LIGHT and the ATOMIC SPECTRA of ELEMENTS

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LIGHT and the ATOMIC SPECTRA of ELEMENTS
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To Recap

• For most purposes light is said to exhibit
  wave-like properties.
• Light consists of electromagnetic waves

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Electromagnetic Radiation includes

• Radio waves, microwaves, infrared, visible light,
  ultraviolet, x-rays, and gamma rays.

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• The product of frequency and wavelength is equal
  to the speed of light.
• C ??
• C speed of light
• ? wavelength
• ? frequency

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• The frequency and wavelength of light waves are
  inversely related. As the wavelength decreases,
  the frequency increases
• ? c
• ?

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• Every element emits light when it is excited by
  the passage of an electric discharge through its
  gas or vapor.
• The atoms first absorb energy, then lose energy
  as they emit light.
• Follow this link to observe.
• http//www.colorado.edu/physics/2000/quantumzone/l
  ines2.html

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• Passing the light through a prism gives the
  atomic emission spectrum of the element.
• White light gives off a continuous spectra.

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• The spectra of most elements shows very few lines
  and are called line spectra or discontinuous
  spectra.
• This is the spectra for mercury.

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• This is the spectra for neon

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• The emission spectra of each element is unique
  and can be used to identify that element.
• Bohrs idea of energy levels resulted in an
  explanation of the hydrogen spectrum.

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• Hydrogen has one electron in its lowest energy
  level-called the ground state. The quantum number
  (n) is 1.
• Exciting the electron raises it to higher energy
  levels such that n 2, 3, 4 and so on.

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• It takes a specific amount of energy (a quantum)
  to raise an electron to a higher energy level.
• The same amount of energy is emitted as a photon
  when the electron drops back to the ground state.
  
• Revisit this tutorial
• http//www.colorado.edu/physics/2000/quantumzone/l
  ines2.html

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• Only electrons in transition from higher to lower
  energy levels lose energy and emit light.
• There are 3 lines in the emission spectrum for
  hydrogen.

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• The 3 groups of lines correspond to the
  transition of electrons from higher energy levels
  to lower energy levels.
• The Lyman series (ultraviolet range) corresponds
  to transitions from high energy levels to n 1
  (ground state).

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• The Balmer series (visible range) correspond to
  transitions from higher energy levels to n 2
  (2nd energy level).

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The Paschen series (infrared range) corresponds
to the transition from higher energy levels to n
3 (or 3rd energy level).
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Consider these questions

• 1- Suppose an electron is excited enough to jump
  to energy level 2. When it returns to the ground
  state, what type of radiation will it emit?

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• 2- If you observed a hydrogen gas discharge tube
  through a diffraction grating, would you be able
  to see the line corresponding to this emission?
  (from previous question)

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• 3- Which series of lines would you be able to
  detect?
• 4- What do you notice about the spacing of the
  energy levels from n 1 to n 7?

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Quantum Mechanics

• Light as waves, light as particles, energy
  absorbed and emitted in packages?
• All that and more
• De Broglie derived an equation that described the
  wavelength of a moving particle. ? h
• m v

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• His equation predicts that all particles of
  matter exhibit wavelike properties.
• Quantum mechanics changes the way observations
  are made. You cant observe something without
  changing it in the process.

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• EX- Suppose you want to find out if there is a
  car in a long tunnel. In quantum mechanics the
  only experiment you could do would be to send
  another car into the tunnel and wait for the
  crash. Although it is possible to detect the
  presence of a car, it is obvious the car will be
  changed by the crash.

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Heisenberg Uncertainty Principle

• This states that it is impossible to know exactly
  both the velocity and the position of a particle
  at the same time.
• If a photon is emitted which tells us the
  position, the velocity will change due to the
  emission of the photon.

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• FOR YOU
• 1- Explain the origin of the atomic emission
  spectrum of an element.
• 2- Compare the ground state and the excited state
  of an electron.

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• 3- Arrange the following in order of decreasing
  wavelength
• Infrared radiation from heat lamp
• Dental x-rays
• A signal from a shortwave radio station