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Dalton's ideas proved foundational to modern atomic theory. However, one of his underlying assumptions was later shown to be incorrect. Dalton thought that atoms were the smallest units of matter-−minustiny, hard spheres that could not be broken down any further. This assumption persisted until experiments in physics showed that the atom was composed of even smaller particles. In this article, we will discuss some of the key experiments that led to the discovery of the electron and the nucleus.
J.J. Thomson and the discovery of the electron
In the late 19^{\text{th}}19
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19, start superscript, start text, t, h, end text, end superscript century, physicist J.J. Thomson began experimenting with cathode ray tubes. Cathode ray tubes are sealed glass tubes from which most of the air has been evacuated. A high voltage is applied across two electrodes at one end of the tube, which causes a beam of particles to flow from the cathode (the negatively-charged electrode) to the anode (the positively-charged electrode). The tubes are called cathode ray tubes because the particle beam or "cathode ray" originates at the cathode. The ray can be detected by painting a material known as phosphors onto the far end of the tube beyond the anode. The phosphors spark, or emit light, when impacted by the cathode ray.
A diagram of a cathode ray tube.
A diagram of a cathode ray tube.
A diagram of J.J. Thomson's cathode ray tube. The ray originates at the cathode and passes through a slit in the anode. The cathode ray is deflected away from the negatively-charged electric plate, and towards the positively-charged electric plate. The amount by which the ray was deflected by a magnetic field helped Thomson determine the mass-to-charge ratio of the particles. Image from Openstax, CC BY 4.0.
To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
Thomson also placed two magnets on either side of the tube, and obser
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