1.2.1Particles
Atomic Models Through Time — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 6
Section 1 of 6Models change when evidence changes
How science works
- How science works A scientific model is an idea used to explain observations.
- When new experiments give results that the current model cannot explain, the model has to be modified or replaced.
- Each new atomic model therefore built on the one before it.
Dalton, 1803
- Dalton, 1803 John Dalton pictured atoms as tiny solid spheres that cannot be created, destroyed or divided.
- He proposed that all atoms of an element are identical, that atoms of different elements differ,
- and that atoms join in different ways to make compounds.
Section 2 of 6From the solid sphere to the nucleus
1Thomson finds the electron (1897)
- Thomson finds the electron (1897) J.J.
- Thomson used a cathode ray tube and showed that the beam was deflected towards a positive plate.
- It was made of negatively charged particles, now called electrons, which are far smaller than atoms.
- So atoms could be divided.
2Plum pudding model
- Plum pudding model Thomson suggested a ball of positive charge with negative electrons embedded throughout it, like fruit in a pudding,
- so that the atom was neutral overall.
3Gold foil experiment (1909)
- Gold foil experiment (1909) Geiger and Marsden fired alpha particles at very thin gold foil under Rutherford's direction.
- Most passed straight through, some were deflected by large angles, and a very few bounced almost straight back.
4Nuclear model
- Nuclear model The plum pudding model predicted only small deflections.
- Rutherford concluded that an atom is mostly empty space, with a tiny, dense, positive nucleus at its centre that holds most of the mass,
- and electrons moving around it.
Section 3 of 6Bohr and later developments
Bohr model, 1913
- Bohr model, 1913 Niels Bohr proposed that electrons orbit the nucleus only in fixed shells (energy levels) at set distances from it.
- Electrons in shells further from the nucleus have higher energy.
- This explained why electrons do not spiral into the nucleus.
Protons and neutrons
- Protons and neutrons Experiments showed the nucleus contains positively charged protons.
- Later, James Chadwick provided evidence for neutral particles of similar mass, called neutrons, in the nucleus.
Why the Bohr model is still taught
- Why the Bohr model is still taught Bohr's results agreed with experimental evidence such as the energies of light emitted by atoms.
- This model of electron shells is still used at GCSE to explain bonding and the Periodic Table.
Section 4 of 6Comparing the models
| Model | Key features | Evidence |
|---|---|---|
| Dalton | Solid, indivisible sphere | Laws of chemical combination |
| Plum pudding | Electrons embedded in a positive sphere | Discovery of the electron by deflection of cathode rays |
| Nuclear | Tiny dense positive nucleus, mostly empty space, electrons around it | Alpha scattering by gold foil |
| Bohr | Electrons in fixed shells at set energies | Atoms emit and absorb light of particular energies |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Atom | The smallest part of an element that still has the properties of that element. |
| Electron | A tiny negatively charged particle found in shells outside the nucleus. |
| Nucleus | The dense, positively charged centre of an atom that contains the protons and neutrons. |
| Alpha particle | A positively charged particle used to probe atoms in the gold foil experiment. |
| Energy level (shell) | A region around the nucleus in which electrons of a given energy are found. |
Section 6 of 6Exam technique
1
Saying Rutherford discovered the electron — Thomson did; Rutherford's work showed the nucleus.
2
Claiming that all alpha particles bounced back — most went straight through, and that is how we know atoms are mostly empty space.
3
Writing that the plum pudding model has a nucleus; it does not, and its positive charge is spread through the whole atom.
4
Describing the old model as 'wrong' without linking the change to new experimental evidence.
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