Carbon is a unique atom that not only has unique bonding properties, but is also the precursor for very large molecules that primarily contain chains and rings.
Organic molecules are necessary for life and are typically derived from living things. Scientists study organic compounds, also called biomolecules. An organic compound is a compound consisting of molecules that contain one or more carbon-hydrogen bonds. Since carbon is the most important element and is considered the foundation for living things, this element can form a diverse set of compounds ranging in size and complexity. A physical property of carbon that contributes to this characteristic includes its desire to achieve the most stable electron arrangement in its outer shell, which is also referred to as a complete octet, where eight electrons are present in the outer shell of an element. The outer shell in a single carbon atom contains four electrons. Outer shell electrons are also called valence electrons. Due to carbon's special bonding behavior, when a carbon atom shares electrons or covalent bonds with other atoms to hold eight valence electrons in its outer shell, carbon is able to form stable compounds. Because these covalent bonds tend to equally share electrons during chemical bonding, they are also classified as nonpolar covalent bonds.
Carbon atoms can form chains, branches, or rings, with other atoms projecting from the carbon backbone. The numerous possible combinations account for the great variety of biological functions in living systems.