It is sometime said that life on our planet is based on carbon. Carbon is an element that is found in all organic molecules. Carbon forms strong covalent bonds, in other words it shares electrons, with other elements. It forms four such bonds that are it has a valency of four. A simple example is methane, whose molecular formula is CH4. The explanation for the importance of carbon lies in the way carbon atoms can join to each other, forming either chain or rings. These chains & rings are the skeletons of organic molecules & hence of life itself. They are very stable because the covalent bonds linking the carbon atoms together are strong. Atoms or particular groups of atoms of other elements (referred to simply as groups) can be attached at various positions to the carbon skeleton.