History of atom

• What was the main issue with the nuclear model of the atom ?

  The electrons should be attracted to the positive nucleus so the atom would just collapse ; failed to explain stability of atoms.
• Summarise John Daltons theory

  • Described atoms as solid spheres , and said that different-sized spheres made up the different elements.
• What is the nuclear model + proposed by who ?


  • The idea that each atom is composed of a small positively charged nucleus at the centre at the centre, surrounding by a ‘cloud’ of negative electrons.
• Summarise JJ Thompson finding and resulting theory.


  
  

  • He found that atoms contain even smaller, negatively charged particles - electrons.
  • He proposed a new theory in which the atom was a ball of positive charge, with discrete electrons throughout it.
  • Known as plum pudding model.
• Summarise experiment carried out by Ernest Rutherford + it’s results.

  • Conducted the alpha particle scattering experiments where they fired positively charged alpha particles at an extremely thin sheet of gold.
  • Rather than all of the alpha particles passing straight through the sheet of gold ( expected from plum pudding model ) some were deflected to the side , small number were even deflected backwards .
• What did James Chadwick discover ?

  Discovered neutral particles called neutrons .
• What did Niels Bohr propose ?

  Electrons orbit the nucleus in shells, which are a fixed distance from nucleus

  This was in contrast to Ernest Rutherford’s theory in which the electrons were spread in a ’general cloud ‘ around the nucleus.