Substance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of orbiting electrons is equal to the number of protons in its nucleus. Each chemical element is denoted by a symbol, which may be amplified by the addition of the atomic mass (A) as a superscript (4He).

Some or all content above used with permission from J. H. Wittke.