IP The atom and the discovery of the nucleus © Oxford University Press 2011 The atom and the discovery of the nucleus

IP The atom and the discovery of the nucleus © Oxford University Press 2011  In the early 1900s scientists were beginning to think about how matter was made up and there were a lot of different ideas as to what atoms looked like.  The ‘Plum pudding’ model was proposed by J. J. Thomson.  He had discovered the existence of the electron some years before (even though he called them a different name).  He said that these electrons were surrounded by a soup of positive charge to balance their negative charge. It was like the ‘plums’ (electrons) surrounded by ‘pudding’ (positive charge).  The electrons were distributed all through the atom.

IP The atom and the discovery of the nucleus © Oxford University Press 2011  In 1909 two students of Ernest Rutherford, Hans Geiger and Ernest Marsden, performed an experiment which Rutherford later used to prove his planetary model of the atom.  The students fired small positively charged ‘alpha’ particles at gold leaf. To their surprise some of the alpha particles were scattered back towards them.

IP The atom and the discovery of the nucleus © Oxford University Press 2011  Rutherford concluded that the plum pudding model could not explain these results, neither the distributed negative charge or pudding-like positive charge were either strong enough by mass or charge to deflect the alpha particles.  He proposed instead that the majority of the atom’s mass and charge were concentrated in a very small region (which was later named the nucleus) and that the majority of the rest of the atom was empty space.  He knew this because only some of the alpha particles were scattered.