The sub-atomic particles protons and neutrons were discovered early in the 20th century. Ernest Rutherford discovered the proton in 1917 as part of his work on atomic structure, marking a significant advancement in atomic theory during the early 20th century. The proton is a positively charged particle found in the nucleus of an atom.

The neutron was discovered later in 1932 by James Chadwick. He identified the neutron as a neutral particle with a mass similar to that of the proton, located in the atomic nucleus along with protons.

Thus, both protons and neutrons were discovered during the 20th century.