Elements of Life: Atomic Models

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The Accepted Model of the Atom Has Changed Throughout History

  • Ancient Greeks: all matter was made from indivisible particles
  • C19th John Dalton: solid spheres, different spheres made up different elements

Experimental Evidence Showed that Atoms Weren't Solid Spheres

  • 1897: J. J. Thompson concluded atoms weren't solid and indivisible
    • measurements of charge and mass showed that an atom must contain smaller negatively charged particles: electrons
    • plum pudding model

Rutherford Showed that the Plum Pudding Model Was Wrong

  • 1909: Ernest Rutherford and students Hans Geiger and Ernest Marsden
    • gold foil experiment
      • fired alpha particles at gold sheet
      • expected most alpha particles to be deflected very slightly by positive "pudding"
      • most alpha particles actually passed straight through gold atoms
      • very few deflected backwards
      • this showed plum pudding model couldn't be right
    • came up with nuclear model
      • tiny, positively charged nucleus at centre
        • most of atoms mass
      • surrounded by a "cloud" of negative electrons
      • most of the atom is empty space

Rutherford's Nuclear Model was Modified Several Times

  • Henry Moseley: charge of nucleaus increased from one element to…


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