Discovering the Electron

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Discovering the Electron

• Chapter 4, Section 2

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Crookes Tube
Crookes tubes were developed in the 1870s -
kind of like early neon lights. Sealed glass
tube with a small amount of gas inside and metal
electrodes (, -) at either end. Pass
electricity through the tube.
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Victorian Party Novelty
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Crookes cathode ray

• Working with a tube that had a coating at the
  end. The coating produced a flash of light when
  it was hit by radiation.
• There were rays (radiation) traveling inside the
  tube from the cathode (-) to the anode ().
• Called a cathode ray.

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Movie of cathode ray tube
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Cathode Rays by late 1800s

• Actually a stream of charged particles.
• Particles carry a negative charge.
• Didnt matter what gas (low P) was inside the
  tube or what metal the electrodes were made of.
  So the negative particles were in all forms of
  matter.

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Divisible!!!

• Negative particle in all forms of matter.
• Called electrons.
• The atom is DIVISIBLE!

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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 in 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. He found
the charge-to-mass ratio of the electron. (He
called it a corpuscle.)
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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. The
negative charge is in bits like chips.
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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
  as 1.76 X 108 coulomb per gram.
• So the mass of the electron is 9.09 X 10-28 grams.

Animation of Oil-Drop Experiment
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Proton Discovered by 1920
animation

• 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 proton.

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Rutherford

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

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Rutherfords Experiment - 1911
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The steel marble will move in a straight line
until it hits something. If it hits something
heavy, like the rim of the table, it will rebound
back.
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Rutherfords expt animation
Compared to an electron, an alpha particle is
massive fast. If Thomsons model was correct,
the alpha particle wouldnt be much affected. No
big deflections. electrons are tiny positive
charge uniformly spread
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Results of Rutherfords Expt

• Most of the alpha particles went straight through
  they didnt bump into anything so most of the
  atom was empty space.
• Some of the alpha particles were deflected back
  they must have hit something really heavy that
  Rutherford called the nucleus.
• Results do NOT match Thomsons model.

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

• If the atom was as big as a football stadium, the
  nucleus would be smaller than a flea on the
  50-yard line!
• If the atom was as big as a period at the end of
  a sentence in a standard textbook, it would have
  the mass of 70 cars!

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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. The orbit
  closest to nucleus has the lowest energy.
• Atoms emit radiation when an electron jumps from
  an outer orbit to an 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 or Modern model.
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Modern 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.
• Also called Quantum Mechanical Model, Wave
  Mechanical Model, or Modern Model
• Orbitals standing wave patterns with definite
  energy.

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The development of atomic theory represents the
work of many scientists over many years.
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James Chadwick - 1932

• Discovered the neutron in cloud chamber
  experiments.
• About the same mass as a proton.
• Electrically neutral.