The correct option is **A. James Chadwick**. ### Detailed Explanation: 1. **Historical Context**: The discovery of the neutron was a significant milestone in the field of nuclear physics. Before neutrons were discovered, scientists had already identified protons and electrons as the fundamental particles of atoms. However, the existence of a neutral particle within the nucleus was still unknown. 2. **James Chadwick's Contribution**: In 1932, James Chadwick conducted experiments that led to the discovery of the neutron. He bombarded beryllium with alpha particles (which are positively charged) and observed that a new type of radiation was emitted. This radiation was not deflected by electric or magnetic fields, indicating that it was neutral. Chadwick concluded that this radiation was composed of neutral particles, which he named neutrons. 3. **Experimental Evidence**: Chadwick's experiments involved measuring the energy and momentum of the emitted particles. He found that these particles had a mass similar to that of protons but carried no charge. This was a groundbreaking discovery because it explained the missing mass in the atomic nucleus that could not be accounted for by protons alone. 4. **Significance of the Neutron**: The discovery of the neutron was crucial for the development of nuclear physics. It helped explain isotopes (atoms of the same element with different numbers of neutrons) and led to advancements in nuclear energy and weapons. ### Why the Other Options Are Incorrect: - **B. Ernest Rutherford**: While Rutherford is known for his discovery of the nucleus and the concept of the nuclear model of the atom, he did not discover the neutron. His work laid the groundwork for understanding atomic structure, but the neutron was identified later by Chadwick. - **C. J.J. Thompson**: J.J. Thompson is famous for discovering the electron and proposing the "plum pudding" model of the atom. He did not discover the neutron and was primarily focused on the study of charged particles. - **D. John Dalton**: Dalton is known for his atomic theory, which proposed that matter is made up of indivisible atoms and that atoms of different elements have different masses. He did not have any involvement in the discovery of neutrons, as his work predates the discovery of subatomic particles. ### Summary of Key Points: - **Correct Answer**: A. James Chadwick discovered the neutron in 1932. - **Chadwick's Experiment**: He bombarded beryllium with alpha particles, leading to the identification of neutral particles. - **Importance of Neutrons**: Neutrons are essential for understanding atomic structure, isotopes, and nuclear reactions. - **Other Scientists**: Rutherford, Thompson, and Dalton contributed to atomic theory but did not discover neutrons. This thorough understanding of the discovery of neutrons and the contributions of various scientists will help you grasp the historical context and significance of this fundamental particle in chemistry and physics.