The ability of carbon atoms to form covalent bonds with other carbon atoms is the most unique of its bonding properties. This enables carbon to form long, continuous chains, branches and loops consisting of carbon and hydrogen in hydrocarbons and only carbon in carbon allotropes such as C60.
The ability of carbon to bond with itself is called catenation. This ability of carbon atoms to combine with other atoms, including itself, with a high degree of versatility gives rise to the possibility of near-infinite combinations. Over 10 million organic compounds consisting of some combination of carbon and other elements have been documented, with new organic compounds being conceived and synthesized every day.
The position of carbon midway in the periodic table makes it neither a metal nor a non-metal, making it equally likely to share electrons with either. The abundance of outer-shell electrons in carbon make it capable of forming a maximum of four covalent bonds.
In addition to sharing electrons with metals and non-metals of moderate electronegativity, carbon can also donate or accept electrons, giving rise to organic salts. Silicon is another element that can bond with itself and form long chains, but the large size of silicon atoms makes it incapable of forming as large a variety of compounds as carbon.