1) A substance that cannot be decomposed into simpler substances by chemical means.
2) 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.
3) A substance that cannot be decomposed; each chemical element is characterized by the number of protons in the nucleus. See the periodic table and the introduction for elements.