The Foundations of Chemistry

gas at a very low pressure. When a high voltage is applied, current flows and rays are
given off by the cathode (negative electrode). These rays travel in straight lines toward
the anode (positive electrode) and cause the walls opposite the cathode to glow. An object
placed in the path of the cathode rays casts a shadow on a zinc sulfide screen placed near
the anode. The shadow shows that the rays travel from the cathode toward the anode.
The rays must therefore be negatively charged. Furthermore, they are deflected by elec-
tric and magnetic fields in the directions expected for negatively charged particles.
In 1897 J. J. Thomson (1856–1940) studied these negatively charged particles more
carefully. He called them electrons,the name Stoney had suggested in 1891. By studying
the degree of deflections of cathode rays in different electric and magnetic fields, Thomson
determined the ratio of the charge (e) of the electron to its mass (m). The modern value
for this ratio is

e/m1.75882 108 coulomb (C)/gram

This ratio is the same regardless of the type of gas in the tube, the composition of the
electrodes, or the nature of the electric power source. The clear implication of Thomson’s
work was that electrons are fundamental particles present in all atoms. We now know that
this is true and that all atoms contain integral numbers of electrons.
Once the charge-to-mass ratio for the electron had been determined, additional exper-
iments were necessary to determine the value of either its mass or its charge, so that the
other could be calculated. In 1909, Robert Millikan (1868–1953) solved this dilemma with
the famous “oil-drop experiment,” in which he determined the charge of the electron.
This experiment is described in Figure 5-2. All of the charges measured by Millikan turned

The coulomb (C) is the standard unit

of quantityof electric charge. It is

defined as the quantity of electricity

transported in one second by a current

of one ampere. It corresponds to the

amount of electricity that will deposit

0.00111798 g of silver in an apparatus

set up for plating silver.

5-2 The Discovery of Electrons 179

Figure 5-2 The Millikan oil-drop experiment. Tiny spherical oil droplets are produced by
an atomizer. The mass of the spherical drop can be calculated from its volume (obtained
from a measurement of the radius of the drop with a microscope) and the known density
of the oil. A few droplets fall through the hole in the upper plate. Irradiation with X-rays
gives some of these oil droplets a negative charge. When the voltage between the plates is
increased, a negatively charged drop falls more slowly because it is attracted by the positively
charged upper plate and repelled by the negatively charged lower plate. At one particular
voltage, the electrical force (up) and the gravitational force (down) on the drop are exactly
balanced, and the drop remains stationary. Knowing this voltage and the mass of the drop,
we can calculate the charge on the drop.

Oil droplets

Atomizer

Microscope

Oil droplet

under observation

Charged plate (+)

Small hole

X-ray beam

Charged plate (–)

Robert A. Millikan (left) was an

American physicist who was a

professor at the University of

Chicago and later director of the

physics laboratory at the California

Institute of Technology. He won the

1923 Nobel Prize in physics.

X-rays are a form of radiation of much

shorter wavelength than visible light

(see Section 5-10). They are

sufficiently energetic to knock

electrons out of the atoms in the air.

In Millikan’s experiment these free

electrons became attached to some of

the oil droplets.