mathematical formula. The fact that the spectrum of hydrogen was so simple
and so unexplainable caused a problem for classical mechanics. It turned out,
about 30 years later, that classical mechanics could not explain it. Other theo-
ries were necessary.9.5 Atomic Structure
In the fourth century B.C., Democritus suggested that matter was composed of
tiny parts called atoms. However, experience suggests that matter is smooth.
That is, it is continuous and not broken into individual pieces. Faced with
mounting evidence, especially from the study of gases, John Dalton (Figure
9.6) revived the atomic theory in a modern version that gradually came to be
accepted. Implicit in this theory was the idea that atoms are indivisible.
In the 1870s and 1880s, certain phenomena were investigated by passing an
electrical current through an evacuated tube having a small quantity of gas in
it. In the 1890s J. J. Thomson (Figure 9.7) performed a series of experiments
in evacuated tubes and showed that the electrical discharge was not composed
of electromagnetic radiation—mistakenly referred to as cathode rays—but was
instead a stream of particles formed from some residual gas left in the tubes.
Further, these particles had electric charges on them, indicated by a deflection
of the stream by a magnetic field. Measurements of the charge-to-mass ratio,
e/m, which could be measured by the amount of magnetic deflection of the
stream, were extraordinarily high. This indicated either a huge charge or a tiny
mass. Thomson speculated that the charge could not be large, leaving the tiny
mass as the only possibility.
The mass of this particle, called the electron, had to be less than one-
thousandth of that of a hydrogen atom (whose mass was known). But this in-
dicated that some particles of matter are smallerthan atoms, an idea that was
supposedly precluded by the modern atomic theory. Obviously, this negatively
charged particle was only a piece of an atom. The implication was that atoms
were not indivisible.
Experiments by Robert Millikan between 1908 and 1917 established the ap-
proximate magnitude of the charge, which was then used with Thomson’s e/m
ratio to determine the mass of the electron. In his famous oil drop experiment,
diagrammed in Figure 9.8, Millikan and coworkers introduced tiny oil droplets
in between charged plates, subjected them to ionizing radiation (X rays), and
varied the voltage over the plates to try to electrostatically levitate the drops.
Knowing the density of the oil, the voltage difference between the plates, the
radius of the droplets, and correcting for air buoyancy, Millikan calculated an
approximate charge of 4.77
10 ^10 electrostatic units (esu) or about 1.601
10 ^19 coulombs (C). From the e/m, Millikan was able to calculate the mass of
the electron as about 9.36
10 ^31 kg, about 1/1800 of the mass of a hydro-
gen atom. (The modern accepted value for the mass of an electron is 9.109
10 ^31 kg.) Since there are negatively charged particles in atoms, there should
also be positively charged particles, so that matter would be electrically neu-
tral. The proton, a positively charged particle, was identified by Ernest
Rutherford in 1911.
Following Rutherford and Marsden’s classic experiments with metal foil
scattering in 1908, Rutherford proposed the nuclear model for atoms. In
the nuclear model the majority of the mass—consisting of the protons and the
later-discovered neutrons—is concentrated in a central region called the
nucleus,and the smaller electrons revolve around the nucleus at some rela-
tively great distance. The experiment and the resulting model are illustrated in
Figure 9.9.9.5 Atomic Structure 251Figure 9.6 John Dalton (1766–1844). In 1803,
Dalton restated the atomic theory of Democritus
(fourth century B.C.) in a more modern form that
with only slight modification is still considered
valid today. In his honor, another name for an
atomic mass unit is the dalton. Also in his honor,
since he was the first person to write a descrip-
tion of color blindness, this affliction is some-
times referred to as daltonism. The original
records of his experiments were destroyed by
bombing in World War II.Figure 9.7 Sir Joseph John Thomson (1856–
1940). Thomson is usually credited as the discov-
erer of the electron, although many people con-
tributed to its identification as a basic building
block of matter. Seven of his research assistants,
who were also heavily involved in understanding
the structure of matter, would eventually win
Nobel Prizes.© CORBIS-Bettmann
© CORBIS-Bettmann