Who discovered neutron? 

James Chadwick 

The correct answer is option 3. James Chadwick.

James Chadwick, a British physicist, is credited with the discovery of the neutron in 1932. Here's an explanation of his discovery:

1. Experimental Setup:  Chadwick conducted experiments involving the bombardment of various elements with alpha particles (positively charged helium nuclei). He aimed to investigate the properties of atomic nuclei and the nature of the radiation emitted during such interactions.

2. Observation of Neutrons:  In one series of experiments, Chadwick bombarded a sample of beryllium with alpha particles and observed an unexpected form of radiation being emitted from the target material. This radiation appeared to consist of neutral particles with no electric charge.

3. Analysis and Identification:  Chadwick carefully analyzed the properties of the neutral radiation and conducted further experiments to characterize its behavior. He determined that these neutral particles had a mass similar to that of a proton but lacked any electric charge. This led Chadwick to conclude that he had discovered a new subatomic particle, which he named the "neutron" due to its neutral (no charge) nature.

4. Significance:  The discovery of the neutron was a significant milestone in the field of nuclear physics. Neutrons, along with protons, are fundamental building blocks of atomic nuclei. The existence of neutrons provided crucial insights into the structure of atomic nuclei and the forces that hold them together. Additionally, neutrons play a key role in nuclear reactions, including nuclear fission and fusion processes.

5. Impact: Chadwick's discovery of the neutron opened up new avenues for research in nuclear physics and had profound implications for various scientific and technological applications. It laid the foundation for the development of nuclear reactors, nuclear weapons, and techniques such as neutron scattering for studying the structure of materials.

In summary, James Chadwick's discovery of the neutron revolutionized our understanding of atomic structure and paved the way for advancements in nuclear physics and related fields.