J.J. Thomson's Discovery of the Electron
On April 30, 1897, J.J. Thomson announced that cathode rays were negatively charged particles which he called corpuscles. He also said that "corpuscles" had a mass about 1000 times smaller than a hydrogen atom, and he said that these corpuscles were the things from which atoms were built up.He had jumped to the conclusion that the particles in the cathode ray, which we now called electrons, were a fundamental part of all matter. This was reaching far beyond what he had actually discovered.
Thomson's corpuscle hypothesis was not generally accepted, even by British scientists, until he said something of it again in 1899. By this time, George Francis FitzGerald an Irish physicist, had said that Thomson's corpuscles making up the cathode ray were actually free electrons. IN fact, this suggestion was published as a commentary to the publication of Thomson's April 30, 1897 lecture in which he first announced his results.
Other people had measured the e/m ration or suggested that the cathode rays were composed of particles, but Thomson was the first scientist to say that the cathode ray was a stepping stone of the atom. It was risky thing, but he was proved right and for his boldness he is remembered as the discoverer of the electron.
The Thomson Model of the Atom
In 1897, Thomson discovered the electron, the first subatomic particle. He also was the first to attempt to incorporate the electron into a structure for the atom. The internal structure of the atom had been a source of speculation for thousands of years. The Greeks thought that the atom was solid. Jon Dalton thought the same.
Thomson faced two big problems:
1. How to account for the mass of the atom when the electron was only about 1/1000 the mass of the hydrogen atom.
2. How to create a neutral atom when the only particle available was negatively charged.
This solution was the gospel of the scientific world for about a decade and Thomson himself would make a major contribution to undermining his own model.
Leadup to Thomson's 1904 Model of the Atom
Thomson said that the view taken by many chemists was that the atoms of the different chemical elements are different aggregations of atoms of the same kind. He also said that he regarded the atom as containing a large number of smaller bodies which he would call corpuscles. He said that these corpuscles were equal to each other and that in the normal atom, this assemblage of corpuscles forms a system which is electrically neutral.
Thomson's Mature Model
His next statement on the structure of the atom comes in a 1904 article. The first half of the article is filled with detailed calculations about the stability of corpuscles moving about in a positive environment. Thomson is only able to make calculations where all of the corpuscles are limited to rotating in a ring. Moving from ring to sphere proves a challenge much to difficult.
This model is often referred to as Thomson's "plum pudding model," where the pudding represents the sphere of positive electricity and the pieces of plum scattered in the pudding are electrons.
Savante Arrhenius was not completely convinced about all of this. He said:
"This conception has hitherto remained only a formal one, and has led to no new results.