For nearly a century after John Dalton announced his atomic theory in 1803, the concept that matter consists of tiny, indivisible particles (atoms) was widely accepted among scientists. That theory explained nearly all physical phenomena involving matter then known.

However, the theory was profoundly shaken in the 1890s with J. J. Thomson's discovery of the electron. It immediately became obvious that atoms are not indivisible, but, in fact, consist of even smaller particles, the electron being one of them. The existence of the electron implied, furthermore, that at least one other subatomic particle existed. Since electrons are negatively charged and atoms are electrically neutral, it follows that some positively charged subatomic particle must also exist. That particle, the proton, was discovered by Ernest Rutherford in 1919.

Strong evidence also existed for the presence of yet a third subatomic particle. The discrepancy between atomic number (number of protons in...