Alpha Particle Scattering (Oxford AQA IGCSE Physics)

Alpha Particle Scattering

• Scientists understanding of what atoms are has changed through time

• Different models have been developed, and then replaced as new evidence from experiments is discovered

  • A model is a way of describing something in order to explain the way it behaves

What is the 'Plum Pudding' model of the atom?

• At the end of the 19th Century, Physicist Joseph Jon Thomson discovered the existence of electrons

• Using this new evidence, Thomson proposed the 'plum pudding' model

  • The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’

The 'Plum Pudding' model

• It was known that electrons were much smaller than atoms, so it made sense that they should be embedded within the larger atom

• Since electrons have a negative charge, it was reasoned that the rest of the atom would be positive, making the atom neutral overall

Alpha particle scattering

• In 1909 a group of scientists were investigating the 'plum pudding' model

  • Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment

• They were directing a beam of alpha particles (He²⁺ ions) at a thin gold foil

• They expected the alpha particles to travel through the gold foil, and maybe change direction by a small amount

• Instead, they discovered that :

  • Most of the alpha particles passed straight through the foil

  • Some of the alpha particles changed direction but continued through the foil

  • A few of the alpha particles bounced back off the gold foil

• Bouncing back of alpha particles could not be explained by the 'plum pudding' model,

• This new evidence led to the creation of a new model

Path of alpha particles through gold foil

The nuclear model

• Ernest Rutherford made different conclusions from the findings of the experiment

• The table below describes the findings and conclusions of A, B and C from the image above:

Alpha Scattering Findings and Conclusions Table

• Rutherford proposed the nuclear model of the atom

• In the nuclear model:

  • Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)

  • The nucleus is positively charged

  • Negatively charged electrons orbit the nucleus at a distance

• Rutherford’s nuclear model replaced the 'plum pudding' model

  • The nuclear model could explain experimental observations better than the 'plum pudding' model

Different models of the atom