Atomic Models


Dalton's & Thomson's Models

  • 19th century: John Dalton - solid spheres, different spheres made up different elements 
  • 1897: J.J. Thomson - concluded from experiments atom must contain even smaller negatively charged particles - electrons 
  • 'Solid sphere' idea of atomic structure had to be changed
  • New model: 'plum pudding model' 
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Rutherford's Model

  • 1909 - Ernest Rutherford: gold foil experiment 
  • fired alpha particles at v thin sheet of gold 
  • expected from plum pudding model: most alpha particles deflected slightly by positive 'pudding' 
  • most of alpha particles passed straight through gold atoms
  • v small number deflected backwards 
  • came up with nuclear model of atom 
  • tiny, positively charged nucleus at centre, surrounded by 'cloud' of negative electrons 
  • most of atom empty space 
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Bohr's Model

  • scientists realised that electrons in 'cloud' around nucleus would quickly spiral down into nucleus: atom collapse
  • Niels Bohr proposed new model with 4 basic principles:
    • electrons only exist in fixed orbits & not anywhere in between
    • each shell - fixed energy
    • when electron moved between shells - electromagentic radiation emitted/absorbed
    • energy of shells is fixed so radiation has fixed frequency 
  • frequencies of radiation emitted & absorbed by atoms was already known from experiments 
  • Bohr model fitted these observations
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Other atomic models

  • scientists later discovered not all electrons in a shell had same energy 
  • meant Bohr model not quite right 
  • refined model fit observations even better
  • refined model is not perfect but widely used to describe atoms because it is simple & explains many observations from experiments 
  • most accurate model is based on quantum mechanics 
  • explains some observations that can not be accounred for by Bohr model, lot harder to get head round & visualise 
  • scientists use whichever model is most relevant to whatever they're investigating 
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