Alpha Particle Scattering (Oxford AQA IGCSE Physics)
Revision Note
Written by: Caroline Carroll
Reviewed by: Lucy Kirkham
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
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
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