The document outlines the history of atomic theory from Democritus to Niels Bohr, including key contributors like John Dalton, JJ Thomson, Ernest Rutherford, and their discoveries about atoms, electrons, protons, and neutrons that led to modern atomic theory. Democritus first proposed atoms as indivisible particles, while Dalton established atoms as the basic unit of elements. Thomson discovered the electron, Rutherford discovered the nucleus with protons, and Bohr incorporated quantum theory with electrons orbiting the nucleus.