Download 1.1 The Changing Atom - Beechen Cliff Science Faculty

1.1 The Changing Atom
• Describe how the model of the atom has
changed over the years, and how it continues
to do so.
• Understand that scientific knowledge is
always evolving.
• Describe how new theories are accepted by
scientists.
1.1 The Changing Atom
The Greek atom
•
The development of modern atomic theory started with the ancient
greek philosophers, who were interested in determining what made up
matter throughout the universe.
•
This lead to development of 4 elements of matter. Democritus and his
teacher, Leucippus, proposed that matter was made of hard, indivisible
particles of matter moving through empty space.
•
Democritus coined the term atom (atomos, indivisible). He reasoned
that if one was to split gold into smaller and smaller pieces, one would
obtain a small particle that was uncuttable. These atoms had shape,
mass, and motion, but no other qualities such as color or flavor.
•
The latter properties were supplied by the observer and are
subjective. He also proposed that atoms were held together by "hooks
and eyes".
•
Atomic theory was not accepted by either Aristotle or Plato
Democritus' idea of atoms would lie dormant for 2000 years, until it
was developed further by John Dalton.
1.1 The Changing Atom
• John Dalton
•
John Dalton (1766-1826) combined the Law of Definite Proportions and
the Law of Conservation of Mass and formulated an Atomic Theory that
linked Democritus' idea of atoms to Boyle's idea of elements.
•
Dalton's Atomic Theory
•
Elements are made of tiny particles called atoms. Atoms are indivisible
and indestructible.
•
Each element is characterized by the mass of its atoms. All atoms of
the same element have the same mass, but different elements have
different masses.
•
Chemical combination of elements to make different substances occurs
when atoms join together in small, whole number ratios.
•
Chemical reactions only rearrange the way that atoms are combined; the
atoms are not changed.
1.1 The Changing Atom
J. J. Thomson (1856-1940) found that if a certain
high voltage is applied to two electrodes, a beam of
cathode rays would flow from the negatively charged
electrode to the positive electrode. This lead to the
development of the cathode ray tube. When the
cathode ray beam was placed between charged
plates, it deflected toward the positively charged
plate, indicating that the beam was made of
negatively charged particles
Thomson called these particles corpusles
(electrons). By applying both an electrical field and a
magnetic field and balancing their effects to make the
beam pass straight through, Thomson calculated the
charge to mass ratio (e/m). This value is currently
1.758819 x 108 C/g.
Thomson also used 20 different metals as the
negative electrode and found that each produced
electrons with the same charge to mass ratio. He
concluded that these negatively charged species are
associated with all atoms of all elements.
1.1 The Changing Atom
Rutherford’s gold-leaf experiment
.
Week 1
Rutherford’s gold-leaf experiment
Tasks
Write brief notes on
• The Greek atom
• John Dalton
• J J Thompson
• Ernest Rutherford
• Neils Bohr and . .
© Pearson Education Ltd 2008
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