Early 20th-century experiments overturned the assumption that atoms are indivisible: J.J. Thomson's cathode-ray deflection experiments established the existence of a light, negatively charged subatomic particle (the electron) by relating electromagnetic deflection to a particle's charge-to-mass ratio, while Ernest Rutherford's alpha-particle gold-foil scattering experiment showed that atomic mass and positive charge are concentrated in a tiny, dense central nucleus surrounded mostly by empty space. Together these findings replaced the "plum pudding" model of the atom with a nuclear model, motivating the subsequent use of classical electrostatics (Coulomb's force law) — and ultimately quantum mechanics — to describe how electrons and nuclei interact within an atom.
Early 20th-century experiments overturned the assumption that atoms are indivisible: J.J. Thomson's cathode-ray deflection experiments established the existence of a light, negatively charged subatom…