atom describes the positions of electrons in an atom in terms of probabilities. An electron can potentially be found at any distance from the nucleus, but, depending on its energy level and angular momentum, exists more frequently in certain regions around the nucleus than others; this pattern is referred to as its atomic orbital. The orbitals come in a variety of shapes—sphere, dumbbell, torus, etc.—with the nucleus in the middle. The shapes of atomic orbitals are found by solving the Schrödinger equation; however, analytic solutions of the Schrödinger equation are known for very few relatively simple model Hamiltonians including the hydrogen atom and the dihydrogen cation. Even the helium atom—which contains just two electrons—has defied all attempts at a fully analytic treatment. See also Spectroscopy History of molecular theory Timeline of chemical element discoveries Introduction to quantum mechanics Kinetic theory of gases Atomism The Physical Principles of the Quantum Theory Footnotes Bibliography Further reading Charles Adolphe Wurtz (1881) The Atomic Theory, D. Appleton and Company, New York. Alan J. Rocke (1984) Chemical Atomism in the Nineteenth Century: From Dalton to Cannizzaro, Ohio State University Press, Columbus (open access full text at http://digital.case.edu/islandora/object/ksl%3Ax633gj985). External links Atomism by S. Mark Cohen. Atomic Theory - detailed information on atomic theory with respect to electrons and electricity. The Feynman Lectures on Physics Vol. I Ch. 1: Atoms in Motion Statistical mechanics Chemistry theories Foundational quantum physics Amount of substance