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The Atom (Cambridge O Level Physics)
Revision Note
Atomic Structure
- Atoms are the building blocks of all matter
- They are incredibly small
- Atoms have a tiny nucleus at their centre, with electrons orbiting around the nucleus
- They consist of small positively charged nuclei, surrounded by negatively charged electrons
The Structure of an Atom
An atom has a small positive nucleus, surrounded by orbiting negative electrons
Rutherford's Experiment
- In 1909 Ernest Rutherford was investigating their ideas about the structure of the atom
- This involved the scattering of alpha (α) particles by a sheet of thin metal supports the nuclear model of the atom
- A beam of alpha particles was directed at a thin gold foil
- They expected the alpha particles to travel straight through the gold foil with only a small amount of deflection
- 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
Alpha Particles Passing Through Atoms in a Gold Foil
When α-particles are fired at thin gold foil most of them pass straight through but a very small number bounce straight back
- When α-particles are fired at thin pieces of gold foil:
- The majority of them go straight through (A)
This happens because the atom is mainly empty space
- Some are deflected through small angles (B)
This happens because the positive α-particles are repelled by the positive nucleus which contains most of its mass
- A very small number are deflected straight back (C)
This is because the nucleus is extremely small
- The majority of them go straight through (A)
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