In 1913 Henry Moseley measured the energy of X-ray's emitted from a metal.  Using this information about the discrete energy levels of an element he reorder the periodic table of elements.  Up until that point the periodic table had been ordered by atomic number.  Using the binding energy of the inner most electron Moseley ordered the periodic table the way we see it today.  Using this theory of discrete excitation energies the order of eight elements will be discovered.