The Development of the Atomic Model

This lesson covers the history of the atomic model and how our understanding of the atom has changed over time — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand how scientific models develop when new experimental evidence is found, and be able to describe the key models in the history of atomic theory.

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Why Do Scientific Models Change?

A model in science is a simplified representation of something that helps us explain observations and make predictions. Scientific models are not permanent — they change when new experimental evidence is discovered that the current model cannot explain.

Key principles:

• Models are based on the best evidence available at the time.
• When new evidence contradicts the current model, scientists must modify or replace it.
• The development of the atomic model is an excellent example of how science progresses through evidence and peer review.

Exam Tip: The Edexcel specification explicitly requires you to understand that scientific models change over time as new evidence is found. You may be asked to describe why a model was changed, not just what the models were. Always link the change to specific experimental evidence.

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Timeline of Atomic Models

flowchart LR
    A["Dalton’s Solid<br/>Sphere Model<br/>(early 1800s)"] --> B["Thomson’s<br/>Plum Pudding Model<br/>(1897)"]
    B --> C["Rutherford’s<br/>Nuclear Model<br/>(1911)"]
    C --> D["Bohr’s<br/>Shell Model<br/>(1913)"]
    D --> E["Modern<br/>Quantum Model<br/>(1920s onwards)"]

    style A fill:#8e44ad,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#c0392b,color:#fff

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1. Dalton's Solid Sphere Model (Early 1800s)

John Dalton proposed that atoms were tiny, solid spheres that could not be divided. Each element was made of a different type of atom, and compounds were formed when atoms of different elements combined.

Key Ideas

• Atoms are indivisible — they cannot be broken down.
• All atoms of the same element are identical.
• Atoms of different elements have different masses.
• Compounds are formed by combining atoms of different elements in fixed ratios.

Limitations

Dalton's model could not explain:

• Why elements could be grouped by similar chemical properties.
• The existence of sub-atomic particles (discovered later).
• Any internal structure of the atom.

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2. Thomson's Plum Pudding Model (1897)

In 1897, J. J. Thomson discovered the electron — a negatively charged sub-atomic particle. This showed that atoms were not indivisible as Dalton had proposed.

Thomson proposed the plum pudding model:

• The atom is a sphere of positive charge with negatively charged electrons embedded within it.
• The electrons are scattered throughout the positive charge, like plums in a pudding (or raisins in a Christmas pudding).
• The positive and negative charges balance out, making the atom electrically neutral overall.

Why This Model Was Proposed

• Thomson knew that atoms were neutral overall but contained negative electrons.
• He reasoned that there must be an equal amount of positive charge spread throughout the atom to balance the negative electrons.

Exam Tip: The plum pudding model is sometimes called the "Christmas pudding model." You need to describe it as a ball of positive charge with electrons embedded in it. Do not confuse it with the nuclear model.

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3. Rutherford's Nuclear Model (1911)

In 1909, Ernest Rutherford, along with Hans Geiger and Ernest Marsden, carried out the famous alpha particle scattering experiment (also known as the Geiger-Marsden experiment or the gold foil experiment).

The Experiment

• They fired a beam of alpha particles (positively charged) at a very thin sheet of gold foil.
• A detector screen surrounded the foil to detect where the alpha particles went.

Results

Observation	Proportion	Meaning
Most alpha particles passed straight through the gold foil	Vast majority	Most of the atom is empty space
Some alpha particles were deflected (bent off course) by small angles	A small number	The alpha particles passed close to something positive in the atom
A very few alpha particles bounced straight back (reflected back by more than 90°)	About 1 in 8,000	The alpha particles hit something very small, very dense and positively charged

Conclusions — Rutherford's Nuclear Model

From these results, Rutherford concluded:

1. The atom is mostly empty space (most alpha particles went straight through).
2. The mass and positive charge are concentrated in a tiny, dense centre — the nucleus (some alpha particles bounced back).
3. The electrons orbit the nucleus at a distance, in the empty space.

Why the Plum Pudding Model Was Replaced

The plum pudding model predicted that alpha particles would pass through with only slight deflections (because the positive charge was spread out). It could not explain why some alpha particles bounced straight back. Only a concentrated positive charge (a nucleus) could cause this.

Exam Tip: This is one of the most commonly examined topics. You must be able to describe the experiment, state the three key observations, and explain what each observation tells us about atomic structure. Always link observations to conclusions.

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4. Bohr's Shell Model (1913)

Shortly after Rutherford's model, Niels Bohr refined it. Rutherford's model had a problem — according to classical physics, orbiting electrons should lose energy and spiral into the nucleus. This clearly does not happen.

Bohr proposed that:

• Electrons orbit the nucleus in fixed energy levels (shells) at specific distances from the nucleus.
• Electrons can only exist in these defined shells — they cannot exist between them.
• Each shell has a fixed energy.
• Electrons can move between shells by absorbing or emitting energy (as electromagnetic radiation).

Evidence for Bohr's Model

Bohr's model was supported by emission spectra — when elements are heated, they emit light at specific wavelengths (colours). These specific wavelengths correspond to electrons dropping from higher energy levels to lower ones, releasing specific amounts of energy. The plum pudding and simple nuclear models could not explain these line spectra.

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5. The Modern Quantum Model (1920s Onwards)

Further research by scientists such as Schrödinger and Heisenberg led to the modern quantum mechanical model: