4.2. Structure of the Atom http://www.ck12.org
Discovery of Cathode Rays
In 1877, William Crookes (1832-1919) was studying how electrical current behaves in a vacuum tube. In one
experiment, he passed an electric current through an evacuated phosphorous-coated glass cylinder with an object in
the center, as shown inFigure4.4.
FIGURE 4.4
Diagram of Crookes tube.Upon passing a current through the tube, Crookes noticed that a "shadow" was cast by the object in the tube. The
phosphorus on the terminal end of the tube became brightly fluorescent, except for the region directly behind the
central object. He interpreted this to mean that the electrical current was blocked by the object. He reasoned that
the electrical current, which he later called cathode rays, was composed of streams of particles. Crookes’s work was
later expanded upon by several other scientists. One scientist in particular, J. J. Thomson, was able to show that
cathode rays could be deflected by a magnetic field, as shown in the following video: http://www.youtube.com/w
atch?v=M1REuKMeI34 (0:58).
MEDIA
Click image to the left for more content.Thomson’s interpretation of this effect was that cathode rays must consist of charged particles that have mass.
Thomson presented his work in 1897, where he referred to these negatively charged particles as corpuscles. Later
on, this name was changed and negatively charged particles became known aselectrons. Thomson revised the model
of the atom into what became known as theplum pudding model. He hypothesized that the atom was comprised of
negatively charged particles in a field of positive charge (positively charged particles had not yet been discovered).
This proposed arrangement was compared to the arrangement of plums in plum pudding, as illustrated inFigure
4.5.