The Development of Atomic Theory

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The Development of Atomic Theory
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I. Early Models of Atomic Structure

• The work of Dalton, Thomson, and Rutherford

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Democritus, "The Laughing Philosopher"

• Democritus was a pre-Socratic philosopher who
  said that all matter is made up of various
  indivisible elements which he called atoma, from
  which we get the English word atom.

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John Daltons Model of the Atom

• John Dalton developed his atomic theory in 1803
• It worked well, until subatomic particles were
  discovered.

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Daltons atomic theory

1. All matter is made of atoms, which are
   indivisible and indestructible particles.
2. All atoms of an element are all identical in mass
   and properties.
3. Atoms of different elements have different masses
   and properties.
4. Compounds are formed by atoms combining in small
   whole number ratios.

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J.J. Thomsons Model of the Atom

• J.J. Thomson discovered the electron while
  studying cathode ray tubes in 1897.
• He received the Nobel Prize in 1906.

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Thomsons cathode ray tube (a vacuum tube with 2
electrodes)
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His discovery of the electron

• Thomson found that the cathode ray was a beam of
  negative particles (electrons) and so atoms were
  NOT indivisible.

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Thomsons Plum Pudding Model of the Atom

• He described his atomic model as negative charged
  electrons scattered in a lump of positively
  charged material, like raisins scattered in plum
  pudding (a popular dessert at the time).

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Rutherfords Model of the Atom

• In 1894, Ernest Rutherford was awarded a
  scholarship to be a research student at the
  Cavendish Laboratory under J.J. Thomson. He
  received the Nobel prize in 1908.
• Rutherfords Gold Foil experiment led to the
  development of his atomic model in 1911.

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The Gold Foil Experiment

• Rutherford fired a beam of positively charged
  particles (called alpha particles) at a sheet of
  gold foil a few particles thick.

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The Gold Foil Experiment

• Rutherford was expecting results in line with
  Thomson's model, with the stream of positive
  particles passing through the foil.

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The Gold Foil Experiment

• Instead, he observed that some of the alpha
  particles were repelled, while most went through
  the foil unchanged.

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The Gold Foil Experiment
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There was only one explanation

• A dense, very positive
• charge was condensed
• into one place, called
• the nucleus.

• The rest of the atom had to be made up of mostly
  empty space. He described the electrons as
  buzzing around the nucleus like bees buzz around
  a hive.

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Rutherfords Model of the Atom

• One particularly memorable quote attributed to
  Rutherford is "All science is either physics or
  stamp collecting.

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II. Understanding the basics

• Lets look at some basic ideas of physics to
  better understand the more recent models of the
  atom. We need to understand wave properties, and
  light energy.

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Wave Properties

• Wavelength, ?, is the distance between two like
  points on a wave the unit of wavelength is the
  meter (m)

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Wave Properties

• Frequency, ?, describes the number of wave cycles
  per second.
• The unit of frequency is cycles/second (s-1), or
  the Hertz (Hz)

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Wave Properties

• Amplitude is the maximum height of a wave,
  measured from the origin line of the wave.
• A wave has zero amplitude at certain intervals
  along the wave, called nodes.

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The Electromagnetic Spectrum

• Short wavelength.long wavelength
• High frequency...low frequency
• High energy...low energy

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The Electromagnetic Spectrum

• All electromagnetic radiation (including visible
  light) travels at the same speed.
• The speed of light (c)
• 3.0 x 108 m/s

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Questions

• Which has the highest frequency, red light or
  green light?
• Which has the longest wavelength, x-rays or
  microwaves?
• Which has the highest energy, yellow light or
  infrared rays?

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The wavelength and frequency of light are
inversely related.

• C lu
• Speed of light wavelength x frequency
• (Since c 3.0 x 108 m/s, you will be asked to
  solve for wavelength or frequency.)
• l c/u u c/l

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Question

• Orange light has a wavelength of 620 nm. What is
  the wavelength in meters? What is the frequency?
• 620 nm (10-9 m/1 nm) 6.2 x 10-7 m
• u c/ l
• u 3.0 x 10 m/s / 6.2 x 10-7 m
• 4.8 x 1014 s-1