Carbon (from "coal") is a chemical element with symbol C and atomic number 6. On the periodic table, it is the first (row 2) of six elements in column (group) 14, which have in common the composition of their outer electron shell. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with and being stable, while is radioactive, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.