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OVERVIEW

The development of the modern atomic model illustrates how modern scientific understanding comes from many
different fields of study. For example, you will learn how studies of electricity led to the discovery that atoms have
subatomic parts called electrons. The discovery of radioactivity led to the discovery of more parts, a central nucleus
that contains protons and neutrons. Information from the absorption and emission of light was used to construct a
model of how these parts are put together, a model resembling a miniature solar system with electrons circling the
nucleus. The solar system model had initial, but limited, success and was inconsistent with other understandings
about matter and energy. Modifications of this model were attempted, but none solved the problems. Then the
discovery of the wave properties of matter led to an entirely new model of the atom (Figure 8.1).
The atomic model will be put to use in later chapters to explain the countless varieties of matter and the changes
that matter undergoes. In addition, you will learn how these changes can be manipulated to make new materials,
from drugs to ceramics. In short, you will learn how understanding the atom and all the changes it undergoes not only
touches your life directly but also shapes and affects all parts of civilization.

chemical reactions. Five state ments will summarize his theory.
8.1 ATOMIC STRUCTURE DISCOVERED
As you will soon see, today we know that statement 2 is not
Did you ever wonder how scientists could know about something strictly correct:
so tiny that you cannot see it, even with the most powerful optical
1. Indivisible minute particles called atoms make up all
microscope? The atom is a tiny unit of matter, so small that 1 gram
matter.
of hydrogen contains about 600,000,000,000,000,000,000,000 2. All the atoms of an element are exactly alike in shape and
23
(six hundred thousand billion billion, or 6 × 10 ) atoms. Even
mass.
more unbelievable is that atoms are not individual units but

3. The atoms of different elements differ from one another in
are made up of even smaller particles. How is it possible that
their masses.
scientists are able to tell you about the parts of something so
4. Atoms chemically combine in defi nite whole-number
small that it cannot be seen? The answer is that these things
ratios to form chemical compounds.
cannot be observed directly, but their existence can be inferred
5. Atoms are neither created nor destroyed in chemical
from experimental evidence. The following story describes the
reactions.
evidence and how scientists learned about the parts—electrons,
the nucleus, protons, and neutrons—and how all the parts are During the 1800s, Dalton’s concept of hard, indivisible
arranged in the atom. atoms was familiar to most scientists. Yet the existence of atoms
The atomic concept is very old, dating back to ancient was not generally accepted by all scientists. There was skepti-
Greek philosophers some 2,500 years ago. The ancient Greeks cism about something that could not be observed directly.
also reasoned about the way that pure substances are put Strangely, full acceptance of the atom came in the early 1900s
together. A glass of water, for example, appears to be the same with the discovery that the atom was not indivisible after all.
throughout. Is it the same? Two plausible, but conflicting, ideas The atom has parts that give it an internal structure. The first
were possible as an intellectual exercise. The water could have part to be discovered was the electron, a part that was discovered
a continuous structure, that is, it could be completely homo- through studies of electricity.
geneous throughout. The other idea was that the water only
appears to be continuous but is actually discontinuous. This
means that if you continued to divide the water into smaller
and smaller volumes, you would eventually reach a limit to DISCOVERY OF THE ELECTRON
this dividing, a particle that could not be further subdivided. Scientists of the late 1800s were interested in understanding the
The Greek philosopher Democritus (460–362 b.c.) developed nature of the recently discovered electric current. To observe a
this model in the fourth century b.c., and he called the indivis- current directly, they tried to produce a current by itself, away
ible particle an atom, from a Greek word meaning “uncutta- from wires, by removing much of the air from a tube and then
ble.” However, neither Plato nor Aristotle accepted the atomic running a current through the rarefied air. When metal plates
theory of matter, and it was not until about 2,000 years later inside a tube were connected to the negative and positive ter-
that the atomic concept of matter was reintroduced. In the minals of a high-voltage source (Figure 8.2), a greenish beam
early 1800s, the English chemist John Dalton brought back was observed that seemed to move from the cathode (nega-
the ancient Greek idea of hard, indivisible atoms to explain tive terminal) through the empty tube and collect at the anode

204 CHAPTER 8 Atoms and Periodic Properties 8-2