GCSE Combined Science (AQA)

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Alpha Scattering

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Atomic structure Atoms and isotopes

Alpha Scattering

7:45 History of the Atom
Spec 4.4.1.3
Previous The Plum Pudding Model Next Bohr Model
  • In 1909, Hans Geiger and Earnest Marsden conducted an experiment directing alpha particles at gold foil.
  • The experiment aimed to investigate unexplained behavior of alpha particles and led to discoveries about atomic structure.
  • Alpha particles are radiation consisting of two protons and two neutrons, similar to helium ions with a strong positive charge.
  • The experiment involved firing alpha particles at a thin gold foil, only a few atoms thick.
  • The plumb pudding model suggested alpha particles would pass through the gold without being affected.
  • Scientists believed atoms consisted of electrons embedded in positive spheres of matter with evenly distributed mass and charge.
  • Contrary to expectations, some alpha particles were slightly deflected, and a small number bounced back.
  • Charged plates detected the paths of alpha particles, showing most passed straight through, but some were deflected.
  • Some particles had large deflections of more than 90 degrees, contradicting the plumb pudding model.
  • In 1911, Ernest Rutherford proposed a nuclear model for the atom, explaining the observations.
  • Rutherford theorized atoms contained a small charged nucleus surrounded by electrons, with the nucleus containing most of the atom's mass.
  • The nucleus was later confirmed to be positively charged, balancing the negative charge of electrons.
  • The nuclear model explained that only a few alpha particles were heavily deflected due to the small size of the nucleus compared to the atom.
  • Most alpha particles passed through the atom unaffected, as they didn't come into contact with the nucleus.
  • The nucleus must have a large mass to not be affected by the forces deflecting the alpha particles.
  • Modern experiments show the nucleus contains over 99% of the atom's mass.
  • The nucleus must be charged to cause deflections of particles passing close by, with more particles undergoing partial deflections.
  • Assuming the nucleus is positively charged, alpha particles experience repulsive electrostatic forces causing path bending.
  • Rutherford initially considered the nucleus could be negatively charged, causing attractive forces and strong deflection through slingshotting.

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