http://www.ck12.org Chapter 4. Atomic Structure
the cathode, becomes negatively charged. Once this happens,a beam called acathode raytravels from the cathode
to the anode.
FIGURE 4.6
To produce a cathode ray, a glass tube
filled with a low-pressure gas is con-
nected to a power source. The beam, or
cathode ray, moves from the cathode to
the anode.Investigations were carried out to determine the nature of the cathode ray. The results of two further experiments
supported the hypothesis that the cathode ray consisted of a stream of particles.
- When an object was placed between the cathode and the opposite end of the tube, it cast a shadow on the glass.
- A cathode ray tube was constructed with a small metal rail between the two electrodes. Attached to the rail was
a paddle wheel capable of rotating along the rail. Upon connecting the cathode ray tube to a power source, the
wheel rotated from the cathode toward the anode. This suggested that the cathode ray was made of particles
that must have mass.
In order to determine if the cathode ray consisted of charged particles, Thomson used magnets and charged plates to
deflect the ray (Figure4.7). His findings are summarized below.
- Cathode rays were deflected by a magnetic field in the same manner as a wire carrying an electric current,
which was known to be negatively charged. - Cathode rays were deflected away from a negatively charged metal plate and toward a positively charged plate.
Thomson knew that opposite charges attract one another, while like charges repel one another. Together, the results of
the cathode ray tube experiments showed that cathode rays are actually streams of tiny negatively charged particles
moving at very high speeds. While Thomson originally called these particles corpuscles, they were later named
electrons.
Thomson conducted further experiments which allowed him to calculate the charge-to-mass ratio (e/me) of the
electron. In units of coulombs to grams,e/me= 1.8× 108 C/g. He found that this value was a constant and did not
depend on the gas used in the cathode ray tube or on the metal used as the electrodes. He concluded thatelectrons
were negatively charged subatomic particles present in atoms of all elements.
Watch a video of a cathode ray tube experiment at http://www.dlt.ncssm.edu/core/Chapter3-Atomic_Str_Part1/cathode
-rm-lg.htm.
Charge and Mass of the Electron
American physicist, Robert Millikan (1868-1953), carried out a series of experiments between 1908 and 1917 that
allowed him to determine the charge of a single electron. Millikan’s experiment was called the oil drop experiment
(Figure4.8).