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General Chemistry
History of Atomic Theory
Name:
Carefully read pages 107 to 112 in your textbook. Answer the following questions as you go. You are
responsible for learning this material on your own; it will not be covered during lectures.
Greeks(~450 BC)
What was Democritus’ main contribution to atomic theory?
Other important Greek philosophers disagreed with Democritus. What did they think?
John Dalton (1766-1844)
What are the four postulates of Dalton’s atomic theory of matter?
a.
b.
c.
d.
Benjamin Franklin (1706-1790)
Benjamin Franklin investigated electricity. He concluded there are two kinds of electric charges an object can
have and:
a. He called them
.
b. Like charges
each other.
c.
each other.
Opposite charges
Michael Faraday (1791-1867)
Early scientists wondered where these charges come from and had questions about their physical properties.
Michael Faraday thought that the structure of atoms was related to electricity. Faraday and other scientists
investigated this possibility using a cathode ray tube.
Cathode Ray Tube (mid-1800s)
Describe a cathode ray tube and draw a diagram of one:
Scientists found that a kind of radiation streams from the
to the
of a
cathode ray tube. One experiment showed that a cathode ray could spin a small paddle wheel, which suggested
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that it was actually a
. They also discovered that a magnet deflects
the cathode ray in the direction expected for
charged particles.
JJ Thomson (1856-1940)
In 1896, JJ Thompson began to systematically study cathode rays. He measured the ratio of electricity
charge to mass of the electrons flowing through a cathode ray tube by deflecting the beam of electrons with a
magnetic field. Thomson discovered that magnetic and electric fields deflected the ray’s path in a
mathematically predictable way. Thomson concluded that a cathode ray is composed of
. This meant that atoms were not indivisible balls but instead had a substructure. Thomson
named these negative particles
. He determined the ratio of an electron’s electrical
charge to its mass, which is
.
JJ Thomson also developed the “plum pudding” model of an atom (see p. 101). In this model, negative charges
(electrons) are distributed evenly throughout an atom’s positively charged interior, similar to the way chocolate
chips are distributed throughout cookie dough.
Robert Millikan (1868-1953)
Millikan built on Thomson’s experiments by successfully measuring the mass of a single electron using oil
droplets. He was able to balance the oil drop against the force of gravity to determine the charge of an
electron. Briefly describe Millikan’s experiment and draw a diagram of the apparatus he used.
Millikan calculated the mass of an electron to be
.
Henri Becquerel (1852-1908)
Henri Becquerel accidentally found that uranium produces an image when placed on photographic film. He
discovered that uranium exhibits
,
.
Marie Curie (1867-1943) and Pierre Curie (1859-1906)
What other two elements were found to be radioactive by Marie and Pierre Curie?
a.
b.
What important observation did they make?
Ernest Rutherford (1871-1937)
Studying the nature of radiation gave scientists further clues about the substructure of the atom. Ernest
Rutherford continued the study of radiation. He used charged plates to deflect particles emitted by
radioactive samples. Draw a diagram of Rutherford’s experimental apparatus.
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Name and describe the three types of radiation emitted by radioactive elements:



In 1909, Rutherford and his colleagues performed a famous experiment called the
. This experiment is also sometimes called the gold foil experiment. In this experiment,
alpha particles (with a +2 charge) from radioactive polonium were aimed at a thin piece of gold foil. Most of
the particles passed through the foil, but some were scattered in all directions. This led Rutherford to reject
Thomson’s plum pudding model. Instead, he proposed that
.
What name, originally proposed by Rutherford, is still used to describe the core of an atom?
Draw a diagram showing the apparatus used in the gold foil experiment.
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