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THE STORY OF
ATOMIC STRUCTURE
About 2500 years ago, certain Greek thinkers proposed that all
matter consisted of extremely tiny particles called atoms. The
sizes and shapes of different atoms, they reasoned, was what
determined the properties of a substance. This early atomic theory,
however, was not widely accepted. Many at the time found these
tiny, invisible particles difficult to accept.
What everyone could observe was that all substances were
liquid, solid, or gas, light or heavy, hot or cold. Everything, they
thought, must then be made of only a few basic substances or
elements. They reasoned these elements must be water, air, fire,
and earth. Different substances contained different amounts of
each of these four substances.
The timeline shows a few of the major events that led
scientists to accept the idea that matter is made of atoms and
agree on the basic structure of atoms. With the revised atomic
theory, scientists were able to explain how elements could be
basic but different.
1661
Boyle Challenges Concept
of the Four Elements
British chemist Robert Boyle
proposes that more than four
basic substances exist. Boyle
also concludes that all matter
is made of very tiny particles
he calls corpuscles.
EVENTS
1600
APPLICATIONS AND TECHNOLOGY
TECHNOLOGY
Collecting and Studying Gases
Throughout the 1600s, scientists tried to
study gases but had difficulty collecting
them. English biologist Stephen Hales
designed an apparatus to collect gases.
The “pneumatic trough” was a breakthrough in chemistry because it allowed
scientists to collect and study gases for
the first time. The pneumatic trough was
later used by such chemists as Joseph
Black, Henry Cavendish, and Joseph
Priestley to study the gases that make up
the air we breathe. The work of these
scientists showed that air was made of
more than a single gas.
386 Unit 3: Chemical Interactions
1620
1640
1660
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1808
John Dalton Says: “Bring Back
the Atom”
English chemist John Dalton revives the
ancient Greek idea that all matter is made
of atoms. Dalton claims that each element
has its own type of atom and that the
atoms combine in fixed and predictable
ratios with one another in different
substances.
1808
Humphrey Davy Shocks Chemistry
English chemist Humphrey Davy applies
an electric current to different materials.
He discovers that many materials once
thought to be elements break apart into
even simpler materials. Davy succeeds in
isolating the elements sodium, calcium,
strontium, and barium.
1800
1820
1840
1897
It’s Smaller Than the Atom!
English physicist Joseph John
Thomson discovers the electron—the
first subatomic particle to be identified. Thomson concludes that these
tiny particles have a negative charge.
Thomson will later propose that
atoms are made of a great many of
these negative particles floating in a
sea of positive charge. Thomson
suggests that each atom resembles a
dish of pudding with raisins in it.
The electrons are the raisins and the
pudding the positive charge in which
they float.
1860
1880
TECHNOLOGY
Chemistry and Electric Charge
In 1800 Italian physicist Alessandro Volta announced
that he had produced an electric current from a
pile, or battery, of alternating zinc and silver discs.
Volta’s invention was important for the study of
atoms and elements in two ways. First, the fact that
the contact of two different metals could produce
an electric current suggested that electric charge
must be part of matter. Second, the powerful
electric current produced by the batteries enabled
chemists to break apart many other substances,
showing that there were more elements than
previously thought.
Timelines in Science 387
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1911
1903
Atoms Release Energy
Polish-born French physicist Marie
Curie and her husband, Pierre, have
won the Nobel Prize for their isolation
of the elements polonium and radium.
These elements are unique because
they release energy. Marie Curie names
this trait “radioactivity.” They share
the award with Henri Becquerel, who
previously observed this trait with the
element uranium.
Atoms Have a Center
By aiming a stream of particles at a piece of gold foil, New Zealandborn physicist Ernest Rutherford finds that atoms are not like a dish
of pudding filled with raisins, as J. J. Thomson had suggested. Atoms
must have a positive center because many of the particles bounce
back. He calls the atom’s center its nucleus.
1913
Bohr Puts Electrons into Orbit
Building on the work of Rutherford,
Danish physicist Niels Bohr claims
that electrons move about the
nucleus only in certain, welldefined orbits. Bohr also says that
electrons can jump to different
orbits and emit or absorb energy
when doing so.
1919
Atoms Share a Common Bond
U.S. chemists G.N. Lewis and Irving Langmuir
suggest that atoms of many elements form bonds
by sharing pairs of electrons. The idea that atoms
could share electrons leads to a greater understanding of how molecules are structured.
1900
1905
1910
1915
1920
APPLICATION
The Chemistry of Communication
The discovery of the electron resulted in more than
a greater understanding of the atom. It also opened
new ways of communicating. In 1906, U.S. inventor
Lee De Forest invented a device for detecting and
amplifying radio signals that he called the audion.
The audion worked by producing a beam of
electrons inside a vacuum tube. The beam was
then made to respond to radio signals that
it received from an antenna. The audion
helped pave the way for later devices
such as the transistor.
388 Unit 3: Chemical Interactions
1940
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1960s
Smaller Particles Discovered
By smashing atoms into one another,
scientists discover that protons and
neutrons are themselves composed
of even smaller particles. In a bit of
scientific humor, these smaller particles are named “quarks,” a nonsense
word taken from a novel. Scientists
detect these particles by observing the
tracks they make in special detectors.
Humans have gone from hypothesizing atoms
exist to being able to see and move them.
People once considered only four substances
to be true elements; today we understand how
there are more than a hundred simple substances. Not only have scientists learned atoms
contain electric charges, they have also
learned how to use these charges.
1980s
Tunneling to the Atomic Level
Scanning tunneling microscopes (STMs) allow
scientists to interact with matter at the atomic
level. Electrons on the tiny tip of an STM “tunnel”
through the gap between the tip and target
surface. By recording changes in the tunneling
current, researchers get an accurate picture.
As scientists learn more and more about
the atom, it is difficult to say what they will
find next. Is there something smaller than a
quark? Is there one type of particle from which
all other particles are made? Will we one day
be able to move and connect atoms in any way
we want? Are there other kinds of atoms to
discover? Maybe one day we will find answers
to these questions.
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1960
1980
TECHNOLOGY
Particle Accelerators
Particle accelerators speed up charged particles by
passing them through an electric field. By smashing
subatomic particles into one another, scientists are
able to learn what these particles are made of as
well as the forces holding them together. The H1
particle detector in
Hamburg, Germany,
can accelerate protons
to 800 billion volts and
is used to study the
quarks that make up
protons.
2000
Explore a Model Atom
The discovery of the nucleus was one
of the most important discoveries in
human history. Rutherford’s experiment, however, was a simple one that
you can model. Take an aluminum pie
plate and place a table tennis ballsized piece of clay at its center. The
clay represents a nucleus. Place the end
of a grooved ruler at the edge of the
plate. Hold the other end up to form a
ramp. Roll a marble down the groove
toward the clay. Move the ruler to
different angles with each roll. Roll the
marble 20 times. How many rolls out
of 20 hit the clay ball? How do you
think the results would be different if
the atoms looked like pudding with
raisins in it, as Thomson suggested?
Writing About Science
Suppose you are an atom. Choose
one of the events on the timeline
and describe it from the atom’s point
of view.
Timelines in Science 389