Chemistry

Thomson used a glass tube with one end connected to a negative electrode (cathode), a positively charged electrode (anode) at the other end, and a tiny paddle wheel in the center. Running an electric current through a variety of gases inside the tube, he discovered that: The current created different colors in different gases; the anode (positive) end glowed; the negative (cathode) did not; and the paddle wheel spun in the direction of the anode. From this, Thomson deduced that atoms of different gases produced different energies, shown by the different colors. In order to make the paddle spin, the gases must have particles; and that the current flowed from negative to positive. This latter finding was one of his most important discoveries.

In 1897, J. J. Thomson announced his discovery of the electron and that atoms must therefore have some structure. At the time not all scientists were convinced that atoms even existed, but Einstein's 1905 explanation of Brownian motion convinced other scientist that atoms were real.

Thomson's model of atomic structure is called the plum pudding model. He assumed that the negatively charged electrons, he had recently discovered, were scattered throughout a cloud of positive charge, like the plums in plum pudding.

The nuclear model described the atom as a nucleus, in which all the mass is concentrated around light. Electrons circulate at some distance. This was discovered by Rutherford.

The nucleus was assumed to be small and dense to account for the alpha particles from thin gold foil. A radioactive source capable of giving off alpha particles was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through an opening in a lead screen. A thin section of gold foil was placed in front of the opening, and a screen coated with zinc sulfide to make it fluorescent served as a counter to detect alpha particles. As each alpha particle struck the fluorescent screen, it would...