The Development of the Atomic Model

This lesson traces the historical development of the atomic model from Dalton through to Chadwick, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how each model improved upon the last and be able to describe the key experiments that led to changes in our understanding of atomic structure.

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Why Models Change Over Time

Scientific models are not fixed — they change as new experimental evidence becomes available. Each new discovery about the atom led scientists to revise or replace the existing model. This is how science progresses: models are tested, and when evidence no longer fits, a better model is proposed.

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Dalton's Atomic Model (Early 1800s)

John Dalton proposed that:

• All matter is made of tiny, solid spheres called atoms.
• Atoms cannot be divided or broken down further.
• Each element is made of a unique type of atom.
• Atoms of different elements have different masses.

Dalton's model was the first to treat atoms as indivisible particles. It explained many observations about chemical reactions, such as the law of conservation of mass.

Limitation: Dalton did not know about sub-atomic particles — his model treated atoms as solid, featureless spheres.

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Thomson's Plum Pudding Model (1897)

J.J. Thomson discovered the electron in 1897 through experiments with cathode rays. Since electrons are negatively charged and atoms are neutral overall, Thomson proposed a new model:

• The atom is a sphere of positive charge with negatively charged electrons embedded in it — like plums in a pudding or raisins in a bun.
• The positive and negative charges balance, making the atom neutral overall.

This became known as the plum pudding model.

graph LR
    subgraph "Plum Pudding Model"
        A["Sphere of positive charge<br/>(the ’pudding’)"]
        B["Electrons scattered<br/>throughout<br/>(the ’plums’)"]
    end

    style A fill:#e74c3c,color:#fff
    style B fill:#3498db,color:#fff

Key advance: Thomson showed that atoms are not indivisible — they contain smaller particles (electrons).

Exam Tip: Thomson discovered the electron, not the proton or the neutron. The plum pudding model has a uniform positive charge with electrons dotted throughout — there is no nucleus in this model.

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Rutherford's Nuclear Model (1911)

Ernest Rutherford (along with Hans Geiger and Ernest Marsden) carried out the famous alpha particle scattering experiment. They fired positively charged alpha particles at a very thin sheet of gold foil and observed what happened.

Results of the Alpha Particle Scattering Experiment

Observation	Percentage	Conclusion
Most alpha particles passed straight through	~99%	Most of the atom is empty space
Some were deflected at small angles	A small fraction	The positive charge is concentrated in a small region
A very few bounced straight back	~1 in 8,000	The nucleus is very small, very dense and positively charged

Rutherford's Nuclear Model

Based on these results, Rutherford proposed that:

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

Limitation: Rutherford's model did not explain why the electrons did not spiral into the nucleus (since opposite charges attract), nor did it explain the arrangement of electrons.

Exam Tip: When describing the alpha particle scattering experiment, you must link each observation to a conclusion. For example: "Most alpha particles passed straight through, which shows that most of the atom is empty space."

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Bohr's Model (1913)

Niels Bohr improved Rutherford's model by proposing that:

• Electrons orbit the nucleus in fixed energy levels (shells).
• Each shell is at a specific distance from the nucleus.
• Electrons cannot exist between shells.
• Electrons in higher shells have more energy.

Bohr's model explained why electrons do not spiral into the nucleus — they are restricted to fixed orbits at set energy levels.

Bohr's Model — Key Points

Feature	Description
Nucleus	Small, dense, positively charged centre
Electrons	Orbit in fixed energy levels (shells)
First shell	Closest to nucleus, lowest energy
Higher shells	Further from nucleus, higher energy

Key advance: Bohr introduced the idea of electron shells, which explained atomic spectra and is the basis of electron configuration used today.

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Chadwick's Discovery of the Neutron (1932)

James Chadwick discovered the neutron in 1932. Before this discovery, scientists knew the nucleus contained protons but could not explain why the mass of the nucleus was greater than expected from protons alone.

Why the Neutron Was Important

• It explained the difference between the atomic number (number of protons) and the mass number (total mass of the nucleus).
• The extra mass was due to neutrons — particles with the same mass as protons but with no charge.
• This completed the model of the atom as we understand it at GCSE level: a nucleus of protons and neutrons, surrounded by electrons in shells.

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Timeline of the Atomic Model

graph LR
    A["Dalton<br/>(~1803)<br/>Solid sphere"] --> B["Thomson<br/>(1897)<br/>Plum pudding"]
    B --> C["Rutherford<br/>(1911)<br/>Nuclear model"]
    C --> D["Bohr<br/>(1913)<br/>Electron shells"]
    D --> E["Chadwick<br/>(1932)<br/>Discovery of<br/>the neutron"]

    style A fill:#8e44ad,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#27ae60,color:#fff

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Comparing the Models

Model	Scientist(s)	Date	Key Feature	Limitation
Solid sphere	Dalton	~1803	Atoms are indivisible spheres	No sub-atomic particles
Plum pudding	Thomson	1897	Electrons embedded in positive sphere	No nucleus
Nuclear model	Rutherford	1911	Small, dense, positive nucleus; mostly empty space	No explanation of electron arrangement
Bohr model	Bohr	1913	Electrons in fixed energy levels (shells)	Simplified picture; real atoms are more complex
Modern model	Chadwick (neutron, 1932)	1932	Nucleus contains protons and neutrons	—

Exam Tip: Questions on the development of the atomic model often ask you to describe what evidence led to a change and how the model was modified. Always link the evidence to the conclusion.

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Summary