electrons, protons and which other sub-atomic particle were discovered in the 19th and early 20th century?

The other main sub-atomic particle is the neutron.

Quick Scoop

• Electrons (J. J. Thomson, 1897) and protons (building on work by Goldstein and Rutherford in the early 1900s) were identified as fundamental components of the atom in the late 19th and early 20th centuries.

• The third basic sub-atomic particle discovered in that early 20th‑century period is the neutron, identified by James Chadwick in 1932.

• Together, electrons, protons, and neutrons are usually listed as the three primary sub‑atomic particles that make up ordinary atoms.

So the answer to “electrons, protons and which other sub-atomic particle…?” is: neutrons.