Each carbon atom forms four chemical bonds. Carbon most commonly forms covalent bonds, which are two atoms sharing electrons, but also sometimes forms ionic bonds found in compounds, such as calcium carbide.
Carbon has six electrons: two in the inner orbit and four in the outer orbit. The outer orbit is complete when it reaches eight electrons, and so carbon atoms form covalent bonds with other atoms with electrons to share. Although carbon is not the only element with four valence, or outer, electrons, it is the smallest, allowing it to fit comfortably as part of much larger molecules.
Any compound that contains covalently bonded carbon is considered an organic compound. More than 10 million organic compounds are known to chemists, who are continually synthesizing more. Carbon's ability to form carbon-to-carbon chains is one reason why carbon can form so many compounds. Carbon atoms can form chains, complex branches and rings. Carbon bonds with elements by sharing one, two or three electrons, which allows it to bond with a wide range of other atoms. Hydrogen is the most common element to bond with carbon, but nitrogen, oxygen, phosphorous, sulfur, and halogens, along with many other kinds of atoms, bond with carbon to form organic molecules.