For most of the nineteenth century, the atom was the end of the line: the smallest possible piece of matter, with nothing inside it.1Science History Institute. John Dalton and the Scientific Method In 1897, J.J. Thomson found particles inside atoms, the first known part smaller than an atom. His real contribution was turning the atom from a solid dot into something with an inside, and the question of what that inside looks like drove atomic physics for the next several decades.
What Dalton’s Atom Looked Like Before Thomson
John Dalton, who lived from 1766 to 1844, proposed that all matter is made of atoms.2Science History Institute. John Dalton and the Scientific Method He said each element has its own kind of atom, all alike within that element and different from the atoms of other elements.3Science Museum. John Dalton: atoms, eyesight and auroras Each type had a fixed mass and could not be destroyed or changed.4Science History Institute. John Dalton and the Scientific Method Chemical reactions, in his view, were atoms joining together and separating, not atoms breaking apart.5Science History Institute. John Dalton and the Scientific Method So nothing could be smaller than an atom. The word “atom” comes from a Greek word meaning uncuttable, and Dalton’s atoms were often pictured as tiny hard balls.
The theory was strong because it made chemistry predictable. If each atom has a set mass, a compound such as water always contains the same proportion of its elements by weight.
How Thomson Discovered the Electron
Thomson worked with a glass tube with most of the air pumped out and metal pieces at each end connected to a high voltage.6Nobelprize.org. Vacuum Tubes A beam called a cathode ray traveled from the negative end through the tube.7Nobelprize.org. Vacuum Tubes Where the beam hit the far end, it made a glowing spot, so its path could be seen and measured.8Philosophical Magazine. Cathode Rays In a darkened room he marked the spot with a needle coated in glowing paint, then read its position on a scale once the lights were on.9Philosophical Magazine. Cathode Rays
Two charged metal plates made the beam bend toward the positive plate.10Science Ready. Thomson’s Discovery of The Electron – HSC Physics Bending toward the positive plate showed the beam carried negative charge.
Coils carrying electric current made a magnetic field that pushed the beam the opposite way.11Science Ready. Thomson’s Discovery of The Electron – HSC Physics He adjusted the magnetic field until the two pushes cancelled and the beam ran straight.12Science Ready. Thomson’s Discovery of The Electron – HSC Physics The electric and magnetic pushes depend on the beam’s speed differently, so when they cancel exactly, the speed can be calculated from the two field strengths. Then he could work out the beam’s speed and the charge-to-mass ratio, meaning how much electric charge its particles carried for their weight.13Philosophical Magazine. Cathode Rays Turning off one field and measuring how far the beam bends under the other gives that ratio.
He got the same figure no matter which metal the ends of the tube were made from.14Purdue University Chemistry. J. J. Thomson It also stayed the same whatever gas filled the tube.15Purdue University Chemistry. J. J. Thomson He concluded the particles are a component of all matter.16Purdue University Chemistry. J. J. Thomson He also concluded they were much smaller than ordinary atoms and called them corpuscles; today they are called electrons.17Science History Institute. Joseph John “J. J.” Thomson
The smallness came from the size of the ratio. It was hugely larger than the figure for charged hydrogen, the lightest charged atom known. A larger ratio means either much more charge or much less mass, and Thomson argued the particles were far lighter than any atom.
Thomson measured only the ratio of charge to mass, not the electron’s mass itself. The full answer came later with Millikan’s work, covered below.
The Plum Pudding Model of the Atom
Atoms are electrically neutral, so the negative electrons needed matching positive charge somewhere. Thomson had no evidence yet for any separate positive particle, so he pictured the atom as a sphere of evenly spread positive charge with electrons scattered through it.18Wikipedia. Plum pudding model
He compared this to plum pudding, with electrons as the fruit dotted through the pudding.19Royal Institution. Subatomic science: JJ Thomson’s discovery of the electron The pudding stands for positive charge with no particular center, and the fruit stands for scattered electrons. Today’s textbooks often use blueberries in a muffin instead.20LibreTexts Chemistry. J.J. Thomson’s Atomic Model
Each electron is pulled toward the center of the positive sphere and pushed away from the other electrons, so they spread out fairly evenly.21Wikipedia. Plum pudding model Thomson worked on how electrons might settle into rings, hoping this would explain why elements repeat in the periodic table.
Around 1900 this model was widely accepted.22PubMed Central. Nagaoka’s atomic model and hyperfine interactions But the Japanese physicist Hantaro Nagaoka proposed a very different model in the same period.23PubMed Central. Nagaoka’s atomic model and hyperfine interactions He pictured electrons in a ring around a heavy positive center, like Saturn’s rings.
Why the Plum Pudding Model Was Replaced
In 1909 Hans Geiger and Ernest Marsden fired alpha particles at thin metal foil.24Wikipedia. Rutherford scattering experiments A screen around the foil flashed where a particle hit. Most particles passed straight through, which showed that an atom is mostly empty space. Only a very small fraction were deflected strongly, but some bounced off at large angles, sometimes more than 90 degrees.25Wikipedia. Rutherford scattering experiments In Thomson’s model, all of them should have gone straight through.26Wikipedia. Rutherford scattering experiments
Spread-out positive charge gives only weak pushes everywhere, so heavy, fast alpha particles would barely swerve. Bouncing back requires charge packed into a tiny point. Rutherford likened it to a shell bouncing off tissue paper. He proposed a small dense positive nucleus with electrons around it. Rutherford had been one of Thomson’s own students.27Trinity College Library, Cambridge. J. J. Thomson Remembered
Thomson’s model still got a lot right. Atoms do contain electrons, and they do balance negative and positive charge. Only the arrangement of the positive charge was wrong.
Thomson’s Discovery of Isotopes
Thomson also studied beams of positively charged atoms in his tubes.28Wikipedia. J. J. Thomson In 1907 he proposed that lighter charged particles in such beams bend more than heavier ones.29International Journal of Mass Spectrometry. Development of mass spectrometers from Thomson and Aston to present Positive atoms were created in the tube and sent through electric and magnetic fields together. The fields spread the beam into curved tracks on a photographic plate, one track per mass, and the plate let him measure them afterward.30EBSCO. Thomson Confirms the Possibility of Isotopes Particles with the same mass and charge landed in the same track, so he could identify different elements from their tracks.31EBSCO. Thomson Confirms the Possibility of Isotopes
Working on neon, he got two tracks, one at mass 20 and one at mass 22.32EBSCO. Thomson Confirms the Possibility of Isotopes At first he thought the heavier one was a new element.33EBSCO. Thomson Confirms the Possibility of Isotopes He concluded instead that he had separated two forms of the same element, now called isotopes: versions of one element with different masses.34EBSCO. Thomson Confirms the Possibility of Isotopes This 1912–1913 work is credited as the first evidence for isotopes of a non-radioactive element.35Wikipedia. J. J. Thomson
Isotopes behave the same chemically and differ only in mass. The difference comes from neutrons, which Chadwick found in 1932.
This explained why atomic weights measured by chemists were often not whole numbers.36International Journal of Mass Spectrometry. Mass spectrometry—The early years A sample mixes isotopes, and the measured weight is an average of them. So chlorine’s weight is about 35.5 rather than a whole number. It also meant Dalton’s idea that every atom of an element has the same mass was wrong.
How Thomson Helped Launch Mass Spectrometry
His work with Francis Aston was the first use of mass spectrometry, sorting charged particles by mass.37Wikipedia. J. J. Thomson It led to Aston’s mass spectrograph.38Wikipedia. J. J. Thomson The field dates its birth to Thomson’s 1907 idea about lighter particles bending more,39International Journal of Mass Spectrometry. Development of mass spectrometers from Thomson and Aston to present and it celebrated its 100th anniversary in 2013.40International Journal of Mass Spectrometry. Mass spectrometry—The early years
The method works the same way in principle today. A sample is turned into charged particles, which are pushed through fields that bend lighter ones more than heavier ones. A detector records where and how many particles land, giving a pattern of masses. The pattern works like a fingerprint, because each substance produces its own set of masses.
That makes it useful well beyond the physics lab. Hospital and workplace drug tests confirm results with it, and forensic labs identify unknown substances. Carbon dating counts isotopes, and space probes analyze planetary atmospheres.
How Millikan Finished Measuring the Electron
Robert Millikan’s oil drop experiment measured the charge of a single electron. Tiny charged oil drops floated between charged plates, and their charges always came in multiples of one basic amount. That basic amount is the electron’s charge. His result came about 1909 to 1913.
Thomson’s ratio had mass and charge tangled together, so knowing either one would give the other. Dividing Millikan’s charge by Thomson’s charge-to-mass ratio gave the electron’s mass.41Brainly. Thomson was able to determine the mass/charge ratio of the electron but not its mass. How did Millikan’s experiment allow determination of the electron’s mass? The electron weighs about 1/1,800 of a hydrogen atom. The mass was calculated rather than weighed because nobody could put a single electron on a scale.
The Name Electron and Thomson’s Wider Legacy
George Stoney coined the word electron in 1891, six years earlier.42Physics Education. J J Thomson and the Discovery of the Electron For Stoney it meant a basic unit of charge, not a particle.43Physics Education. J J Thomson and the Discovery of the Electron So Thomson kept calling his particles corpuscles for many years.44Physics Education. J J Thomson and the Discovery of the Electron
Thomson won the 1906 Nobel Prize in Physics for his work on how electricity passes through gases.45World Patent Information. J.J. Thomson, the electron and the birth of electronics Nine of his students and assistants later won Nobel Prizes too.46Trinity College Library, Cambridge. J. J. Thomson Remembered They included Niels Bohr, whose atom model is still taught, and Rutherford.47Trinity College Library, Cambridge. J. J. Thomson Remembered