Final answer:
J.J. Thomson discovered electrons through his research on cathode rays, disproving Dalton's Atomic Theory that stated atoms were indivisible. He also discovered that atoms were composed of electrons and a positive nucleus, which contributed to the development of the atomic model.
Step-by-step explanation:
J.J. Thomson discovered electrons in 1897 through his research on cathode rays. He observed that the small, negatively charged particles in the cathode ray were actually pieces of atoms, which he called 'corpuscles' but are now known as electrons. This discovery provided evidence that atoms were not indivisible as proposed by Dalton's Atomic Theory.
Dalton's Atomic Theory was proven incorrect in two aspects. First, it stated that atoms were indivisible, but the discovery of electrons by Thomson showed that atoms have subatomic particles. Second, Dalton's theory suggested that atoms of different elements have identical properties, which was disproven by advancements in atomic theory and the discovery of isotopes, which have different numbers of neutrons.
J.J. Thomson discovered the electron through his research on cathode rays. He conducted experiments that demonstrated that the cathode rays were made up of small, negatively charged particles. This discovery contributed to the development of the atomic model, showing that atoms are composed of electrons and a positive nucleus.