The development of the model of the atom (common content with physics): The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

The idea

Before electrons were discovered, atoms were imagined as indivisible solid spheres. When J J Thomson identified the electron in 1897, scientists had to explain how a negatively charged particle could exist inside a neutral atom. Thomson proposed the plum pudding model: a sphere of positive charge with negative electrons scattered through it, like fruit in a pudding. The positive charge balances the total negative charge, so the atom overall is neutral. Crucially, this model has no nucleus and no empty space; the positive charge is spread evenly throughout the whole atom. A useful mental picture is a sponge cake with currants: the cake is the positive charge, the currants are the electrons. This model was accepted because it accounted for the electron's existence while keeping the atom neutral, and it made no claim about a dense centre.

What earns the marks

• States that the electron was discovered before the plum pudding model was proposed, and that the model was created to explain how negative electrons fit inside a neutral atom.
• Describes the plum pudding model as a ball or sphere of positive charge, not as a nucleus surrounded by empty space.
• Explains that negative electrons are embedded in, or scattered through, the positive sphere rather than orbiting a centre.
• States that the positive charge is spread throughout the atom and balances the total negative charge, so the atom is neutral overall.
• Uses the pudding analogy correctly: positive charge is the pudding and electrons are the plums or currants distributed inside it.

Common mistakes and how to put them right

• Drawing electrons orbiting a central nucleus.

  Put it right: Show electrons embedded within a uniform positive sphere.

• Saying the atom contains only positive charge or only negative charge.

  Put it right: The model has both, with the positive charge spread out and the electrons embedded in it, giving overall neutrality.

• Claiming the plum pudding model included a nucleus.

  Put it right: The nucleus was proposed only after the alpha particle scattering results, so it is absent from the plum pudding model.

Exam tips

• When asked to describe the model, name both components: the positive sphere and the embedded negative electrons.
• If asked to draw the model, use a large circle for the positive charge and small dots inside it for electrons, with no central dot.
• Link the model to the evidence available at the time: the electron had been discovered, but no evidence yet existed for a nucleus.