Unit 7.1

• Discrete energy


  Energy is discrete in the microscopic world
• E = hf


  Formula for energy of a photon
• c = hc/λ


  Formula for wavelength of a photon
• The geopolitics of the past 60+ years have been greatly influenced by the existence of nuclear weapons
• Plum pudding model of the atom


  Atoms have a spherical positive charge with electrons embedded in it
• Gold leaf is like tin foil, but it can be made much thinner so that the alpha particles only travel through a thin layer of atoms
• Alpha particle


  A doubly-positive charged particle emitted by radioactive materials
• Rutherford proposed that alpha particles would travel more or less straight through the atom without deflection if Thomson's "Plum pudding" model was correct
• Instead of observing minimal scattering as in the Plum Pudding model, Rutherford observed the scattering as shown on the next slide
• Scintillation screen


  A screen used to detect the scattering of alpha particles
• Rutherford proposed that the positive charge of the atom was located in the center, and he coined the term nucleus
• Rutherford model


  The atom has a dense, positively charged nucleus at the center
• This experiment is called the Geiger-Marsden scattering experiment
• Only by assuming a concentration of positive charge at the center of the atom, as opposed to "spread out" as in the Plum Pudding model, could Rutherford's team explain the results of the experiment
• When a gas in a tube is subjected to a voltage, the gas ionizes, and emits light
• Spectroscope


  An instrument that separates light into its constituent wavelengths
• Emission spectrum


  The spectrum produced by the light emitted from a hot gas, which is an elemental fingerprint
• Absorption spectrum


  The spectrum produced by the absorption of light by a cool gas, which is also an elemental fingerprint
• In the late 1800s a Swedish physicist by the name of J.J. Balmer observed the spectrum of hydrogen – the simplest of all the elements
• Energy levels


  The discrete energy levels that an electron in an atom can occupy
• Principal quantum number (n)


  The number that labels the energy levels of an atom, going from 1 to infinity
• Ground state


  The lowest energy state of an atom
• Excited state


  A higher energy state of an atom after absorbing a photon
• De-excitation


  The process of an electron dropping back down to a lower energy level, emitting a photon
• The human eye is only sensitive to the Balmer series of photon energies (or wavelengths)
• The previous energy level diagram was NOT to scale. This one is. Note that none of the energy drops of the other series overlap those of the Balmer series, and thus we cannot see any of them. But we can still sense them!
• Transition energy


  Measured in eV due to the tiny amounts involved
• Discrete energy levels


  Discontinuous or separated energy levels in an atom
• The previous energy level diagram was NOT to scale. This one is.
• None of the energy drops of the other series overlap those of the Balmer series, and thus we cannot see any of them. But we can still sense them!
• Transition energy


  Measured in eV because of the tiny amounts involved
• Energy levels of hydrogen atom

  • n = 1 -13.6 eV
  • n = 2 -3.40 eV
  • n = 3 -1.51 eV
  • n = 4 -0.850 eV
  • n = 5 -0.544 eV
  • n = 0 0.00 eV
• Wave-particle duality


  Light acts like both a wave and a particle (photon)
• Photon


  A particle of light
• Energy E of a photon

  E = hf, where h is Planck's constant and f is the frequency
• Electron transition in hydrogen atom

  Electron jumps from energy level n = 3 to energy level n = 2
• Balmer series


  Series of spectral lines produced by electron transitions in the hydrogen atom
• Electron jumps from energy level n = 3 to energy level n = 2


  Produces a visible photon
• The hydrogen atom lost 3.02*10^-19 J of energy
• From conservation of energy a photon was created having E = 3.02*10^-19 J