Section 1 of 7Why models change
What a model is
- What a model is A model is a simplified description used to explain observations and make predictions.
Science is revised
- Science is revised When new experimental evidence cannot be explained by the current model, the model is changed or replaced.
Section 2 of 7Timeline of atomic models
1Early idea
- Early idea Democritus suggested matter is made of tiny indivisible particles; the word atom comes from the Greek for 'cannot be cut'.
- Dalton later pictured atoms as solid spheres.
2Plum pudding
- Plum pudding After J.
- J.
- Thomson discovered the electron, he proposed a ball of positive charge with negative electrons embedded in it, like plums in a pudding.
3Nuclear model
- Nuclear model Rutherford's team found that the positive charge and most of the mass is concentrated in a tiny nucleus.
4Bohr model
- Bohr model Bohr showed electrons orbit at fixed distances, explaining the energy levels seen in chemistry.
5Neutron
- Neutron Chadwick provided evidence for neutrons in the nucleus, completing the model used today.
Section 3 of 7Alpha scattering experiment
1
Geiger and Marsden, working with Rutherford in 1909, fired positively charged alpha particles at very thin gold foil.
2
A detector screen recorded where the alpha particles went after passing through the foil.
3
The plum pudding model predicted that nearly all particles would pass through with only tiny deflections.
4
Most particles did pass straight through, some were deflected through large angles, and a very small number bounced back.
5
This showed that the model was wrong and a new one was needed.
Section 4 of 7Observations and conclusions
| Observation | Conclusion |
|---|---|
| Most alpha particles went straight through | The atom is mostly empty space |
| Some were deflected | The nucleus is positive and repels the positive alpha particles |
| A few bounced straight back | The positive charge is concentrated in a tiny, dense nucleus that contains most of the mass |
Section 5 of 7Comparing the two models
| Feature | Plum pudding | Nuclear model |
|---|---|---|
| Positive charge | Spread through whole atom | Concentrated in the nucleus |
| Electrons | Embedded in the positive ball | Orbit outside the nucleus |
| Mass | Spread out | Almost all in the nucleus |
| Explains alpha scattering? | No | Yes |
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Model | A simplified representation used to explain and predict how something behaves. |
| Alpha particle | A helium nucleus made of two protons and two neutrons. |
| Nucleus | The tiny, dense, positively charged centre of an atom. |
| Plum pudding model | Thomson's model of the atom as a positive ball with embedded electrons. |
| Deflect | To change direction due to a force. |
Section 7 of 7Exam technique
1
Saying Rutherford discovered the electronThomson did, and Rutherford's team found the nucleus.
2
Claiming the alpha particles bounced back because they hit electrons: electrons are far too light, so a heavy, positive nucleus is needed.
3
Writing that old models were stupideach fitted the evidence of the time and was replaced when new evidence appeared.
4
Mixing up what was observed with what was concluded: say what happened first, then explain what it shows.