The Discovery of the Neutron: James Chadwick’s 1932 Experiment Explained
After the discovery of electrons and protons, a major puzzle remained: atoms were much heavier than the sum of their protons and electrons. In 1932, British physicist James Chadwick solved this mystery by discovering the neutron —the final missing piece of the atomic nucleus. The Atomic Weight Mystery By the 1920s, scientists knew the atom had a dense positive nucleus surrounded by negative electrons. However, the numbers didn't add up: A helium nucleus had a charge of +2 (meaning 2 protons), but its mass was 4 times that of a hydrogen atom. Where was that extra mass coming from if there were no extra electric charges? Ernest Rutherford hypothesized that an electrically neutral particle—a bound pair of a proton and an electron—must exist inside the nucleus to add weight without changing the charge. The Experiment That Unlocked the Answer In 1930, German physicists Walther Bothe and Herbert Becker noticed that when beryllium was bombarded with alpha particles, it emitted an unusual...