In the spring of 1889, the World’s Fair opened in Paris in the shadow of the newly-built Eiffel Tower. Six years later, while his fellow Parisians were still craning their necks to study this architectural marvel, the physicist Henri Bequerel was sitting across town, in a darkened room, wrapping rocks in paper.
He was studying the properties of phosphorescent rocks, which were known to “trap” light and then re-emit radiation when the lights were switched off. When he wrapped up a piece of uranium salt, though, he was surprised to see that it emitted radiation even without any initial exposure to the room’s lights. A year later, this phenomenon was named radioactivity by a pair of colleagues, Marie and Pierre Curie.
Radioactivity is the process through which one type of atom spontaneously converts (or decays) into a different type of atom. The process is accompanied by the energetic emission of either small, charged particles (called alpha and beta particles) or electromagnetic radiation (called gamma rays).
Though scientists needed a few years to interpret the scientific basis of radioactivity, they eventually concluded that the uranium atoms were undergoing a transformation to a different type of atom called thorium. In the process, they emitted miniscule and energetic charged particles. If true, this hinted that the atom was not truly fundamental after all. Instead, this evidence pointed to a divisible atom that was composed of even smaller building blocks.
For fans of the fundamental atom, the next sign of trouble came in 1897, across the English Channel at the University of Cambridge. Here, a physicist named J. J. Thomson was studying something known as the cathode ray. This is essentially a type of radiation emitted from the negative pole of an open electric circuit. Thomson discovered that by applying an electromagnetic field to a beam of cathode rays, he could deflect their straight-path trajectories. The larger the electromagnetic field, the larger the deflection.
Since an electromagnetic field could deflect the rays, they could not have been electromagnetic waves themselves. Instead, these rays behaved as if they were small, negatively charged electric particles. Therefore, Thomson named them electrons. By applying Newton’s laws of motion and Maxwell’s electromagnetic equations, he was able to infer that electrons were contained within atoms themselves. This provided a definitive blow to the concept of the atom as nature’s smallest and most fundamental particle.
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