Mod 8

• Cathode rays


  Glows and 'discharges' in electrified vacuum tubes came from the negative electrode
• Evidence for cathode rays being waves

  • Travel in straight lines like light waves
  • Cause fluorescence, like ultraviolet waves
  • Can 'expose' photographic film, as light does
  • Have momentum
  • Have mass
• Evidence for cathode rays having both wave and particle properties

  • Carry kinetic energy and momentum, and therefore must have mass
  • Carry negative electric charge
• Thomson's charge-to-mass experiment


  1. Deflect cathode rays with electric and magnetic fields

  2. Measure velocity

  3. Measure centripetal motion under magnetic field

  4. Calculate charge to mass ratio
• In 1897, when Tompson did his famous experiment, little was known about atomic structure. No subatomic particles were known - protons, neutrons and electrons were unheard of. The debate about whether cathode rays were particles or waves still raged in the scientific community.
• Thomson's experiment proved that cathode rays consisted of particles with a large charge:mass ratio, smaller than the smallest atom (hydrogen), proving the existence of subatomic particles.
• Millikan's oil drop experiment

  1. Mist of oil sprayed (charged) into an electric field

  2. Strength of electric field varied until single oil drop was suspended between plates

  3. Determined charge of electron
• Types of radiation


  • α ~ He4 nuclei (2 protons, 2 neutrons), not very penetrating, quite large, positive charge
  • β ~ Electron or positron, smaller and more penetrative
  • γ ~ Very penetrating. High energy radiation, very high frequency electromagnetic radiation
• Geiger-Marsden experiment


  1. α particles directed at a sheet of gold foil

  2. Most went straight through (empty spaces)

  3. Some deviated from path (significantly) - indicating a small nucleus with positive charge
• Rutherford's atomic model


  • Positive charged particle concentrated in an extremely small volume
  • Most of the mass in concentrated in this small volume → called this the nucleus
  • Negative electrons orbit in circular orbits
  • Atom held together by electrostatic forces
• Limitations of the Rutherford Model: Could not explain the stability of the atom, could not explain arrangement of electrons, could not explain spectral lines
• Chadwick's discovery of the neutron


  1. α radiation (from polonium) directed onto Beryllium

  2. High energetic radiation produced, assumed to be γ radiation

  3. Radiation fell on paraffin, protons of very high energy were ejected

  4. Chadwick proposed this radiation was uncharged and had approximately the same mass as a proton → neutron
• The nuclear model of the atom was essentially complete with Chadwick's discovery of the neutron.
• Bohr's Postulates


  • Electrons in atoms exist in stable circular orbits
  • Electrons in stable orbits do not emit radiation
  • Electrons emit or absorb specific quanta of energy when they move from one stable orbit to another
• Limitations of Bohr's Model: Accelerating electrons should emit radiation, could not explain spectra of larger elements, only works well for hydrogen
• Bohr's Model


  1. Could be used to predict the wavelengths of hydrogen spectra lines (Balmer and Rydberg)

  2. Predicted ionisation energy of Hydrogen

  3. Predicted other spectral lines not yet observed (Lyman)

  4. Explained why not all absorption spectra lines are present
• E=hc/λ


  Relationship between energy, Planck's constant, speed of light, and wavelength
• 1/λ = R[1/n_i^2 - 1/n_f^2]


  Rydberg equation to calculate wavelength of emitted/absorbed radiation in hydrogen spectra
• Strengths of Bohr's Atom

  • Explained quanta
  • Could explain emission spectra of Hydrogen (Balmer series)
  • Used Rydberg's equation to predict other spectral lines
• de Broglie's matter waves


  • Electron orbits contain whole numbers of standing waves
  • λ = h/p
• Evidence for de Broglie's matter waves


  1. Electrons have been observed to demonstrate diffraction

  2. Davisson-Germer experiment showed electrons reflected off a crystal
• Schrödinger's electron cloud model


  • Described the electron structure of the atom in terms of the mathematical probability of finding electrons in certain regions of space around the nucleus
  • Electron shells have shells within shells, known as sub-shells or sub-orbitals (S, P, D, F)
  • Supports Pauli's exclusion principle