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Physics · Atomic structure

The development of the model of the atom

Describe how the model of the atom has changed with new experimental evidence, including the plum pudding model, the alpha scattering experiment and the nuclear model.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    Who discovered the electron?

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    J. J. Thomson

  2. 2

    What is a scientific model?

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    A simplified representation used to explain observations

  3. 3

    What is an alpha particle?

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    Two protons and two neutrons (a helium nucleus)

  4. 4

    What is evidence?

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    Results from experiments that support or reject an idea

  5. 5

    Who discovered the neutron?

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    James Chadwick

Learning Objectives

  1. 1Describe how the atomic model has changed over time.
  2. 2Describe the plum pudding model and the nuclear model.
  3. 3Explain why the alpha particle scattering experiment led to the nuclear model.
  4. 4Describe the contributions of Bohr, the discovery of the proton and Chadwick's neutron.

CHANGING MODELS

New experimental evidence may lead to a scientific model being changed or replaced.

The atom has been thought of as a solid sphere, a plum pudding, a nuclear atom, then with electrons in specific orbits, and finally with protons and neutrons in the nucleus.

Evidence and Conclusions

Match each result to its explanation.

  • Most alpha particles went straight through

    Conclusion: Most of the atom is empty space

  • Some were deflected through small angles

    Conclusion: The nucleus is positive and repels the positive alpha particles

  • A very few bounced back

    Conclusion: The mass is concentrated in a tiny, dense, charged nucleus

Plum Pudding or Nuclear?

Plum pudding model

  • A ball of positive charge.
  • Negative electrons embedded in it.
  • Charge and mass spread throughout the atom.

Nuclear model

  • A tiny, positively charged nucleus at the centre.
  • Electrons orbit at a distance.
  • Most of the atom is empty space.

Why the Model Changed

Explain why the results of the alpha scattering experiment led to the plum pudding model being replaced.

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  1. 1 Plum pudding prediction Alpha particles should pass through with only small deflections
  2. 2 Result Some were deflected by large angles and a few rebounded
  3. 3 Conclusion A concentrated, positive nucleus must exist, so the nuclear model replaced the plum pudding model

AnswerThe scattering results could not be explained by the plum pudding model, so the nuclear model replaced it.

The Story

Learn the sequence.

  • Before the electron

    Atoms were thought to be tiny spheres that could not be divided.

  • Discovery of the electron

    Led to the plum pudding model.

  • Alpha scattering

    Led to the nuclear model; the nucleus was charged.

  • Bohr

    Adapted the nuclear model with electrons orbiting at specific distances; his calculations agreed with experiments.

  • Protons

    The nucleus's charge was divided into whole-number units: protons.

  • Chadwick

    Provided evidence for neutrons about 20 years after the nucleus was accepted.

Predict the Result

If the plum pudding model were correct, what would happen to alpha particles fired at gold foil? What actually happened, and what did it show?

1. Predict with plum pudding.

2. Compare with the result.

A good answer shows: They would pass through with small deflections. In fact most passed straight through, some were deflected by large angles and a few bounced back, showing a small dense positive nucleus.

Can I...?

  1. 1Describe the plum pudding model.
  2. 2Describe the nuclear model.
  3. 3Describe the alpha scattering results.
  4. 4Explain why the model changed.
  5. 5Say what Bohr added.
  6. 6Say what Chadwick showed.
  7. 7Explain why models change.
  8. 8Order the models.

Summary & Exam Focus

  • Plum pudding: ball of positive charge with electrons in it.
  • Nuclear: small positive nucleus with electrons around it.
  • Alpha scattering evidence changed the model.
  • Bohr: specific orbits. Chadwick: neutrons.

Exam focus

Describe the difference between the plum pudding model and the nuclear model of the atom. (3 marks) (3 marks)

Say where the positive charge and mass are in each.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Model
A representation used to explain observations.
Plum pudding model
A ball of positive charge with electrons embedded.
Alpha scattering
An experiment in which alpha particles are fired at gold foil.
Nuclear model
An atom with a small, dense, positive nucleus.
Bohr model
Electrons orbit the nucleus at specific distances.
Evidence
Experimental results that support or contradict a model.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Describe 2 marks

    Describe the plum pudding model of the atom.

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    Model answer

    The atom is a ball of positive charge with negatively charged electrons embedded in it.

    Mark scheme

    • Ball of positive charge — 1 mark
    • Electrons embedded in it — 1 mark
  2. Question 2 Explain 4 marks

    The diagram shows the paths of alpha particles fired at thin gold foil. Explain how these results led to the nuclear model of the atom.

    Paths of alpha particles through thin gold foil, mostly straight through, a few deflected and one bouncing back.
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    Model answer

    Most alpha particles went straight through, so most of the atom is empty space. Some were deflected, so the nucleus is positively charged (repelling alpha particles). A few bounced back, so the mass is concentrated in a small, dense nucleus. The plum pudding model could not explain this.

    Mark scheme

    • Most went straight through so the atom is mostly empty space — 1 mark
    • Some deflected: the nucleus is positive — 1 mark
    • A few rebounded: mass concentrated in a small nucleus — 1 mark
    • Plum pudding could not explain this — 1 mark
  3. Question 3 Describe 3 marks

    Describe how the nuclear model of the atom differs from the plum pudding model.

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    Model answer

    In the nuclear model the positive charge and most of the mass are concentrated in a tiny nucleus with electrons around it; in the plum pudding model the positive charge is spread throughout the atom with electrons embedded in it.

    Mark scheme

    • Nuclear: positive charge and mass in a small nucleus — 1 mark
    • Electrons outside the nucleus — 1 mark
    • Plum pudding: charge spread out — 1 mark
  4. Question 4 State 2 marks

    State two ways in which Bohr changed the nuclear model, or what the later experiments showed.

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    Model answer

    Bohr suggested electrons orbit the nucleus at specific distances. Later experiments showed the nucleus contains protons and neutrons (Chadwick for neutrons).

    Mark scheme

    • Electrons orbit at specific distances — 1 mark
    • Protons and neutrons in the nucleus — 1 mark
  5. Question 5 Explain 6 marks

    Describe how the model of the atom has changed, from the idea of a solid sphere to the model with protons and neutrons in the nucleus. Explain why each change was made.

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    Model answer

    See levels-of-response scheme.

    Mark scheme

    • Level 3 (5 to 6 marks): a correct sequence (solid sphere, plum pudding, nuclear model, Bohr, protons and neutrons) with the evidence for at least three changes, including alpha scattering — 5 to 6 marks
    • Level 2 (3 to 4 marks): at least three models named with some evidence — 3 to 4 marks
    • Level 1 (1 to 2 marks): one or two models described — 1 to 2 marks

Quick check

  1. The plum pudding model was replaced after...

    1. Athe discovery of the neutron
    2. Bthe discovery of X-rays
    3. CBohr's model
    4. Dthe alpha scattering experiment
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    D: the alpha scattering experiment

    The results could not be explained by it.

  2. Most alpha particles passed through gold foil because...

    1. Athe atom is mostly empty space
    2. Bthe foil was thick
    3. Calpha particles are neutral
    4. Dthe nucleus is large
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    A: the atom is mostly empty space

    Most of the atom is empty.

  3. Some alpha particles bounced back because...

    1. Athe electrons are heavy
    2. Bthe nucleus is small, dense and positive
    3. Cthe foil is negative
    4. Dgold is magnetic
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    B: the nucleus is small, dense and positive

    They hit or were repelled by the nucleus.

  4. Bohr suggested that electrons...

    1. Aare inside the nucleus
    2. Bare neutral
    3. Corbit at specific distances
    4. Ddo not exist
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    C: orbit at specific distances

    That is the Bohr model.

  5. Neutrons were shown to exist by...

    1. AMarie Curie
    2. BBohr
    3. CThomson
    4. DJames Chadwick
    Show answerHide answer

    D: James Chadwick

    About 20 years after the nucleus was accepted.

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