Modern Theories of the Atom

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Modern Theories of the Atom

• Discovery of the Electron through the Neutron

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Cathode-Ray Tubes

• Sealed glass tube with a small amount of gas
  inside (low pressure) and metal electrodes at
  either end.
• Developed in the 1870s looks like neon lights.

Crooke's Tubes
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Victorian Party Novelty
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Movie of cathode ray tube
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An electric field or a magnetic field will
deflect a beam of charged particles.
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J.J. Thomson discovered the electron!
1897
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Thomsons Cathode Ray Tube
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Thomson proposed that cathode rays were streams
of particles much smaller than atoms. Found the
charge-to-mass ratio of the electron.
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Atom is Divisible!

• Thomsons discovery meant that the atom was
  divisible!
• He knew there had to be an equal amount of
  positive charge because matter is neutral.

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Thomsons Plum-Pudding Model
The positive charge is evenly smeared out- like a
pudding. The negative charge is in bits like
raisins.
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Robert Millikan - 1909

• Oil-Drop Experiment
• determined the charge of the electron
• 1.60 X 10-19 coulomb
• Thomson had determined the
• charge-to-mass ratio
• 1.76 X 108 coulomb per gram
• So Millikan calculated the mass of the electron
  to be 9.09 X 10-28 grams.

Animation of Oil-Drop Experiment
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Proton Discovered by 1920

• Thomson Goldstein 1907
• Discovered a heavy particle with a positive
  charge in some cathode ray tube experiments.
• Rutherford 1918
• Shot alpha particles at nitrogen gas and got
  hydrogen. Figured out that the hydrogen had to
  come from the nitrogen. Suggested that the
  hydrogen nucleus was an elementary particle.
  Named it the proton.

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Rutherford

• Famous for many experiments.
• Discovered the proton.
• Figured out ? and ? radiation.
• Changed our idea of the atom!
• One of the most elegant experiments in the
  history of science!

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Rutherfords Experiment - 1911
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What did Rutherford conclude from the particles
that went straight through? What did Rutherford
conclude from the particles that bounced back?

• Animation

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Rutherford's Results

• The atom is mostly empty space!
• The size of the nucleus is small compared to the
  size of the entire atom.
• The mass of the atom is concentrated in the
  nucleus of the atom.
• The nucleus has a positive charge.

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So how big is the nucleus compared to the entire
atom?

• Analogy
• If the atom was as big as a football stadium, the
  nucleus would be smaller than a flea on the
  50-yard line!

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Rutherford proposed the nuclear atom.
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Rutherford did not speculate on how the electrons
were arranged around the nucleus.
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Neils Bohr - 1913
Planetary Model
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• Electrons travel only in specific orbits.
• Each orbit has a definite energy (lowest inner).
• Atoms emit radiation when an electron jumps from
  an outer ? inner orbit.
• Outer orbits hold more electrons than inner
  orbits.
• Outer orbits determine atoms chemical
  properties.

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• Schrodinger 1926
• Mathematically - treated electrons as waves
  rather than particles!
• Quantum Mechanical Model
• Modern Model
• Cloud Model

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Cloud Model
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Modern (Cloud) Model

• Electrons energy has only certain values it
  is quantized. (Bohr model had quantization too!)
• Electrons are located in probability regions
  or atomic orbitals. These are not circular
  orbits!
• Electrons move around the nucleus at near the
  speed of light.

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Schrodingers Model

• We talk about the probability of locating an
  electron at a certain place.
• Orbital a region in an atom in which an
  electron of a particular amount of energy is
  most likely to be located.
• standing wave patterns with definite energy.

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James Chadwick - 1932

• Discovered the neutron in cloud chamber
  experiments.
• About the same mass as a proton.
• Electrically neutral.
• The neutron changes atomic mass but not the
  element.

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The development of atomic theory represents the
work of many scientists over many years.
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