The History of the Atom

• What is John Dalton’s theory?

  Atoms were described as solid spheres, and said that different-sized spheres made up the different elements
• What is J J Thompson’s findings and resulting theory?


  Found that atoms contain even smaller, negatively charged particles - electrons.

  He proposed a new theory in which the atom was a ball of a 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 J J Thompson in which the atom is a ball of positive charge, with discrete electrons throughout it. Like a ‘pudding’ with bits of ‘plum‘ spread throughout it
• What is the experiment carried out by Ernest Rutherford, and its results?

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

  Rather than all the alpha particles passing straight through the sheet of gold as you would expect from the plum pudding model, some we deflected to the side, and a small number were even deflected backwards
• What is the nuclear model of the atom, and who proposed it?


  Ernest Rutherford proposed the idea that each atom is composed of small positively charged nucleus at the centre, surrounding by a ‘cloud’ of negative electrons
• 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
• What did Niels Bohr propose?

  Electrons orbit the nucleus in shells, which are 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
• What did James Chadwick discover?

  Neutral particles called neutrons