular tetrahedron by covalent bonds to form a rigid three dimensional structure. While in case of graphite, each carbon atom is attached to three other carbon atoms forming a hexagonal planar structure. Physical properties of graphite and diamond are different but their chemical properties are the same. In diamond, there are no mobile electrons in the system and hence diamond crystal is a non conductor of electricity. On the other bond, in graphite, free electrons move throughout the entire layers, and hence graphite is a good conductor of electricity.

        Uses: Diamond is used as precious stone in jewellery. Black diamonds are used for cutting glass, while graphite is used in making electrodes and carbon, lubricants and lead pencils.

Digram (Diamond and Graphite)
Structure of Graphite
Structure of Graphite
 
Structure of Diamond
Structure of Diamond
Fig. 9.1

9.2.2 Parent hydrocarbons:
        Take the example of methane CH4. It is a saturated hydrocarbon and an alkane. If we increase the number of carbon atoms by 1 and the number of hydrogen atoms by 2, then we will get a compound C2H6. This is ethane and is the next member in the family of alkanes. If we add one more carbon atom and two more hydrogen atoms to ethane, we get the compound propane C3H8.

From this you can infer that the general formula for alkanes is CnH2n+2 where n is the number of carbon atoms.
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