The results from the alpha particle scattering experiment led to the…

The idea

In the alpha particle scattering experiment, alpha particles were fired at thin gold foil. Most passed straight through, showing that most of the atom is empty space. Some were deflected through small angles, and a very small number bounced back. The bouncing back could only happen if they hit something dense and positively charged. This led Rutherford to conclude that the mass and positive charge are concentrated in a tiny central nucleus. The nucleus is much smaller than the atom but contains most of its mass. The results also showed that the nucleus is positively charged, because alpha particles, which are positive, were repelled. This replaced the plum pudding model with the nuclear model.

What earns the marks

• State that most alpha particles passed straight through the gold foil.
• Explain that this shows the atom is mostly empty space.
• Describe that a few alpha particles were deflected or bounced back.
• Conclude that the mass and positive charge are concentrated in a small central nucleus.
• Explain that the nucleus must be positively charged because it repels positive alpha particles.

Common mistakes and how to put them right

• Saying most alpha particles bounced back.

  Put it right by stating most passed straight through and only a few bounced back.

• Concluding the nucleus is negative.

  Put it right by explaining that positive alpha particles were repelled by a positive nucleus.

• Thinking the nucleus fills most of the atom.

  Put it right by stating it is tiny compared with the whole atom.

Exam tips

• Link each observation to a conclusion: straight through means empty space, bouncing back means dense positive centre.
• Use the term nucleus and state that it is charged and contains most of the mass.
• Avoid saying the experiment proved electrons orbit the nucleus; it only gave evidence for the nucleus.