Which sub-atomic particle was identified by J. Chadwick in 1932?

The correct option is

Neutron

Explanation

The structure of the atom was elucidated through the successive discovery of its constituent sub-atomic particles: electrons, protons, and neutrons. The neutron was the last major particle to be identified due to its lack of electric charge.

• Electron is Incorrect: The electron was discovered by J.J. Thomson in 1897 through cathode ray discharge tube experiments.
• Proton is Incorrect: The proton is associated with the discovery of canal rays by E. Goldstein in 1886 and the subsequent characterization of the nucleus by Ernest Rutherford.
• Neutron is Correct: James Chadwick discovered the neutron in 1932. He observed that when light elements like beryllium were bombarded with alpha particles, electrically neutral particles were emitted. These particles had a mass slightly greater than that of protons.
• Positron is Incorrect: The positron, which is the antiparticle of the electron, was discovered by Carl Anderson in 1932.

Key Takeaway: James Chadwick discovered the neutron in 1932, a fundamental neutral particle located in the nucleus of an atom.