Organic Chemistry is the Chemistry of Carbon Compounds. Several millions organic compounds are known compared to no more than 100, 000 compounds that contain no carbon. There is, however, a small number of carbon compounds (e.g. CO, CO2, Metal Carbonates) that are not classified as organic.
The vast number of organic compounds reflects the exceptional ability if carbon to form covalent bonds with other carbon atoms to form chains, branched chains and rings structures. Straight chain compounds are called Aliphatic Compounds and ring compounds are called Cyclic Compounds.
In Organic Compounds, Carbon always has a valency of four. Apart from carbon, a small number of other elements is widely found in organic compounds. The most important of these elements are Hydrogen, Oxygen, Nitrogen, Sulfur and the Halogens.