4.1.8Atomic Structure
Bohr’s Model of the Atom — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 9 of 23
Part 1 of 5Where the Bohr model fits
1Nuclear model
- Nuclear model Rutherford's model placed a tiny, dense, positive nucleus at the centre with electrons somewhere in the space around it.
2Gap in the evidence
- Gap in the evidence The nuclear model did not explain why atoms give out or take in radiation only at certain energies,
- so some experimental results could not be fully accounted for.
3Bohr's proposal (1913)
- Bohr's proposal (1913) Niels Bohr, a Danish physicist, kept the nuclear model but added a rule about where the electrons are allowed to be.
4Accepted model
- Accepted model Because it matched experimental results better, the Bohr model became the accepted picture of atomic structure.
Part 2 of 5Key features of the model
Electrons in fixed orbits
- Electrons in fixed orbits Electrons travel around the nucleus only at certain distances from it, not at any distance they like.
- Each allowed distance is called an energy level (or shell).
Different distances
- Different distances Energy levels are at different distances from the nucleus, so electrons in different levels have different amounts of energy.
- The further a level is from the nucleus, the more energy an electron there has.
Nucleus unchanged
- Nucleus unchanged The central nucleus is still positive and very dense;
- the Bohr model changes only the description of the electrons.
Part 3 of 5Filling the energy levels
| Energy level | Maximum electrons | Example |
|---|---|---|
| First (closest to nucleus) | 2 | Helium has 2 electrons, so its first level is full |
| Second | 8 | Neon has 2, 8 electrons in total |
| Third | 8 (at GCSE) | Argon has 2, 8, 8 electrons |
Part 4 of 5Why scientists accepted it
Strengths of the Bohr model
Explained experiments
- Explained experiments It accounted for results that the plain nuclear model could not explain.
Radiation absorbed and emitted
- Radiation absorbed and emitted It explained how atoms absorb and emit electromagnetic radiation: an electron can move between energy levels when it gains or loses energy.
Calculations matched
- Calculations matched Predictions calculated using the model agreed closely with measurements from experiments.
Part 5 of 5Answering an exam question
1
State what the two models have in common: both have a positively charged nucleus with electrons moving around it.
2
State the difference: in the Bohr model electrons are only found at specific distances, called energy levels.
3
State why that matters: the Bohr model explained experimental evidence better than the nuclear model did.
4
Link to the scientific method: a model is kept only while it fits the evidence, and it is changed when new evidence does not fit.
Key terms
| Term | Meaning |
|---|---|
| Energy level | One of the allowed orbits at a fixed distance from the nucleus where electrons can be found. |
| Nuclear model | The model with a small, dense, positive nucleus surrounded by mostly empty space containing electrons. |
| Absorption | An atom taking in electromagnetic radiation, which can raise an electron to a higher energy level. |
| Emission | An atom giving out electromagnetic radiation, which can happen when an electron falls to a lower energy level. |
| Model | A simplified idea used to explain observations; it is improved or replaced when evidence disagrees with it. |
Avoid these mistakes
1
Saying electrons can orbit at any distance in the Bohr model - they are restricted to specific energy levels.
2
Putting more than 2 electrons in the first energy level; remember the pattern 2, 8, 8 for the first three levels.
3
Claiming the Bohr model was proved to be perfect - it was accepted because it agreed with evidence better than earlier models.
4
Forgetting that Bohr kept the nucleus from the nuclear model; he refined the electron arrangement rather than starting again.
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