Download The History of Atomic Theory Lexile 1260L 8.5A: Atoms

8.5A: Atoms
Matter and Energy
The History of Atomic Theory
Lexile 1260L
1
Since the ancient Greek civilization,
people have wondered about the nature
of the atom. The atom is the universe’s
most basic building block. The word
“atom” comes from the Greek
language. “A…” is a prefix meaning
“not,” and the word “tomos means
cut. Therefore, atomos or atoms
means “cannot be cut or divided.”
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Many people have discovered important aspects of the atom. In the 4th
century BC, a Greek named Democritus thought that all matter was
made of tiny particles. He thought that empty space filled the area
between the solid atoms. That concept of the atom continued until 1803
when an English scientist, John Dalton, proposed an “atomic theory.”
Atomic theory says that solid atoms have an atomic mass and can react
because of forces. In 1873, James Clerk Maxwell proposed that electric
and magnetic fields filled the empty space in atoms.
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In 1897, J. J. Thomson discovered subatomic particles called electrons.
He did experiments and found that these electrons orbited the nucleus of
each atom. He then calculated they had a much smaller mass than that
of other particles, protons and neutrons.
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Matter and Energy
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Building upon Thomson’s work, Ernest Rutherford was the first to
show the nuclear model of the atom. This model shows that the center
of an atom contains positively charged protons (p+) surrounded by the
negatively charged electrons (e-).
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In 1932, Sir James Chadwick found the neutrally charged neutron (n0).
The atomic model now seemed complete after all these discoveries.
However, scientists continue to study with much more accurate
instruments now. They have found many more mysterious things about
atoms.
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Many people had a hand in developing atomic theory as we know it
today. Scientists are still studying the tiniest building block of our
universe as the nuclear model of the atom continues to change.
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Matter and Energy
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2
The word “atom” is derived from two Greek words meaning ____ and
_____.
A
“not”, “large”
B
“not”, “cut”
C
“tiny”, “world”
D
“unexplained”, “component”
The Greek _____ theorized that all matter is composed of tiny, indivisible
particles called atoms.
A
Democritus
B
Aristotle
C
Epicurius
D
Demosthenes
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Matter and Energy
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4
Who was the first to theorize that electrons orbited the nucleus?
A
Ernest Rutherford
B
James Maxwell
C
J. J. Thompson
D
Albert Einstein
Which scientist discovered the neutron and gave us the current basic
model of the atom?
A
James Maxwell
B
John Dalton
C
James Chadwick
D
Sir Isaac Newton
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Matter and Energy
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Around the ________ is when the atom was first theorized.
A
5th century AD
B
1st century BC
C
4th century BC
D
12th century AD
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