Modern Atomic Theory: Electrically Charged Particles

Approximately fifty years after John Dalton's proposal of the atom, evidence began to accumulate which suggested that the atom might not be the solid sphere that Dalton had envisioned. This evidence came in the form of the discovery of electrically charged particles and radioactive materials. Based on these new discoveries, Dalton's proposal of a solid, indestructible atom became unacceptable. Listed below, are a few of the significant discoveries that were clues that led to the development of the modern theory of the atom.

 In the 1830's, Michael Faraday, a British physicist, made one of the most significant discoveries that led to the idea that atoms had an electrical component. Faraday placed two opposite electrodes in a solution of water containing a dissolved compound. He observed that one of the elements of the dissolved compound accumulated on one electrode, and the other element was deposited on the opposite electrode. It was clear to Faraday that electrical forces were responsible for the joining of atoms in compounds. 

In 1879, Sir William Crookes studied the effects of sending an electric current through a gas in a sealed tube. The tube had electrodes at either end and a flow of electrically charged particles moved from one of electrodes. This electrode was called the cathode, and the particles were known as cathode rays. The particles were first believed to be negatively charged atoms or molecules. However, subsequent experiments showed that these particles could penetrate thin sheets of material which would not be possible if the particles were as large as atoms or molecules.

In 1895, Wilhelm Roentgen, experimenting with cathode rays, discovered new and different kinds of rays. Roentgen discovered that if he directed these rays toward a paper plate coated with barium platinocyanide, the plate became fluorescent. During subsequent experiments, he found the rays created an image on a photographic plate. These "new" rays were originally known as Roentgen rays. We know them today as x-rays which are part of the electromagnetic spectrum.