4.1.3 The Development of the Model of the Atom

• What may lead to a scientific model being changed/replaced?

  New experimental evidence.
• What were atoms thought to be before the discovery of the electron?

  Tiny indivisible spheres.
• What model did the discovery of the electron lead to?

  The plum pudding model.
• What did the plum pudding model suggest?

  The atom is a ball of positive charge which negative electrons embedded in it.
• What conclusion did the results from the alpha particle scattering experiment lead to?

  That the mass of an atom was concentrated at the centre and that the centre (nucleus) was charged. This model was called the nuclear model.
• How did Bohr adapt the nuclear model?

  He suggested that electrons orbit the nucleus at specific distances.
• How were protons discovered?

  Experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge.
• Who proved the existence of neutrons 20 years after the nucleus became an accepted scientific idea?

  James Chadwick.
• Who first conceived of atomic theory in 500BC?

  Democritus.
• What did John Dalton say about atoms in the 1800s?

  He described them as solid spheres, and suggested that different types of spheres make up the different elements.
• Who came up with the plum pudding model?

  J.J. Thomson.
• How Rutherford developed the nuclear model:

  • In Rutherford's experiments, alpha particles were fired at a thin sheet of gold foil.
  • Most particles passed through, but some were deflected off course. 
  • This caused him to hypothesise that there was a dense region of positive charge at the centre of the atom that repelled the alpha particles.
  • As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surrounded by negative electrons. 
• One issue with Rutherford's nuclear model was that the atom should collapse as the negative electrons would be attracted to the positive nucleus, causing them to rush inwards. In response to this, in 1913, Bohr suggested that electrons ___________________, which kept the atom from collapsing.


  Orbit the nucleus in shells.