The Atom (Cambridge (CIE) 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