Alpha Particle Scattering (Oxford AQA IGCSE Physics)

Revision Note

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

Electrons are embedded within a positively charged sphere
J J Thomson thought of the atom as being a positively charged mass embedded with small negatively charged electrons – a bit like a plum pudding
  • 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

Path of alpha particles fired at gold foil
When alpha particles are fired at thin gold foil, most of them go straight through, some are deflected and a very small number bounce straight back

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

Finding

Conclusion

A

Most of the alpha particles passed straight through the gold foil

Atoms are mostly empty space

B

A few alpha particles were deflected from their path but continued through the gold foil

The nucleus of the atom has a strong positive charge

C

A small number of alpha particles rebounded

The atoms contain a small, dense nucleus

  • 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

Plum pudding and the nuclear model, downloadable IGCSE & GCSE Physics revision notes
The Nuclear model replaced the 'plum pudding' model as it could better explain the observations of Rutherford’s Scattering Experiment

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