Transcript Day 1: History of the Atom

Atoms and Atomic Theory
Day 1: History of the Atom
•Chemistry Mrs. Kam
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Chapter 3
Section 1 Substances Are Made
of Atoms
Your group will examine the 3 boxes.
• Can you figure out a way to determine what is in the
box without opening it?
• Indirect evidence:
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Chapter 3
Section 1 Substances Are Made
of Atoms
Objectives
• State the three laws that support the existence of
atoms.
• List the five principles of John Dalton’s atomic
theory.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory
• The idea of an atomic theory is more than 2000 years
old.
• Until recently, scientists had never seen evidence of
atoms.
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Chapter 3
Section 1 Substances Are Made
of Atoms
Atomic Theory, continued
• The figure on the right is a more accurate
representation of an atom than the figure on the left.
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Brief History of Atomic Theory
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The monster in the cave
• There once was a monster who lived in a dark cave.
• You are a knight and have been sent to fight this
monster.
• First, you need to know how big your opponent is.
• How can you find out the size of this monster (he will
not come out of the dark cave).
• Hint: Lord Ernest Rutherford in the early 1900s used
this equation:
• Hits divided throws= the size of the nucleus (in your
case the cave)
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To Fight the Monster:
• 1. Cover the eyes of the person rolling the marbles at
the monster.
• 2.Roll the marbles into the box (cave).
• 3. Move the monster each time it is hit by a marble.
• 4. Record the number of throws (only marbles that
made it into the cave) and the number of hits.
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How does this cave monster tie into the discovery
of the atom’s nucleus??
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles
Rutherford Discovered the Nucleus
• Thomson proposed that the electrons of an atom
were embedded in a positively charged ball of matter.
His model of an atom was named the plum-pudding
model.
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• Ernest Rutherford performed the gold foil experiment,
which disproved the plum-pudding model of the atom.
• A beam of small, positively charged particles, called alpha
particles, was directed at a thin gold foil.
• Rutherford’s team measured the angles at which the
particles were deflected from their former straight-line paths
as they came out of the foil.
• Rutherford found that most of the alpha particles shot
at the foil passed straight through the foil. But very
few were deflected, in some cases backward.
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Chapter 3
Gold Foil Experiment
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• Rutherford reasoned that only a very concentrated
positive charge in a tiny space within the gold atom
could possibly repel the fast-moving, alpha particles
enough to reverse the alpha particles’ direction.
• Rutherford also hypothesized that the mass of this
positive-charge containing region, called the nucleus,
must be larger than the mass of the alpha particle.
• Rutherford argued that the reason most of the alpha
particles were undeflected, was that most parts of the
atoms in the gold foil were empty space.
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Chapter 3
Gold Foil Experiment on the Atomic Level
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Chapter 3
Section 2 Structure of Atoms
Subatomic Particles, continued
Rutherford Discovers the Nucleus, continued
• The nucleus is the dense, central portion of the atom.
• The nucleus is made up of protons and neutrons.
• The nucleus has all of the positive charge, nearly all
of the mass, but only a very small fraction of the
volume of the atom.
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Chapter 3
Section 3 Electron Configuration
Atomic Models
Rutherford’s Model Proposed Electron Orbits
• The experiments of Rutherford’s team led to the
replacement of the plum pudding model of the atom
with a nuclear model of the atom.
• Rutherford suggested that electrons, like planets orbiting the
sun, revolve around the nucleus in circular or elliptical orbits.
• Rutherford’s model could not explain why electrons did not
crash into the nucleus.
• The Rutherford model of the atom was replaced only
two years later by a model developed by Niels Bohr.
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