Which one of the following particles in the nucleus of an atom was discovered by J. Chadwick ?

James Chadwick discovered the neutron in 1932, a breakthrough that earned him the Nobel Prize in Physics in 1935 [2]. Before this discovery, the atomic nucleus was known to contain protons, but scientists like Ernest Rutherford had speculated about a neutral particle to explain discrepancies in atomic mass. Chadwick demonstrated that radiation produced by alpha particles hitting beryllium consisted of uncharged particles with a mass similar to protons. This discovery clarified the structure of the atomic nucleus as being composed of both protons and neutrons [1]. In contrast, the electron was discovered by J.J. Thomson in 1897, the proton by Ernest Rutherford in 1919, and the positron by Carl Anderson in 1932 [1]. The neutron's discovery was pivotal for understanding isotopes and nuclear fission.

Sources

1. [1] https://en.wikipedia.org/wiki/Timeline_of_particle_discoveries
2. [2] https://www.osti.gov/servlets/purl/6513355-bv4Uex/