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Defining the Atom
The lab technician shown here is using a magnifying lens to examine a bacterial culture in a petri dish. When scientists cannot see the
details of what they study, they try to obtain
experimental data that help fill in the picture.
Slide 3 of 18 Defining the Atom > Early Models of the Atom
Early Models of the Atom
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An atom is the smallest particle of an element that retains its identity in a chemical reaction.•
Philosophers and scientists have proposed many ideas on the structure of atoms.Defining the Atom >
Slide 4 of 18 Early Models of the Atom
Democritus’s Atomic Philosophy
How did Democritus describe atoms?
Democritus
Slide 5 of 18 Defining the Atom > Early Models of the Atom
Democritus believed that atoms were indivisible and indestructible.
Democritus’s ideas were limited because they didn’t explain chemical behavior and they lacked experimental support.
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Dalton’s Atomic Theory
How did John Dalton further Democritus’s ideas on
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By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory.
The result was Dalton’s atomic
theory.
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All elements are composed of tiny indivisible particles called atoms.
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Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
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Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form
compounds.
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Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of
one element are never changed into atoms of another element in a chemical reaction.
Defining the Atom >
Slide 12 of 18 Sizing up the Atom
Sizing up the Atom
What instruments are used to observe individual atoms?
Slide 13 of 18 Defining the Atom > Sizing up the Atom
Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes.
Slide 14 of 18 Defining the Atom > Sizing up the Atom
Iron Atoms Seen Through a Scanning Tunneling Microscope
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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
4.1 Section Quiz
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4.1 Section Quiz
1. The ancient Greek philosopher credited with suggesting all matter is made of indivisible atoms is
a. Plato.
b. Aristotle.
c. Democritus.
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4.1 Section Quiz
2. Dalton's atomic theory improved earlier atomic theory by
a. teaching that all matter is composed of tiny particles called atoms.
b. theorizing that all atoms of the same element are identical.
c. using experimental methods to establish a scientific theory.
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4.1 Section Quiz
3. Individual atoms are observable with
a. the naked eye.
b. a magnifying glass.
c. a light microscope.
