Short answers, pulled from the story.
Henri Becquerel published his findings on radioactivity in 1896 while investigating phosphorescence in uranium salts. This discovery marked the beginning of nuclear physics as a distinct field from atomic physics.
Ernest Rutherford presented his theory of the atomic nucleus to the Royal Society in 1911 after analyzing scattering data. Hans Geiger and Ernest Marsden fired alpha particles at thin gold foil during experiments conducted between 1909 and 1911.
James Chadwick identified the neutron in 1932 as a neutral particle with a mass similar to that of the proton. The neutron's spin properties explained why nitrogen-14 possessed a net spin of one rather than zero.
Eighty elements possess stable isotopes totaling approximately 251 nuclides that never decay. Thousands of other isotopes exist as unstable radioisotopes decaying over timescales ranging from seconds to trillions of years.
Natural nuclear reactors operated in Oklo, Gabon over 1.5 billion years ago before human intervention. Measurements indicate that half the heat emanating from Earth's core results from radioactive decay rather than primordial formation.