Atomic theory, first put in a quantitative conceptual framework by John Dalton, and quantum theory, which emerged in the 1920s as a result of the work of Werner Heisenberg, Erwin Schrödinger, and Max Planck, are the cornerstones of our present-day view on atomic structure. Atomic theory holds that matter consists of vast numbers of small particles called atoms which combine together to form molecules existing in the three main states of matter as gases, liquids or solids. Atoms are very small, their diameter lies between 0.7 and 6 Ångstroms (1 Ångstrom = 10-10 m), as revealed by x-ray crystallography, the method of choice to determine the size of atoms from a pattern of x rays diffracted on a crystal. At the turn of the century, the work of J.J. Thomson, Robert Millikan and Ernest Rutherford showed that atoms are electrically neutral and consist of a...