The prevailing scientific understanding of matter in the late 19th century was rooted in John Dalton’s atomic theory, which proposed that the atom was the fundamental, indivisible, and indestructible unit of matter, often conceptualized as a solid, uniform sphere. As the director of the Cavendish Laboratory at the University of Cambridge, J.J. Thomson inherited this static view. His experimental work on the conduction of electricity through gases fundamentally challenged the belief that the atom was the ultimate building block, paving the way for a new understanding of atomic structure.
Investigating Cathode Rays
Thomson’s breakthrough began with his study of cathode rays, streams of luminescence observed in highly evacuated glass tubes when a high voltage was applied across two electrodes. He designed an apparatus that incorporated a near-perfect vacuum and a fluorescent screen to track the ray’s path. He first confirmed that the rays carried a negative electrical charge by constructing a tube with an electrometer positioned to collect the beam. When he used a magnet to bend the ray’s path away from the electrometer, the charge reading dropped significantly, demonstrating that the negative charge was inextricably linked to the rays themselves.
Previous experiments by other scientists had failed to definitively bend the rays using an electric field, leading many to believe the rays were a form of electromagnetic wave or light. Thomson hypothesized that residual gas within the tubes used by his predecessors was ionizing and shielding the rays. He overcame this by creating a much higher vacuum, which allowed him to successfully deflect the cathode rays with an applied electric field. Observing the rays bend toward a positively charged plate and away from a negative plate confirmed that the rays consisted of negatively charged particles. This deflection by both electric and magnetic fields provided definitive evidence that the cathode rays were composed of discrete, material particles.
Identifying the Electron
The ability to deflect the particles with both magnetic and electric fields allowed Thomson to perform a calculation: determining the mass-to-charge ratio ($e/m$) of the particles. By adjusting the magnetic field strength to precisely cancel the deflection caused by the electric field, he could calculate the velocity of the particles. He then used the deflection in the electric field alone to solve for the ratio of the charge to the mass. This precise measurement yielded a ratio that was over a thousand times greater than the ratio for the hydrogen ion, the lightest known charged particle at the time.
This finding meant the particles were thousands of times smaller in mass than the hydrogen atom. Thomson concluded that these particles, which he initially termed “corpuscles,” were subatomic components of the atom itself. He repeated the experiment using different cathode materials and different gases inside the tube, yet the mass-to-charge ratio remained constant. This universality proved that the same negatively charged particle was a fundamental constituent of all atoms, thereby shattering the concept of the atom as an indivisible unit.
Proposing the Plum Pudding Model
The discovery of a negatively charged subatomic particle necessitated a new theoretical model for the atom that could account for its existence while preserving the atom’s overall electrical neutrality. Thomson proposed the “Plum Pudding Model” in 1904. In this conception, the atom was visualized as a sphere of uniform, diffuse positive charge, much like a pudding.
Scattered throughout this positively charged sphere were the newly discovered, minuscule negative particles (electrons), analogous to plums or raisins. The total negative charge of the embedded electrons was precisely balanced by the magnitude of the surrounding positive charge. This structural arrangement satisfied the observation that atoms possess no net electric charge, providing the first attempt to incorporate the new subatomic reality into atomic theory.
Pioneering Mass Spectrometry
Thomson’s experimental focus later shifted from the negative cathode rays to the positively charged ions, which were also generated within his discharge tubes. These positive rays, sometimes called canal rays, were formed when the cathode rays struck the residual gas atoms inside the tube, stripping them of an electron and leaving behind a positive ion. He adapted his experimental method by passing these positive ions through parallel electric and magnetic fields, causing them to deflect based on their mass and charge.
This technique involved adjusting the fields so that ions with the same mass-to-charge ratio would trace out a parabolic curve on a photographic plate. This method was the conceptual and physical precursor to modern mass spectrometry, allowing for the precise analysis of the particles’ composition.
Using this apparatus in 1912, Thomson and his colleague Francis William Aston investigated a stream of neon ions. They observed two distinct parabolic traces on the photographic plate, one corresponding to a mass of 20 and a smaller, fainter trace corresponding to a mass of 22. This observation provided the first experimental evidence that atoms of the same element could possess different masses, a phenomenon now known as isotopy.