3.1 The History of the Atom

  • Democritus: (460 - 370 BC), Greek philosopher who predicted Atoms, it took 2000 years to prove them
  • Lavoisier: (460 - 370 BC), First to apply stoichiometry (moles) , father of modern chemistry
  • Dalton: (1766 - 1844), came up with the Modern Atomic Theory, elements consist of atoms that can not be created. divide or destroyed
  • Thompson: (1856 - 1940), %%named the electron%%, discovered the electron with the Cathode Ray Tube, mass to charge ratio
      * Ratio of charge to mass: -1.76 x 10^8 C/g
      * Plum pudding: cloud of positivity with negativity embedded inside
  • Milikan: (1909) came up with the mass of the electron
      * Mass of the electron: 9.11 x 10^-31 kg
  • Henri Becquerel: (1896), uranium could produce an image on a photographic plate in the absence of light, concept of radiation
  • Rutherford: (1871-1937), %%named the proton%%, radioactivity is the spontaneous decay, discovered the alpha & beta particles and the gamma ray
  • James Chadwick: %%named the neutron%%, figured this out because of the different masses of nuclei
The Gold Foil Experiment
  • Alpha particles fired at a thin sheet of gold foil
      * Most of the particles passed through meaning the atoms in the foil were mostly empty space
      * Some scattered so the there must be a dense area as well to be hit off of
  • There must be a proton

Atoms:

  • All 3 subatomics

  • electrically neutral

  • Atoms of different elements contain different numbers of electrons, which provides unique properties

  • Isotopes: Same protons but diff # of neutrons

  • Radioisotopes: their neutrons decay releasing radiation

Classical Theories of Light
  • Particle theory of light
      * started by greek philosophers in 300 BC
      * light exists as a steam of particles
  • Wave theory of light
      * 1968, Christian Huygens
      * Light as a wave by reflection, refraction & diffusion of light
      * Newton still supported the other one
  • Electromagnetism
      * James Clark Maxwell
      * Light is an electromagnetic wave made of electric and magnetic fields, explaining how light could have an effect on charged particles
Photoelectric Effect

  • Light on a metal causes emission from the metal

  • Frequency (colour) was important, not the intensity (brightness)

  • Threshold frequency: the minimum frequency required to emit an electron

  • Planck:
      * black body: absorbing all light, no reflection (stove coil)
      * Energy is given off by vibrating atoms, as energy increased the colour changed
      * Ultraviolet catastrophe: predicts that increasing temperature will result in objects emitting infinitely high intensity UV light, however, no matter how high the temperature, there was always a peak
      * Energy must be quantized: gained or lost in packets not as a wave
      * h: 6.63 x 10^-34

Einstein’s Quantum Theory
  • Photons = nhf
  • Threshold frequencies for ejection