The document summarizes the major historical theories and models of the atom. It describes how ancient Greek philosophers first proposed the idea of indivisible atoms. In the early 1900s, scientists including J.J. Thomson, Ernest Rutherford, Niels Bohr, and Erwin Schrödinger developed models of the internal structure of atoms. Thomson's plum pudding model depicted atoms as positively charged spheres containing electrons. Rutherford's gold foil experiment led to the planetary model with a small, dense nucleus surrounded by orbiting electrons. Bohr added that electrons can only orbit in fixed energy levels. Later, Schrödinger developed the probabilistic cloud model still used today.