The Development of a New Atomic Model

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The Development of a New Atomic Model
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Objectives

• Explain the mathematical relationship between the
  speed, wavelength, and frequency of
  electromagnetic radiation.
• Discuss the dual-wave particle nature of light.
• Discuss the significance of the photoelectric
  effect and the line-emission spectrum of hydrogen
  to the development of the atomic model.
• Describe the Bohr model of the hydrogen atom.

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Links

• http//www.quantumintro.com/
• http//newton.ex.ac.uk/research/qsystems/people/je
  nkins/mbody/mbody2.html

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A New Model Was Formed

• Why wasnt the positive nucleus attracted to the
  negative electron cloud?
• Investigations into light led to understanding of
  relationship between light and electrons.
• Led to revolutionary theory about the nature of
  light, matter, and atoms

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Properties of Light

• Electromagnetic spectrumall forms of radiation
  exhibit wavelike behavior as they travel through
  space.
• ?-rays (gamma), X-rays, ultraviolet, visible
  light, infrared light, microwaves, radio waves
• Travel at 3.0 x 108 m/s

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Waves

• Wavelength? (lambda)distance between crests of
  adjacent waves (nanometers 1 nm1 x 10-9 m)
• Frequency? (nu)the number of waves that pass a
  given point in one second (Hzhertz)
• Speed of lightwavelength is inversely
  proportional to frequency
• c ? ?

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Photoelectric Effect

• Electrons are given off from a metal when light
  of a certain frequency is shined on it
• If light is a wave, light of any frequency should
  make electrons be given off
• ??why only certain frequencies??

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Light as Particles

• Max PlanckGerman physicist1900
• Proposed that objects give off energy in small,
  specific amounts
• Quantumminimum quantity of energy that can be
  lost or gained by an electron

http//nobelprize.org/physics/laureates/1918/planc
k-bio.html
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Albert Einstein

• Electromagnetic radiation is both a wave and a
  particledual wave-particle nature
• Light has wavelike properties
• Light can behave as a stream of particleseach
  particle with a quantum of energy
• Photonsparticle of electromagnetic radiation
  having zero rest mass and carrying a quantum of
  energy
• Amount of energy depends on frequency

http//www-groups.dcs.st-and.ac.uk/history/PictDi
splay/Einstein.html
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Line-Emission Spectrum

• Ground statelowest energy state of an atom
• Excited statestate where an atom has higher
  potential energy
• Neon signcurrent passes through a gas at low
  pressure and is excitedwhen it returns to ground
  state it gives off energy as electromagnetic
  radiation

http//www.neonsign.com/brands/index.html?itemrea
dy
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Line-Emission Spectrum

• When light from an excited state atom is passed
  through a prism, it is separated into a series of
  specific wavelengths (not all like a rainbow)
• When an atom falls back from excited state to
  ground stateit gives off a photon of radiation
• Led to Quantum Theory

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Line-Emission Spectrum

• Specific lines in spectrum mean that the energy
  differences between energy states was fixed
• Electrons exist only in specific energy states

Animation of H line spectrum
http//www.glencoe.com/sites/common_assets/science
/cmc/cim/animations/ch5_1.rm
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http//www.quantumintro.com/
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Bohr Model

• Neils BohrDanish physicist1913
• Linked model of atom with the emission spectrum
• Electrons can only travel in specific paths
  (orbits)
• Electrons in an orbit have a fixed amount of
  energy
• Orbits closest to nucleus have smallest energy

http//nobelprize.org/physics/laureates/1922/bohr-
bio.html
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Bohr Model

• Atoms gain energy electrons move to higher
  orbits (excited state)
• As electrons fall back to lower energy orbits
  they give of photons with a fixed amount of
  energy
• Amount of energy photon has is seen as lines in
  emission spectrum