OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how and why the atomic model has changed over time (to include:…
Full specification wording
The particle model: describe how and why the atomic model has changed over time (to include: the Thomson, Rutherford (alongside Geiger and Marsden) and Bohr models)
The atomic model developed as new evidence challenged old ideas.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The atomic model developed as new evidence challenged old ideas. Thomson proposed that atoms contain small negative electrons embedded in a spread of positive charge, often pictured as a plum-pudding model, after electrons were discovered. Rutherford, working with Geiger and Marsden, fired alpha particles at thin gold foil; most passed straight through but some were deflected strongly, so he concluded that positive charge and most mass sit in a tiny central nucleus with mostly empty space around it. Bohr then suggested that electrons orbit the nucleus in fixed shells or energy levels, explaining why atoms emit and absorb particular energies. Each change happened because experimental evidence did not fit the previous model.
What earns the marks
- Describes the Thomson model as negative electrons set in a spread of positive charge.
- Explains that Thomson's model followed the discovery of the electron.
- Describes the Geiger and Marsden alpha-particle scattering result, with most particles passing through and a few deflected strongly.
- Explains that Rutherford interpreted the scattering as a tiny, dense, positive nucleus surrounded by mostly empty space.
- Describes the Bohr model as electrons in fixed shells or energy levels around the nucleus.
- Explains that each model changed because new experimental evidence did not fit the earlier model.
Common mistakes and how to put them right
Credits Thomson with the nuclear model.
Put it right: Keep Thomson as electrons in a positive spread and give the nucleus to Rutherford.
Says all alpha particles were deflected.
Put it right: Most passed through and only a few were deflected strongly.
Places electrons anywhere in the atom in the Bohr model.
Put it right: Put them in fixed shells or energy levels.
Treats the models as guesses with no evidence.
Put it right: Link each change to experimental evidence that did not fit.
Exam tips
- Name each scientist with the key feature of their model so the examiner can match them quickly.
- Use the gold-foil scattering result as the evidence that supports the nuclear model.
- State the reason for change: new evidence contradicted the previous model.
Try the lesson
The start of the The particle model: describe how and why the atomic model has changed over time (to include: the Thomson, Rutherford (alongside Geiger and Marsden) and Bohr models) lesson, made from checked teaching notes for this specification point.
Have a go first
In the plum pudding model of the atom, how was the positive charge arranged?
Listen
The atomic model developed as new evidence challenged old ideas. Thomson proposed that atoms contain small negative electrons embedded in a spread of positive charge, often pictured as a plum-pudding model, after electrons were discovered. Rutherford, working with Geiger and Marsden, fired alpha particles at thin gold foil; most passed straight through but some were deflected strongly, so he concluded that positive charge and most mass sit in a tiny central nucleus with mostly empty space around it. Bohr then suggested that electrons orbit the nucleus in fixed shells or energy levels, explaining why atoms emit and absorb particular energies. Each change happened because experimental evidence did not fit the previous model. A strong answer earns marks for these points. Describes the Thomson model as negative electrons set in a spread of positive charge. Explains that Thomson's model followed the discovery of the electron. Describes the Geiger and Marsden alpha-particle scattering result, with most particles passing through and a few deflected strongly. Explains that Rutherford interpreted the scattering as a tiny, dense, positive nucleus surrounded by mostly empty space. Exam tip: Name each scientist with the key feature of their model so the examiner can match them quickly.
Key words
Say what each word means before you turn the card.
Thomson's plum-pudding model
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Rutherford, Geiger and Marsden alpha-particle scattering
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Bohr's electron shells and energy levels
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Role of evidence in changing scientific models
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See every definition
- Thomson's plum-pudding model
- Atoms contain small negative electrons embedded in a spread of positive charge, often pictured as a plum-pudding model.
- Rutherford, Geiger and Marsden alpha-particle scattering
- Alpha particles fired at thin gold foil mostly passed straight through but some were deflected strongly.
- Bohr's electron shells and energy levels
- Electrons orbit the nucleus in fixed shells or energy levels, explaining why atoms emit and absorb particular energies.
- Role of evidence in changing scientific models
- Each change happened because experimental evidence did not fit the previous model.
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