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than one particular value (1.60 × 10 coulomb) and that larger Nucleus
charges on various droplets were always multiples of this unit of
charge. Since all of the droplets carried the single unit of charge
or multiples of the single unit, the unit of charge was under-
stood to be the charge of a single electron.
Knowing the charge of a single electron and knowing the Atoms
charge-to-mass ratio that Thomson had measured now made it
possible to calculate the mass of a single electron. The mass of
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an electron was thus determined to be about 9.11 × 10 kg, or Particles
from
about 1/1,840 of the mass of the lightest atom, hydrogen. source
Thomson had discovered the negatively charged electron,
and Millikan had measured the charge and mass of the electron.
But atoms themselves are electrically neutral. If an electron is Deflected
part of an atom, there must be something else that is positively particle
charged, canceling the negative charge of the electron. The next
step in the sequence of understanding atomic structure would FIGURE 8.6 Rutherford’s nuclear model of the atom
explained the scattering results as positive particles experiencing
be to find what is neutralizing the negative charge and to figure a repulsive force from the positive nucleus. Measurements of the
out how all the parts are put together. percentage of particles passing straight through and of the various
Thomson had proposed a model for what was known angles of scattering of those coming close to the nuclei gave
about the atom at the time. He suggested that an atom could Rutherford a means of estimating the size of the nucleus.
be a blob of massless, positively charged stuff in which elec-
trons were stuck like “raisins in plum pudding.” If the mass of a
hydrogen atom is due to the electrons embedded in a massless, container, so only a narrow beam of the massive, fast-moving
positively charged matrix, and since an electron was found to particles would penetrate a very thin sheet of gold. The parti-
have 1/1,840 of the mass of a hydrogen atom, then 1,840 elec- cles were detected by plates that produced small flashes of light
trons would be needed together with sufficient positive stuff when struck.
to make the atom electrically neutral. A different, better model Rutherford found that most of the particles went straight
of the atom was soon proposed by Ernest Rutherford, a British through the foil. However, he was astounded to find that
physicist. some were deflected at very large angles and some were even
reflected backward. He could account for this only by assum-
ing that the massive, positively charged particles were repelled
THE NUCLEUS
by a massive positive charge concentrated in a small region of
The nature of radioactivity and matter were the research inter- the atom (Figure 8.6). He concluded that an atom must have
ests of Rutherford. In 1907, Rutherford was studying the scat- a tiny, massive, and positively charged nucleus surrounded by
tering of radiation particles directed toward a thin sheet of electrons.
metal. As shown in Figure 8.5, the particles from a radioactive From measurements of the scattering, Rutherford esti-
source were allowed to move through a small opening in a lead mated electrons must be moving around the nucleus at a dis-
tance 100,000 times the radius of the nucleus. This means the
volume of an atom is mostly empty space. A few years later
Rutherford was able to identify the discrete unit of positive
Lead Radioactive
container source charge which we now call a proton. Rutherford also specu-
Gold Particles lated about the existence of a neutral particle in the nucleus,
foil a neutron. The neutron was eventually identified in 1932 by
James Chadwick.
Today, the number of protons in the nucleus of an atom
is called the atomic number. All of the atoms of a particular
element have the same number of protons in their nuclei, so all
atoms of an element have the same atomic number. Hydrogen
has an atomic number of 1, so any atom that has one proton
Detecting
screen Light flashes in its nucleus is an atom of the element hydrogen. In a neutral
atom, the number of protons equals the number of electrons, so
FIGURE 8.5 Rutherford and his coworkers studied alpha par- a neutral atom of hydrogen has one proton and one electron.
ticle scattering from a thin metal foil. The alpha particles struck the
Today, scientists have identified 117 different kinds of elements,
detecting screen, producing a flash of visible light. Measurements
of the angles between the flashes, the metal foil, and the source of each with a different number of protons.
the alpha particles showed that the particles were scattered in all The neutrons of the nucleus, along with the protons,
directions, including straight back toward the source. contribute to the mass of an atom. Although all the atoms

206 CHAPTER 8 Atoms and Periodic Properties 8-4