1. Dalton's atomic theory from 1808 proposed that elements are composed of atoms and that atoms of the same element are identical. Compounds are composed of atoms of more than one element combined in simple whole number ratios. 2. J.J. Thomson discovered the electron in experiments in 1906. Rutherford's gold foil experiment in 1911 led to the nuclear model of the atom with a small, dense positively charged nucleus surrounded by electrons. Neutrons were discovered by Chadwick in 1932. 3. Atoms are characterized by atomic number and mass number. Isotopes are atoms of the same element with different numbers of neutrons. Molecules and ions are composed of multiple atoms or groups of atoms. Chemical n