Lesson 13, Topic 2
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Experimental Evidence Supporting the Nuclear Model

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In his famous gold foil experiment, Rutherford fired alpha particles (helium nuclei consisting of two protons and two neutrons) at a thin sheet of gold foil. If Thomson’s plum pudding model was correct, the alpha particles would have travelled straight through the foil with only occasional, minor deflections.

  • Most of the alpha particles passed through undeflected.
  • Some were very significantly deflected, with a few bouncing straight back towards the source.
  • These deflections were caused by a highly-concentrated positive charge at the centre of the atom (the nucleus) which electrically repelled the alpha particles.

Later, Rutherford observed hydrogen as a product of a reaction between nitrogen and alpha particles. From this, he concluded that one of them must have hydrogen within it, and that hydrogen nuclei (i.e. protons) were present inside nuclei, and hence inside all atoms.