Download History of Atomic Theory

General Chemistry
History of Atomic Theory
Name:
Pe
Carefully read pages 101 to 107 and 127 to 129 (time-line) 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 he called them positive and negative. He concluded that like (same) charges repel each other while
opposite charges attract each other.
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
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
.
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JJ Thomson also developed the “plum pudding” model of an atom (see pp. 106 and 128). 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 was able to determine the quantity of charge carried by the electron. He was able to balance the oil
drop against the force of gravity to determine that charge.
Millikan further built on Thomson’s experiments by using the values Thomson obtained for the charge-to-mass
ratio. With these values he was able to successfully calculate the mass of a single electron by using his oil
droplet experiment.
The electron carries exactly
unit of
Millikan calculated the mass of an electron to be
charge.
.
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.
In 1909, Rutherford and his colleagues performed a famous experiment 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.
Draw a diagram showing the details of what
went on in the gold foil experiment.
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