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Plum-Pudding Model

In 1859 the German physicist, Julius Plucker, developed a vacuum tube that allowed a current to flow across a gap from the cathode (-) to the anode (+). Eugene Goldstein realized this was a type of radiation and named the new radiation cathode rays. Thus the cathode ray tube (CRT) was born, and would eventually lead to the development of radio, television and computers.

Later in 1879, English physicist Sir William Crookes proposed that cathode rays were actually streams of particles flowing from the cathode to the anode, and these particles had the same properties regardless of the cathode material used to produce the cathode rays.

Then in 1891, Irish physicist George Stoney proposed that a negatively charged particle associated with the atom existed, and he suggested the new particle be called an electron.

Stoney derived this name from the Greek word "elektron" meaning amber-- a substance known to become electrically charged. However, he was unable to prove the existence of this particle.

Soon after in 1897, English physicist, J. J. Thomson performed a set of experiments that showed cathode rays could be deflected away from a negative charge by both electric and magnetic fields.

"I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter."
- J.J. Thomson

This discovery led Thomson to suggest that cathode rays are negatively charged particles which he called corpuscles (electrons). He also determined that the charge was about -1 x 10-19 coulombs.

In another set of experiments Thomson was able to determine the charge to mass ratio (e/m) of the cathode rays and using this data he calculated the mass of the electron.

Finally in 1909, American physicist Robert Millikan performed his now famous oil drop experiment. Millikan's experiment determined the exact charge and mass of the electron.

The electron was no longer an idea but a real particle, but its mass is 1836 times smaller than the hydrogen ion, the smallest known ion. Thus, the electron is just a small part of the atom, in other words the electron was a subatomic particle. Dalton's theory of an indivisible particle was no longer acceptable.

I was told long afterwards by a distinguished physicist who had been present at my lecture that he thought I had been pulling their leg
-J. J. Thomson

Now, a theory that allowed for subatomic particles was needed. Thomson proposed that the atom was a mass of positive charge with negative electrons embedded into it like raisins in plum pudding, so it was dubbed the Plum Pudding model.

Thomson's Plum Pudding model, although attractive, nevertheless had its own shortcomings. Philipp Lenard had noted that cathode rays could pass through very thin pieces of matter.

Thus, in 1903, Lenard suggested that the atom was not a large mass but mostly space.