Joseph John Thomson’s contributions to science helped revolutionize the understanding of atomic structure. Although a mathematician and an experimental physicist by training, J. J. Thomson contributed extensively to the field of chemistry by discovering the existence of electrons, developing the mass spectrometer and determining the presence of isotopes.
Thomson’s Early Interest in Science
J. J. Thomson was born in Manchester, England, in 1856. His father expected him to be an engineer. When an engineering apprenticeship did not materialize, he was sent, at 14 years old, to Owen College. After the death of J. J.’s father, the cost of an engineering apprenticeship was unmanageable. Instead, in 1876, he received a scholarship to Trinity College at Cambridge to study mathematics.
After attending Trinity College, Thomson went on to become a Fellow of Trinity College in 1880. He remained as a professor at Trinity for the entirety of his career. At the age of 28, he succeeded Lord Rayleigh (discoverer of argon and investigator of densities of gases) as the Cavendish Professor of Experimental Physics at Cambridge in 1884.
J.J. Thomson: Experiment Beginnings
Thomson, as professor of experimental physics, attempted to build mathematical models to explain the nature of atoms and electromagnetism.
He began studying cathode rays in 1894. Little was understood at the time about cathode rays beyond being a glowing beam of light in a high-vacuum glass tube. A cathode ray tube is a hollow glass oblong container where the air is removed to create a vacuum. At the cathode, a high voltage is applied, and this causes a green glow at the opposite end of the glass tube.
The idea that tiny particles transmitted electricity had been proposed in the 1830s. When Thomson allowed the cathode rays to travel through air versus a vacuum, he found they traveled a far distance before being stopped; they traveled even farther in a vacuum. He thought the particles must be smaller than the estimated size of atoms.
J.J. Thomson: Experiments With Cathode Ray Deflection
To test his hypothesis that the cathode ray particles were smaller than the size of atoms, Thomson improved his experimental apparatus and began to deflect the cathode rays with electric and magnetic fields. His goal was to find whether these particles held a positive or negative charge. Also, the angle of deflection would allow him to estimate mass.
After measuring the angle at which these rays were deflected, he calculated the ratio of electrical charge to the mass of the particles. Thomson found the ratio remained the same regardless of which gas was used in the experiment. He postulated that the particles contained within the gases were universal and not dependent on the composition of the gas utilized.
J.J. Thomson: Model of Atom
Up until J. J. Thomson’s experiments with cathode ray particles, the scientific world believed that atoms were the smallest particles in the universe. For over 2,000 years, the atom was considered the minutest possible particle, and the Greek philosopher Democritis named this smallest particle atomos for uncuttable.
The world now had its first glimpse at a subatomic particle. Science would be forever changed. Any new model of the atom must contain subatomic particles.
Thomson called these particles corpuscles. And while he was correct about the existence of the particles, the name he gave them changed: These negatively charged particles are now known as electrons.
J.J. Thomson: Atomic Theory
With this new subatomic particle, J. J. Thomson produced a new atomic model, or atomic theory, concerning the structure of the atom.
Thomson’s theory is now known as the plum pudding atomic model or Thomson atomic model. The atom was visually thought of as a uniformly positively charged mass (the “pudding” or “dough”) with the electrons scattered throughout (like “plums”) to balance the charges.
The plum pudding model proved incorrect, but it offered the first attempt at incorporating a subatomic particle into an atomic theory. In 1911, Ernest Rutherford — a former student of J. J. Thomson — proved this theory incorrect by experimenting and hypothesizing the nucleus.
Invention of Mass Spectrometer
A mass spectrometer is similar to a cathode ray tube, though its beam is made of anode rays, or positive charges, rather than electrons. As in J. J. Thomson’s electron experiments, the positive ions are deflected from a straight path by electric and magnetic fields.
Thomson improved the known anode ray tube by attaching an oscilloscope-like screen at the detection point. The screen was coated with a material that fluoresced when hit by the rays.
Once a charged particle passes by a magnetic field, it is deflected. This deflection is proportional to the mass to charge ratio (m/e). The deflections, which are portions of a parabola, could be recorded accurately against the screen. Each species sent through the anode ray tube has a separate parabola.
When lightweight species penetrated the screen too deeply, J. J. Thomson constructed a slit in the tube where the screen would sit. This allowed him to plot intensity against relative mass and created the first mass spectrometer.
Thomson developed the mass spectrometer along with his student researcher Francis William Aston. Aston continued this research and won a Nobel Prize in 1922 for his work.
Discovery of Isotopes
J. J. Thomson and Aston used the mass spectrometer to identify positive ions of hydrogen and helium. In 1912, they fired ionized neon into the electric and magnetic fields. Two separate patterns for the beam emerged: one with atomic mass of 20 and a weaker parabola of mass 22.
After suggesting impurities, he realized that this weaker parabola was a heavier form of neon. This indicated two atoms of neon with different masses, better known as isotopes.
Recall that an isotope is the change in the number of neutrons within the nucleus. With an isotope, the identity of the element remains the same, but it has a different number of neutrons in the nucleus. J. J. Thomson and Aston concluded the higher mass of another neon isotope without having the benefit of knowing the existence of neutrons (discovered by James Chadwick in 1932).
J.J. Thomson: Contribution to Science
In 1906, J. J. Thompson received the Nobel Prize in Physics “in recognition of the great merits of this theoretical and experimental investigations on the conduction of electricity by gases.” Thomson is credited with identifying electrons as particles of an atom.
Although many other scientists made observations of atomic particles during the time of Thomson's experiments, his discoveries led to a new understanding of electricity and atomic particles.
Thomson is rightfully credited with the discovery of the isotope and his experiments with positive-charged particles led to the development of the mass spectrometer. These accomplishments contributed to the evolution of knowledge and discovery in physics and chemistry that have continued to the present.
J. J. Thomson died in August 1940 in Cambridge and is buried in Westminster Abbey near Isaac Newton and Charles Darwin.
About the Author
Rosann Kozlowski is currently a freelance writer and tutor. She has a Master's Degree in Chemistry from the University of Oregon and has previously worked in the pharmaceutical industry and has taught at the middle school, high school, and college levels.