Of the six million compounds known, five million are organic. Carbon is unique for two main reasons. First, long chains can be built up. Carbon can form two more bonds having made a chain, hence allowing enormous variation in the side chains. Second, carbon is at the top of Group 14 of the periodic table and so is the smallest atom in the group.
Carbon atoms can therefore make stronger bonds than silicon atoms can.
The higher bond strength means that carbon compounds are much more stable kinetically than the corresponding silicon compounds. The most important division within organic chemistry is into aromatic chemistry , based around rings such as those in benzene, and aliphatic chemistry , where molecules do not have benzene rings. Examples of the resulting classes include alkanes , alkenes , alcohols and halogenoalkanes . The New Penguin Dictionary of Science 2. The New Penguin Dictionary of Science 3.