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Quantum Mechanics

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Joseph John "J.J." Thompson (1856 - 1940)

After Thompson’s discovery of the electron with his cathode ray tube experiment, he developed a new view of the atom. It wasn’t entirely correct, but offered good progress.

Recall that John Dalton had suggested that atoms were like hard billiard balls, indestructible and indivisible. But Thompson’s electron gave evidence that the atom came to us in separate parts.

However, the question was raised: “Since atoms are electrically neutral, and since the electron is negatively charged, what is the counterbalancing factor in the atom to offset the electron?”

To answer this, Thompson theorized in 1904 that the atom is like a British “plum pudding,” what Americans would call a Christmas cake.

The negatively charged electrons, Thompson thought, were stuck in the “pudding,” the dough of the dessert, like plums or raisins embedded in a cake. The “dough” was presumed to be positively charged, thereby rendering the atom as a whole electrically neutral.

This model of the atom didn’t last long as, within a few years, Rutherford and Bohr would present alternate explanations.