The first of the fundamental particles to be discovered was the electron. J.J. Thomson carried out experiments with cathode rays and established the electron's charge and mass. Beginning in 1896, the , and rays were discovered and investigated by Becquerel, the Curies, Rutherford, Thomson, and their collaborators. While the rays soon turned out to be electrons and the rays helium nuclei, the rays were not proven to be electromagnetic radiation until 1914. Quantum mechanics had its beginnings when Planck quantized material oscillations in 1900. Soon after, Einstein formally proposed quanta of light with particle-like characteristics

During the 1910's, the nuclear model (Rutherford) and the old quantum theory (Bohr) of the atom were developed. In the 1920's, physicists began a search for an understanding of the nucleus. Chadwick and Bieler recognized that a new kind of strong force was needed to explain what was known about the nucleus. In 1925, Goudsmit and Uhlenbeck attributed an intrinsic angular momentum (spin) to the electron, and two years later Dennison found that the proton has the same spin. During this decade, it was widely believed that nuclei were made of protons and electrons, although with the all-important development of quantum mechanics there were problems (nuclear spin and statistics) with this model. The resolution came with the discovery of the neutron by Chadwick in 1931.