The Nature of Matter
Toward a Modern Atomic Theory
Rutherford’s model of the atom raised some difficult questions. If the nucleus of
an atom contained several positive protons that repelled each other, how did it
stay together? Why didn’t the negatively charged electrons rush toward and crash
into the positively charged nucleus? In response to the first question, Rutherford
suggested the idea of a nuclear force—an attractive force within the nucleus—
that was much stronger than the electrostatic force of repulsion. The answer to
the second question—why the electrons did not fall in to be “captured” by the
nucleus—required a bold new theory created by a young Danish physicist named
Niels Bohr (
What are the relationships among the number of protons,
number of neutrons, and number of electrons in an atom?
Which of the numbers in (a) is/are related to the atomic
Which of the numbers in (a) is/are related to the mass number?
If the atomic mass of an element is not the same as the mass
number of one atom of the element, what conclusions can
you draw about that element?
2. Construct a graphic organizer to indicate the relationships among
atomic number, mass number, atomic mass, isotopes, and
3. Two atoms respectively have
= 26 and
Can these two atoms be classified as isotopes of the same
element? Give reasons for your answer.
Suggest an alternative classification for these atoms. Justify
4. Isotopes are classified as radioisotopes because they demon-
strate the property of radioactivity. How does this property differ
from other properties that we have used to classify elements?
Applying Inquiry Skills
5. When a sample of uranium-238 decays, alpha particles are
emitted and the uranium nuclei are converted to thorium nuclei.
Thorium-234 has a half-life of 24.10 d.
Plot a graph to predict the radioactive decay of 700.0 g of
Use your graph to determine how much time must pass
before only 24.0 g of the sample will remain as thorium-234.
Give at least three examples of ways in which radioisotopes
Suggest at least two safety precautions that may be used
when handling radioisotopes.
Sections 1.2–1.3 Questions
Niels Bohr (1885–1962) developed a new
theory of atomic structure and communicated
this theory with an innovative model of the
atom. For this work he won the Nobel Prize
for Physics in 1922.