Although his discovery would provide the basis for every piece of electronics in our modern households, Thomson was not satisfied to stop there. Instead, he wanted to understand how exactly the electron fit into the atom’s structure. He surmised that the atom was a small sphere of matter, with a positive charge distributed uniformly throughout, into which a large number of negatively charged electrons were embedded. On a microscopic scale, he imagined the negative electrons swimming about like fish in a vast sea of positive charge.
Given its likeness to a popular dessert of his day, he called this the “plum pudding” model of the atom. We have never eaten plum pudding ourselves, but we gather its texture must have resembled that of chocolate chip cookie dough: a gooey blob of dough into which many small chocolate chips are distributed throughout.
Thomson’s “plum pudding” model depicts an atom as a sphere of positively charged matter embedded with negatively charged electrons.
Thomson’s model of the atom is not only simple and logical; it also helped to explain a phenomenon we discussed in the last the emission spectrum of heated solid materials.
Because of the interaction between the positively charged background and the negatively charged electrons, the electrons would normally settle into positions that were uniformly distributed throughout the positive sphere. If the atom were heated, the electrons would begin to jiggle about their usual positions. And, since Maxwell had shown that moving charges emit radiation, this motion could emit light when the solids were heated high enough.
Indeed, the plum pudding model could provide simple qualitative explanations for many phenomena (e.g., why heated metals glow). However, it was unfortunately not able to give accurate quantitative predictions (e.g., why metals heated to 2,700 degrees Celsius produce light at a wavelength of 0.0001 cm). There must have been a better explanation.
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