(a) alpha-particle scattering experiment; evidence of a small charged…

The idea

In the alpha-particle scattering experiment, alpha particles are directed at a very thin gold foil inside an evacuated chamber. A detector, such as a movable microscope and scintillation screen, records where particles arrive. Most alpha particles pass almost straight through, some are deflected through large angles, and a very small number bounce back. This pattern shows that the atom is mostly empty space, with a tiny central nucleus that is positively charged and contains most of the atom's mass. The rare large deflections occur when an alpha particle passes close to a nucleus and experiences a strong electrostatic repulsion.

What earns the marks

• Most alpha particles pass straight through the foil, showing that the atom is mostly empty space.
• A small number are deflected through large angles or bounce back, showing that the positive charge and most of the mass are concentrated in a very small nucleus.
• The deflections are caused by electrostatic repulsion between the positive alpha particle and the positive nucleus.
• The experiment used a thin gold foil and a detector to record the scattering pattern at different angles.
• The results rule out a uniform spread of positive charge throughout the atom, because that would not produce large-angle scattering.

Common mistakes and how to put them right

• Saying alpha particles are attracted to the nucleus.

  Put it right: Both the alpha particle and the nucleus are positively charged, so the force is repulsive.

• Believing most alpha particles are deflected.

  Put it right: Most pass through with little or no deflection, and only a very small fraction are strongly deflected.

• Thinking the nucleus is large.

  Put it right: The nucleus is extremely small compared with the atom, which is why large-angle scattering is rare.

Exam tips

• Link each observation to a conclusion: straight-through paths show empty space, large deflections show a small charged nucleus.
• Use the word electrostatic repulsion rather than a general force when explaining the deflection.
• Avoid saying the alpha particles hit the nucleus; they are deflected by the electric field near it.