|Atomic Structure, Periodicity, and Matter: Development of the Atomic Theory|
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
be the solid sphere that
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.
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