4.4.1.3 - The Development of the Model of the Atom

Models change with evidence

A scientific model is a simplified picture or description that helps scientists explain observations and make predictions. A model is useful only while it fits the evidence. If new experimental evidence does not fit the model, scientists may change the model or replace it.

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. This is sometimes called the solid sphere model. It treated atoms as indivisible particles, so it could not include smaller charged particles inside the atom.

[DIAGRAM: asset_name: evidence-driven-atomic-model-timeline - diagram 1; asset_slug: 041_4_4_1_3_development_of_the_atomic_model_diagram1; file: diagram_assets/041_4_4_1_3_development_of_the_atomic_model_diagram1.png; recommended_method: image_gen; description: Assessed timeline showing the sequence tiny indivisible sphere, plum pudding model, nuclear model, Bohr model, and nucleus with protons and neutrons. Each stage has one short evidence trigger: electron discovered, alpha scattering evidence, Bohr calculations matched observations, later evidence for protons and neutrons. The visual teaches model change over time and uses exact GCSE labels.]

The diagram is a roadmap. The rest of the lesson explains what evidence caused each change and what the new model said about the atom.

The electron and plum pudding model

The discovery of the electron showed that atoms were not indivisible solid spheres. Electrons are negatively charged particles, so the model of the atom had to include smaller parts inside the atom.

The plum pudding model suggested that an atom is a ball of positive charge with negative electrons embedded in it. In this model, the positive charge is spread throughout the atom, with electrons sitting inside the positive material.

The name is only an analogy. The important assessed physics is the charge arrangement: positive charge spread through the atom, with negative electrons embedded in it.

Alpha scattering and the nuclear model

The alpha particle scattering experiment gave evidence that did not fit the plum pudding model. Positive alpha particles were directed at a very thin metal foil. Most alpha particles passed straight through the foil, some were deflected, and a very small number were deflected through large angles or came back.

[DIAGRAM: asset_name: alpha-scattering-evidence-for-the-nuclear-model - diagram 2; asset_slug: 041_4_4_1_3_development_of_the_atomic_model_diagram2; file: diagram_assets/041_4_4_1_3_development_of_the_atomic_model_diagram2.png; recommended_method: image_gen; description: Assessed two-panel diagram comparing alpha particle paths if positive charge were spread out in the plum pudding model with the observed nuclear-model explanation. Left panel shows most alpha particles predicted to pass through with little deflection. Right panel shows most passing through empty space, some deflected by the small charged nucleus, and one rare large deflection. Labels must include positive alpha particles, small charged nucleus, mass concentrated in nucleus, and atom mostly empty space.]

The key reasoning is:

This led to the nuclear model. The nuclear model replaced the plum pudding model because it placed the mass of the atom at the centre in a small charged nucleus.

Do not say that alpha particles "hit" the nucleus as the main explanation. In GCSE answers, the important idea is that positive alpha particles are repelled by the charged nucleus, especially when they pass close to it.