This nuclear model replaced the plum pudding model.

The idea

The plum pudding model pictured the atom as a ball of positive charge with negative electrons embedded in it, like fruit in a pudding. Rutherford's alpha-particle scattering experiment changed this: most alpha particles passed straight through thin gold foil, but a few were deflected strongly, and some bounced back. To explain this, Rutherford proposed the nuclear model, in which a tiny, dense, positively charged nucleus sits at the centre and most of the atom is empty space. Because the nuclear model explained the deflections that the plum pudding model could not, it replaced the older model. Scientific models are provisional: new evidence causes models to be modified or replaced.

What earns the marks

• Describe the plum pudding model as a sphere of positive charge with negative electrons embedded in it.
• Describe the nuclear model as a small, dense, positively charged nucleus surrounded by mostly empty space, with electrons outside the nucleus.
• Link the alpha-particle scattering results to the nuclear model: most particles pass straight through (empty space), a few deflect strongly or bounce back (concentrated positive charge).
• Explain that the nuclear model replaced the plum pudding model because it accounted for the experimental observations.
• Recognise that scientific models change when new experimental evidence does not fit the existing model.

Common mistakes and how to put them right

• Thinking the nucleus contains electrons.

  Put it right by stating that electrons are outside the nucleus and the nucleus contains protons and neutrons.

• Believing the plum pudding model had a nucleus.

  Put it right by explaining that the plum pudding model had positive charge spread throughout the atom, with no nucleus.

• Saying most alpha particles bounced back.

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

Exam tips

• Use the words tiny, dense and positively charged when describing the nucleus.
• When explaining the evidence, pair each observation with what it shows about the atom.
• State clearly that the nuclear model replaced the plum pudding model because it explained the evidence better.