Which postulate of Dalton's Atomic Theory was proven wrong by the discovery of subatomic particles?

Dalton's Atomic Theory stated that atoms are the smallest indivisible particles of matter, but later discoveries showed that atoms contain smaller particles such as electrons, protons, and neutrons. J.J. Thomson discovered the electron in 1897 through cathode ray experiments, proving that atoms have internal structure and are not indivisible. The proton was identified by Ernest Rutherford, while the neutron was discovered by James Chadwick in 1932. The discovery of isotopes also modified Dalton's idea that all atoms of the same element have identical mass, because isotopes have the same atomic number but different mass numbers.