The Bohr Model of the Atom

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Chapter 4

• The Bohr Model of the Atom
• Part 1

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Visible Light
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Niels Bohr

• In 1913 Bohr published a theory about the
  structure of the atom based on an earlier theory
  of Rutherford's.
• Bohr expanded upon this theory by proposing that
  electrons travel only in certain successively
  larger orbits.

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Bohr Model of the Atom

• Electrons orbit the nucleus in orbits that
  represent specific quantities of energy.
• The energies of the electrons in the atom are
  quantized.
• Only certain electron orbits (energy levels) are
  allowed.

The Bohr Atom
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Niels Bohr

• Bohr also described the way atoms emit radiation
  by suggesting that when an electron jumps from an
  outer orbit to an inner one, that it emits light.

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Ground State

• The lowest energy state of an atom.

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Excited State

• Any energy state of an atom that is higher in
  energy than the ground state.

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Energy Absorbed
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Absorption (Dark Line) Spectra
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Energy Emitted
Electron jumps to a lower orbit
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Emission (Bright Line) Spectra
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Emission Spectra
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Spectral Lines Cassiopeia Project
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The lines present in an emission spectrum are the
lines missing in an absorption spectrum.
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Demonstration

• Gas Tubes (He, Hg, Ne) and Diffraction Grating

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Describe the process that occurs inside atoms
that produces the spectra?

• Energy is absorbed by the atom.
• The electrons jump into higher orbits and the
  atom becomes excited.
• The electrons return to their original orbits and
  the atom returns to the ground state.
• The electrons release energy in the form of
  visible light.
• The specific energies (wavelengths, frequencies)
  of visible light absorbed and released produce
  the spectra.

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Northern lights (Auroras) are one of nature's
most beautiful manifestations of the ability of
electrons to jump between quantum states
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What Causes the Aurora

• High speed energetic particles collide with atoms
  in Earth's atmosphere to cause the aurora. These
  high speed particles, which are usually
  electrons, originate from space, specifically
  from the solar wind, blowing outward from the
  Sun.
• When the electrons from space strike an atom or
  molecule in Earth's atmosphere, they give one of
  the electrons in the atom an energy boost. In
  scientific jargon, the electron jumps to a higher
  energy level and the atom is in an excited state.

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What Causes the Aurora

• After a while, the electron in the excited atom
  jumps back down to its original lower energy
  level. It releases this energy as light causing
  the auroral glow. This process is the same
  mechanism that causes emission line spectra and
  aurora are in fact emission line spectra of the
  atoms in Earth's upper atmosphere.
• The color of emission line spectra depends on the
  type of atom that is excited, and each type of
  atom produces its own unique pattern of colors.
  Hence, the different colors in auroral displays
  originate from different elements in Earth's
  atmosphere.

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What Causes the Aurora

• Oxygen molecules cause the green Aurora, and
  oxygen atoms cause the red colors. Blue auroral
  displays result from nitrogen molecules.
  Molecular nitrogen and oxygen are the most common
  constituents of Earth's atmosphere, so these are
  the most common auroral colors. Mixtures of these
  colors form the other colors of the auroras.

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Northern Lights Video
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Star Finder Video - Fingerprints of Light
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Homework

• Chapter 4 Worksheet 3