Section 1 of 6Timeline of models
1Solid sphere
- Solid sphere Early in the 1800s Dalton pictured atoms as tiny, solid, indivisible spheres, with a different type for each element.
2Plum pudding
- Plum pudding After the electron was discovered, Thomson proposed a ball of positive charge with negative electrons embedded in it.
3Nuclear model
- Nuclear model Rutherford's team showed the positive charge and most of the mass are concentrated in a tiny central nucleus.
4Energy levels
- Energy levels Bohr suggested electrons orbit the nucleus in fixed shells, which fitted observations of chemical behaviour.
5Protons and neutrons
- Protons and neutrons Later work identified the proton, and Chadwick's discovery of the neutron explained the extra mass of the nucleus.
Section 2 of 6The alpha scattering experiment
1
A beam of alpha particles (positively charged) was aimed at a very thin sheet of gold foil.
2
A detector screen around the foil recorded where the particles ended up.
3
Most alpha particles passed straight through, so most of the atom is empty space.
4
A few were deflected through large angles, and a very small number bounced almost straight back.
5
Strong repulsion could only come from a tiny, dense, positively charged nucleus.
Section 3 of 6What each observation showed
| Observation | Conclusion |
|---|---|
| Most particles went straight through | The atom is mostly empty space |
| Some were deflected | The centre has positive charge, repelling positive alpha particles |
| A few rebounded backwards | The positive charge and mass are packed into a very small nucleus |
Section 4 of 6Why models change
Evidence drives change
- Evidence drives change A model is accepted only while it explains all the available results.
- New experimental evidence that does not fit forces scientists to revise it.
Peer review
- Peer review Other scientists check and repeat experiments before the new idea is accepted.
Plum pudding rejected
- Plum pudding rejected It predicted only small deflections, yet some alpha particles were turned through large angles, so it was replaced.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Nucleus | The tiny, dense, positively charged centre of an atom. |
| Alpha particle | A positively charged particle made of two protons and two neutrons. |
| Plum pudding model | Thomson's model of electrons spread through a ball of positive charge. |
| Energy level | A fixed shell in which electrons orbit the nucleus. |
| Evidence | Experimental results used to support or challenge a model. |
Section 6 of 6Exam technique
1
Saying the plum pudding model had a nucleus; it did not.
2
Crediting alpha scattering to Bohr. The experiment was carried out under Rutherford; Bohr developed energy levels afterwards.
3
Writing that atoms are mostly solid; the experiment showed they are mostly empty space.
4
Forgetting to link the evidence to the conclusion in explanation questions.