Thomson's Theory Of Negatively Charged Atoms

Although Joseph John (J.J.) Thomson (1856 - 1940) was mainly a physicist, his discoveries are closely connected to the chemical community. Thomson used Crookes high vacuum cathode ray tube in his discovery of electrons. He found the green beam produced by the cathode ray tube was a completely negatively charged material. Experimenting with different electrically charged plates and magnets in the cathode ray tube and observing the deflection of the rays he had enough evidence to say that particles smaller than atoms existed. By doing this experiment multiple times he compiled enough data to conclude that the mass of one of these particles, which he called an electron, was lighter than an atom of hydrogen by around 2,000 times. He also found the ratio of charge to mass (e/m) would be the same for any electron.
His hypothesis about the charge of the electron being the same as the charge found in hydrogen lined up with this experiment. He formulated the idea that electrons would be found inside an atom, proved that Dalton’s theory about the indivisibility of atoms was wrong. Thomson knew these negatively charged electrons needed to have a positive that would balance them out. A positively charged material would have to surround them and this theory became known as the Plum in the Pudding Theory. The “plums” (negatively charged) were inside the "pudding" (positively charged). From this he drew up one of the first models of an atom [see fig. 1 on p. 8]. His theory was the beginning of explaining chemical bonding and molecular