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 = 1.661 × 10−27 kg. See also: Atomic mass unit; Atomic number; Coulomb's law; Electron; Mass number; Neutron; Proton
Atoms are the smallest objects that have a chemical identity. Atoms are thus the basic building blocks of matter. 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. (An element is the name of an atom having a given atomic number.) See also: Element (chemistry); Periodic table
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.
A Rydberg atom has one electron in a highly excited orbit. Its radius grows and binding energy decreases with the square and the inverse square, respectively, of the principal quantum number n. A neutral Rydberg atom with n = 100 has a huge diameter, approximately 10−6 m or 4 × 10−5 in. (near that of a bacterium). Its miniscule binding energy, approximately 1.36 millielectronvolts, explains its fragility in collisions with other particles. Occurring naturally in the interstellar medium, Rydberg atoms with n-values up to several hundred emit spectral lines that have been detected with radio telescopes. They have also been produced and studied in the laboratory. See also: Rydberg atom
An exotic atom has a different charged particle substituted for an electron, its nucleus, or both. For example, positronium consists of an electron and its antiparticle, a positron. See also: Atomic structure and spectra; Hadronic atom; Muonium; Pionium; Positronium