Answer:
1. Ernest Rutherford ---> discovery of the atomic nucleus
2. James Chadwick ---> discovery of the neutron
3. Henri Becquerel ---> discovery of radioactivity
4. J.J. Thomson ---> discovery of the electron
Explanation:
Ernest Rutherford discovered that an atom consists of a dense region of which contains protons at approximately at the center of the atom called the nucleus. His conclusion was derived from the results of the Geiger–Marsden experiment.
James Chadwick discovered the neutron by measuring the radiation of scattered particles. The neutrons did not emit gamma radiation and had no charge.
Henri Becquerel discovered radioactivity by realizing that the images formed by exposing them to radioactive material did not require an external source of radiation.
J.J. Thomson detected a stream of particles was being ejected inside his cathode ray tube leading to the discovery of the electron.
