Models of the Atom: Practice Questions

The story of how scientists pieced together the structure of the atom is a favourite exam topic because it shows how science really works: a model is proposed, new evidence appears, and the model is changed. These questions test both the facts (who, when, what) and the reasoning (why each model was replaced). Answers are at the end.

For a narrative version, read the history of atomic models.

Section A: who proposed what?

Q1. Match each scientist to the model or discovery.

Scientist	Options
(a) John Dalton	1. Electrons in fixed energy levels
(b) J. J. Thomson	2. Discovered the neutron
(c) Ernest Rutherford	3. Atoms as solid, indivisible spheres
(d) Niels Bohr	4. Plum pudding model
(e) James Chadwick	5. Nuclear model with a tiny, dense nucleus

Q2. Put these models in the order they were proposed: Bohr model, plum pudding model, Dalton’s solid sphere model, nuclear model, quantum mechanical model.

Q3. Describe the plum pudding model in one or two sentences.

Section B: the key experiments

Q4. Thomson discovered the electron using cathode rays. Name two observations that showed cathode rays were made of negatively charged particles.

Q5. In the gold foil experiment, alpha particles were fired at a very thin sheet of gold. Describe what was observed for: (a) most of the alpha particles (b) a small number (c) a very small number (about 1 in 8,000)

Q6. For each observation in Q5, state what Rutherford concluded about the atom.

Q7. Why would the plum pudding model predict that no alpha particles bounce back?

Q8. Why was gold foil a good choice for the experiment?

Section C: refining the model

Q9. What problem with Rutherford’s nuclear model did Bohr’s model solve?

Q10. What evidence supported Bohr’s idea of fixed energy levels?

Q11. Why did scientists suspect that the nucleus contained particles other than protons before the neutron was found?

Q12. Why was the neutron harder to discover than the proton or electron?

Section D: the modern model and the nature of science

Q13. State two ways in which the quantum mechanical model differs from the Bohr model.

Q14. Dalton said atoms could not be divided. Is his model therefore “wrong”? Explain how scientists view older models.

Q15. A student says: “Scientists changed the model of the atom because the old scientists made mistakes.” Explain why this is not a good description of what happened.

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Answer key

A1.

(a) → 3, (b) → 4, (c) → 5, (d) → 1, (e) → 2.

A2.

Dalton’s solid sphere (1803–1808) → plum pudding (Thomson, 1904) → nuclear model (Rutherford, 1911) → Bohr model (1913) → quantum mechanical model (1920s). Chadwick’s discovery of the neutron came in 1932.

A3.

The atom is a ball of positive charge with negative electrons scattered through it, like currants in a pudding, so the charges balance overall. See Thomson’s plum pudding model.

A4.

Any two of:

• The rays were deflected towards a positive plate in an electric field.
• The rays were deflected by a magnetic field in the direction expected for negative charge.
• The rays were the same whatever metal the cathode was made of, so the particles were a part of all atoms.
• Thomson measured their charge-to-mass ratio, which was about 1,800 times larger than for hydrogen ions, so the particles were far lighter than atoms.

See the cathode ray tube experiment.

A5.

(a) Most passed straight through the foil. (b) Some were deflected through small angles. (c) A very few bounced back towards the source.

A6.

(a) Most of the atom is empty space. (b) There is a concentration of positive charge that repels the positive alpha particles passing nearby. (c) The positive charge and most of the mass are concentrated in a tiny, dense nucleus. Only a direct approach to a nucleus could turn an alpha particle back.

Full details: Rutherford’s gold foil experiment.

A7.

In the plum pudding model, positive charge is spread thinly through the whole atom. A fast, heavy alpha particle would feel only weak, balanced forces and pass through with at most a tiny deflection. Nothing is concentrated enough to reverse its direction.

A8.

Gold is extremely malleable, so it can be hammered into foil only a few hundred atoms thick. The alpha particles then interact with only a few layers of atoms, making each deflection easier to interpret. Gold atoms are also heavy, with a large nuclear charge, giving strong deflections.

A9.

In Rutherford’s model, electrons orbiting a nucleus should, according to classical physics, continuously radiate energy and spiral into the nucleus, so atoms would collapse. Bohr proposed that electrons exist only in fixed energy levels where they don’t radiate, which made atoms stable. See the Bohr model.

A10.

Line spectra. Elements emit and absorb light only at particular wavelengths. Bohr explained each line as an electron jumping between two fixed energy levels, releasing a photon with exactly the energy difference. His model predicted the wavelengths of hydrogen’s lines very accurately (see the hydrogen emission spectrum).

A11.

The mass of most atoms was about twice the mass of their protons. For example, helium has 2 protons but a mass of about 4 u. Something else in the nucleus had to account for the extra mass.

A12.

Neutrons have no electric charge, so they are not deflected by electric or magnetic fields and don’t leave tracks in detectors the way charged particles do. Chadwick had to detect them indirectly, by the protons they knocked out of paraffin wax. See Chadwick and the discovery of the neutron.

A13.

Any two of:

• Electrons are described by orbitals (regions of probability), not fixed circular orbits.
• An electron’s exact position and path cannot be known (the uncertainty principle).
• Energy levels contain subshells (s, p, d, f) with different shapes and energies.
• It works for all atoms, not just hydrogen.

See the quantum mechanical model.

A14.

Dalton’s model is incomplete rather than simply wrong. It correctly explained why elements combine in fixed ratios and why mass is conserved, and it is still useful for balancing equations. Scientists judge models by how well they explain the evidence available. When new evidence (such as the electron) appears, the model is refined or replaced. Older models often remain useful for simpler problems.

A15.

Each scientist built the best model the evidence of their time supported. Models changed because new experiments produced results the old models couldn’t explain: cathode rays revealed electrons, alpha scattering revealed the nucleus, line spectra revealed energy levels. That’s how science is supposed to work: models are tested against evidence and improved.

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Summary timeline

Year	Scientist	Contribution
c. 400 BCE	Democritus	Idea of indivisible atoms (story)
1803–1808	Dalton	Atomic theory; atoms as solid spheres (details)
1897	Thomson	Discovered the electron
1904	Thomson	Plum pudding model
1909–1911	Geiger, Marsden, Rutherford	Gold foil experiment; nuclear model
1913	Bohr	Electrons in fixed energy levels
1919	Rutherford	Identified the proton
1926	Schrödinger	Wave equation; basis of the quantum model
1932	Chadwick	Discovered the neutron

Key takeaways

• Atomic models developed in steps: solid sphere → plum pudding → nuclear → Bohr → quantum mechanical.
• Each change was driven by new experimental evidence: cathode rays, alpha scattering, line spectra.
• The gold foil experiment showed that atoms are mostly empty space with a tiny, dense, positive nucleus.
• The neutron explained the “missing mass” of nuclei and was hard to detect because it has no charge.
• Older models are still useful where they work; science improves models rather than simply discarding them.