In the 19th and early 20th centuries, the three main subatomic particles discovered were electrons, protons, and neutrons.

• Electrons were discovered by J.J. Thomson in 1897 through his cathode ray tube experiment, where he identified negatively charged particles smaller than atoms.
• Protons were identified primarily by Ernest Rutherford in 1917, building on earlier observations of canal rays (anode rays) by Eugen Goldstein, showing the existence of positively charged particles in atoms.
• Neutrons were discovered by James Chadwick in 1932 when he detected neutral particles in nuclear reactions, which had no electric charge but similar mass to protons.

These discoveries fundamentally reshaped the understanding of atomic structure by revealing that atoms consist of these smaller constituents.