Short answers, pulled from the story.
Henri Becquerel discovered radioactivity in 1896 while investigating phosphorescence in uranium salts. Marie Curie, Pierre Curie, and Ernest Rutherford then investigated radioactivity extensively; Becquerel, Marie Curie, and Pierre Curie shared the 1903 Nobel Prize in Physics for these discoveries.
In 1909, Hans Geiger and Ernest Marsden, working under Rutherford at the University of Manchester, fired alpha particles at a thin gold foil. Some particles bounced back at large angles, disproving the plum pudding model and leading to Rutherford's 1911 conclusion that the atom has a very small, dense, positively charged nucleus surrounded by orbiting electrons.
James Chadwick identified the neutron in 1932, recognizing that radiation observed by Bothe, Becker, and the Joliot-Curies came from a neutral particle with roughly the same mass as the proton. The neutron's discovery allowed scientists to calculate nuclear binding energies and resolved the long-standing mystery of nuclear spin measurements, including nitrogen-14's spin of 1.
In 1935, Hideki Yukawa proposed that a virtual particle, later called a meson, mediates a force between all nucleons, both protons and neutrons. This force prevents proton repulsion from tearing nuclei apart and operates over a shorter range than electromagnetic repulsion. The later discovery of the pi meson confirmed it had the properties Yukawa predicted.
Elements heavier than iron cannot be built by stellar fusion without an energy input, so they are produced by neutron capture. The slow s-process occurs in asymptotic giant branch stars over hundreds to thousands of years, building up to lead and bismuth. The rapid r-process is thought to occur in supernova explosions, where extreme temperatures and neutron flux drive the formation of the heaviest elements.
Natural nuclear fission reactors were active in two regions of Oklo, in Gabon, Africa, more than 1.5 billion years ago. These are the only known naturally occurring fission chain reactors. They operated because conditions there allowed a self-sustaining neutron-initiated chain reaction in the local uranium deposits.