One of the 116 presently known substances that cannot be decomposed by chemical reaction into a simpler substance. Elements comprise all matter at and above the atomic level. All elements heavier than lead (Pb) are radioactive and unstable. In addition, there are no stable elements with odd numbers of both protons and neutrons heavier than nitrogen (N). Elements with even numbers of protons and an even number of neutrons make up about 90% of the earth's crust.