We now know, of course, that the nucleus of an atom holds protons and neutrons, but that knowledge didn’t come easily.
Not long after the invention of the Crookes Tube, Thomson investigated the negative particles traveling across the tube and named them electrons. Not long after that, it was discovered that positive particles were moving in the opposite direction.
By moving the cathode to the middle of the tube and perforating it, these positive particles could be studied.
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| Image from Bartleby.com |
This stream of particles was initially called a canal ray, because they moved through the “canals” drilled through the cathode. After Rutherford’s discovery of the nucleus, these particles were studied extensively and the lightest of them were declared to be hydrogen nuclei. This made sense since their mass was (measurably) identical to the mass of hydrogen.
In 1917, Rutherford was experimenting with alpha particle (known at this point to be positive and heavy). Passing them through nitrogen gas, he discovered that the alpha particles could break the nitrogen nuclei into smaller pieces. The smallest of these pieces was identical to the hydrogen nucleus.
Rutherford declared that the hydrogen nucleus must, in fact, be a universal particle found in other nuclei. He named this particle the proton.
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