Why the new evidence from the scattering experiment led to a change in the…

The idea

Before the scattering experiment, the plum pudding model pictured the atom as a ball of positive charge with negative electrons embedded in it, so charge was spread evenly throughout. In the scattering experiment, positively charged alpha particles were fired at thin gold foil. Most passed straight through, but some were deflected through large angles and a few bounced back. This was impossible if positive charge were spread out, because a diffuse charge could not repel an alpha particle strongly enough. The evidence therefore forced a new model: a small, dense, positively charged nucleus surrounded by mostly empty space, with electrons outside it. Because the evidence contradicted the plum pudding model, scientists replaced it with the nuclear model.

What earns the marks

• State that the plum pudding model has positive charge spread throughout the atom with electrons embedded in it.
• Describe the scattering experiment: alpha particles fired at thin gold foil, with most passing straight through.
• Explain that some alpha particles were deflected through large angles and a few were repelled back towards the source.
• Link the large deflections to a small, dense, positively charged region, since like charges repel and a concentrated charge produces a strong repulsive force.
• Conclude that the results were inconsistent with spread-out positive charge, so the plum pudding model was replaced by the nuclear model.
• Describe the nuclear model as a tiny central nucleus containing most of the atom's mass and positive charge, with electrons occupying the surrounding space.

Common mistakes and how to put them right

• Saying alpha particles are negative.

  Put it right by stating they are positively charged, so the nucleus must also be positive to repel them.

• Claiming most alpha particles were deflected.

  Put it right by stating most passed straight through, showing the atom is mainly empty space.

• Describing the nucleus as spread throughout the atom.

  Put it right by stating it is small, dense and central, with electrons outside it.

Exam tips

• Use the pattern of results: most straight through, some deflected, a few bounced back, then link each pattern to a feature of the nuclear model.
• Name the model change explicitly: plum pudding model replaced by the nuclear model, because the new evidence did not fit the old model.
• When explaining repulsion, refer to like charges repelling and to the nucleus being positive, rather than saying the particles simply hit something.