Atomic Theory (Unit 8)

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Atomic Theory (Unit 8) Introduction
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Atomic Theory Theories in science are proposed
to explain the evidence available at the time.
As new evidence is discovered, theories are
adapted to explain new data. This is the nature
of Science. In the future this will continue.
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Plato
Democritus
460-370 BC
First Greek philosopher to suggest that tiny
different pieces of particles exist atoms
Atoms make up the world His idea was refuted
by Aristotle
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Plato
Teacher of Aristotle
Aristotle
384-322 BC
Four Elements
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The Four Element Theory
Properties
Forces
Aristotle
wetness
conflict
harmony
Water
Air
coldness
hotness
Fire
Earth
dryness
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The Four Element Theory lasted for about 2
thousand years because no one tested the theory
with scientific experiments.
It was not a scientific theory- which is tested
by experiment. Science back then was more of a
philosophical moralistic field that philosophers
rather than experimentation decided.
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John Dalton
1766-1844
Re-visited the idea of Atoms Atomic Theory
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Daltons Atomic Theory- 1808 Evidence 1. The
Law of Conservation of Mass 2. The Law of
Constant Composition- water is 11 H and 89
O Theory 1. Each atom is an indestructible and
unique spherical particle 2. Atoms combine in
simple whole number ratios to form compounds
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Hydrogen
Oxygen
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Dalton Theory- Water
1 g H
1 g H
16 g O
18g O
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Dalton Theory- Water
18g O
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Dalton Theory- Water
2 g H
11 H
x 100
89 O
18 g H2O
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Having a molecule (atoms combine in simple whole
ratios) explains the law of constant
composition.
Still 11 H 89 O
No matter how much water you have its still 11 H
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2H2 O2 ? 2H2O
If the atoms are not destroyed then the mass does
not change
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J. J. Thomson 1871-1937 He discovered the
electron Plum Pudding model/blueberry muffin
model
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The Thomson Theory of the Atom The atom is made
up of positive material with negative particles
throughout- like blueberries in a blueberry
muffin. We now know this is not trueelectrons
are not found inside the nucleus
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Ernest Rutherford 1871-1937 He was J. J
Thomsons student
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The Rutherford Atom 1911 Evidence Gold Foil
Experiment 1. 99.99 of alphas are not
deflected 2. 0.01 of alphas are radically
deflected Theory 1. Most of the atom is empty
space. 2. There is a small dense nucleus in the
center of the atom that makes up most of the
mass. Electrons circle the nucleus randomly.
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The Rutherford Atom 1911 Scale nucleus is home
plate atom is the baseball field
Be
Nucleus- is small but has most of the mass

Nucleus- 4 protons and 5 neutrons
Electrons
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Niels Bohr

• Electrons are restricted to having certain
  specific energies and are restricted to following
  specific paths called orbits at a fixed
  distance from the nucleus.
• Electrons emit energy when they move from one
  orbit to the other

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Niels Bohr

• Electrons _________ energy/light when they move
  from a high energy level to a low energy level.
• Electrons __________ energy/light when they move
  up from a low energy level to a high energy level.

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Niels Bohr

• Limitations of his model is that .
• It only worked for the hydrogen atom ( only atoms
  with one electron)
• Electrons do not really move in circular
  motionthe correct description requires quantum
  mechanics to answer this question

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Discovery of the Nucleus
Size of the Atom
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Read pages 139-144 from your textbook. They cover
the theories we just talked about. It is your
responsibility to know what each scientist
did/contributed to the understanding of the
atom. You can do this by writing out a
summary/list of each scientists accomplishments.