CARBON

(C), a nonmetallic chemical element in Group IVa of the periodic table. Although widely distributed in nature, carbon is not particularly plentiful (it makes up only about 0.025 percent of the Earth's crust); yet it forms more compounds than all the other elements combined. In 1961 the isotope carbon-12 was selected to replace oxygen as the standard relative to which the atomic weights of all the other elements are measured; carbon-14, which is radioactive, is the isotope used in radiocarbon dating and radiolabeling.