A constituent of matter consisting of z negatively charged electrons bound predominantly by the Coulomb force to a tiny, positively charged nucleus consisting of Z protons and (A - Z) neutrons. Z is the atomic number, and A is the mass or nucleon number. The atomic mass unit is u = 1.6605397 x 10-24 g. Electrically neutral atoms (z = Z) with the range Z = 1 (hydrogen) to Z = 92 (uranium) make up the periodic table of the elements naturally occurring on Earth. Isotopes of a given element have different values of A but nearly identical chemical properties, which are fixed by the value of Z. Certain isotopes are not stable; they decay by various processes called radioactivity. Atoms with Z greater than 92 are all radioactive but may be synthesized, either naturally in stellar explosions or in the laboratory using accelerator techniques.
Atoms with Z - z ranging from 1 to Z - 1 are called positive ions. Those having z - Z = 1 are called negative ions; none has been found with z - Z greater than 1.