P1.1 History of the atom

• Summarise the experiment carried out by Ernest Rutherford, and its results.

  Experiment: Rutherford and his student conducted the alpha particle scattering experiments where they fired positively charged alpha particles at an extremely thin sheet of gold.

  Results: Rather than all the alpha particles passing straight through the sheet of gold as you would expect from the plum pudding model, some were deflected to the side, and a small number were even deflected backwards.
• Summarise JJ Thompsons findings and resulting theory.

  JJ Thomson found that atoms contain tiny, negatively charged particles- electrons.

  He proposed a new theory in which the atom was a ball of positive charge, with discrete electrons throughout it. It was known as the 'plum pudding model'.
• What was the 'plum pudding model', and who proposed it?

  A theory proposed by JJ Thompson that the atom is a ball of positive charge, with discrete electrons throughout it.
• What is the nuclear model of the atom, and who proposed it?

  Proposed by Ernest Rutherford.

  The nuclear model of the atom is the idea that each atom is composed of a small positively charged nucleus at the centre, surrounded by a 'cloud' of negative electrons.
• What did Niels Bohr propose?

  Niels Bohr suggested that electrons orbit the nucleus in shells, which are a fixed distance from the nucleus.

  This was in contrast to Ernest Rutherford's theory in which the electrons were spread in a 'general cloud' around the nucleus.
• Summarise John Dalton's theory.

  John Dalton described atoms as solid spheres and said different-sized spheres made up the different elements.
• What did James Chadwick discover?

  James Chadwick discovered neutral particles called neutrons.
• What was the main issue with the nuclear model of the atom?

  The electrons should be attracted to the positive nucleus, so the atom should just collapse.