The Development of the Atomic Model

This lesson covers the historical development of the atomic model as required by AQA GCSE Combined Science Trilogy (8464, Chemistry 4.1.1). You need to know how the model of the atom has changed over time as new experimental evidence was discovered. This is a very common exam topic — AQA frequently asks 6-mark questions on how and why the model has evolved.

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Why Do Scientific Models Change?

A model in science is a simplified representation used to explain observations. Models change when new experimental evidence is discovered that the existing model cannot explain. Each new model builds on the previous one and provides a better explanation of the behaviour of atoms.

Exam Tip (AQA 8464): When discussing the development of the atomic model, always link each change to the new evidence that caused it. Simply listing the models without explaining why they changed will lose you marks.

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Timeline of the Atomic Model

graph LR
    A["Dalton<br/>Early 1800s<br/>Solid sphere"] --> B["Thomson<br/>1897<br/>Plum pudding model"]
    B --> C["Rutherford<br/>1909<br/>Nuclear model"]
    C --> D["Bohr<br/>1913<br/>Electron shells"]
    D --> E["Chadwick<br/>1932<br/>Discovery of neutron"]

    style A fill:#1abc9c,color:#fff
    style B fill:#f39c12,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#9b59b6,color:#fff

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1. John Dalton — The Solid Sphere Model (Early 1800s)

Dalton proposed that:

• All matter is made of tiny, indivisible spheres called atoms.
• Atoms of the same element are identical.
• Atoms of different elements are different.
• Atoms cannot be created, destroyed or divided.
• Chemical reactions involve the rearrangement of atoms.

Dalton's model was useful but incomplete — it did not explain the existence of sub-atomic particles.

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2. J.J. Thomson — The Plum Pudding Model (1897)

Thomson discovered the electron — a tiny, negatively charged particle — using cathode ray tube experiments. He proposed the plum pudding model:

• The atom is a ball of positive charge with negative electrons embedded throughout it (like plums in a pudding).
• The positive charge was spread evenly through the atom.
• The negative electrons were scattered within this positive "dough."

This was the first model to include sub-atomic particles. It explained that atoms were not indivisible — they contained smaller charged particles.

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3. Rutherford — The Nuclear Model (1909)

Ernest Rutherford, together with Geiger and Marsden, performed the famous alpha particle scattering experiment:

The Experiment

1. A beam of alpha particles (positively charged, made of 2 protons and 2 neutrons) was fired at a thin sheet of gold foil.
2. A detector screen surrounding the foil recorded where the alpha particles hit.

Observations

Observation	What It Showed
Most alpha particles passed straight through	The atom is mostly empty space
Some alpha particles were deflected at small angles	They passed close to a concentrated positive charge
Very few alpha particles bounced straight back (about 1 in 8000)	They hit something very small, dense and positively charged

Conclusions — The Nuclear Model

• The atom has a small, dense, positively charged nucleus at the centre.
• Most of the atom is empty space.
• The electrons orbit the nucleus at a distance.

This disproved the plum pudding model — if positive charge were spread evenly, no alpha particles would have bounced back.

Exam Tip (AQA 8464): The alpha particle scattering experiment is one of the most commonly examined topics. You must be able to describe the experiment, state the observations, and explain how each observation led to a conclusion about atomic structure.

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4. Niels Bohr — Electron Shells (1913)

Bohr modified Rutherford's model by suggesting that:

• Electrons orbit the nucleus in specific energy levels (shells) at fixed distances from the nucleus.
• Electrons cannot exist between energy levels.
• Each energy level can hold a fixed maximum number of electrons.

Bohr's evidence came from atomic emission spectra — when elements are heated, they emit light at specific frequencies (seen as coloured lines in a spectrum). Each line corresponds to an electron dropping from a higher energy level to a lower one, releasing a specific amount of energy as light.

Exam Tip: Bohr explained why atoms emit light at only specific frequencies — the fixed energy levels mean that only certain energy transitions are possible, producing discrete lines rather than a continuous spectrum.

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5. James Chadwick — Discovery of the Neutron (1932)

Chadwick discovered the neutron — an uncharged particle in the nucleus with approximately the same mass as a proton. This completed our understanding of the nucleus and explained the existence of isotopes (atoms of the same element with different numbers of neutrons).

Scientist	Date	Key Contribution	How the Model Changed
Dalton	Early 1800s	Proposed atoms as indivisible spheres	First atomic theory
Thomson	1897	Discovered the electron	Atom contains charged sub-atomic particles
Rutherford	1909	Alpha scattering experiment	Atom has a small, dense, positive nucleus
Bohr	1913	Proposed electron energy levels	Electrons orbit in fixed shells, not randomly
Chadwick	1932	Discovered the neutron	Explained isotopes and completed nuclear model

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Worked Example

Question: Explain how the results of the alpha particle scattering experiment led to the replacement of the plum pudding model with the nuclear model. (6 marks)

Model Answer:

1. In the plum pudding model, positive charge is spread evenly throughout the atom, so alpha particles should pass through with only slight deflections.
2. However, most alpha particles passed straight through the gold foil, showing the atom is mostly empty space.
3. Some alpha particles were deflected at angles, indicating they passed near a concentrated region of positive charge that repelled them.
4. A very small number bounced back, meaning they hit something very small, dense and positively charged — the nucleus.
5. These results could not be explained by the plum pudding model, where no concentrated positive charge exists.
6. Rutherford therefore proposed the nuclear model: a small, dense, positively charged nucleus surrounded by electrons at a distance.

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Common Mistakes

Mistake	Correction
Saying Dalton discovered the atom	Dalton proposed a model — atoms were already known to exist
Confusing Thomson and Rutherford	Thomson = plum pudding; Rutherford = nuclear model
Saying "most" alpha particles were deflected	Most passed straight through — only a very few were deflected back
Forgetting to explain why each model was replaced	Always link to new experimental evidence
Saying Bohr discovered electron shells	Bohr proposed them based on emission spectra evidence

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Summary