The Development of the Model of the Atom

From a solid sphere to the nuclear atom — and the experiments that forced each change. AQA tests not just what each model says, but why it was replaced. Get the experimental evidence and the model names exactly right.

SPEC 4.1.1.3 • ATOMIC STRUCTURE

Why This Lesson

This is one of the most accessible question topics in the whole spec — but also one of the most marked-down. Why? Because it's a definitions topic, and students almost get the words right. "Ball of positive energy" instead of "ball of positive charge". "Atomic model" instead of "nuclear model". Each substitution loses a mark.

What AQA tests

Two skills: (1) describe each model precisely, using the spec's own language, and (2) explain why new experimental evidence forced each model change. The alpha particle scattering experiment is the centrepiece — every result links to a specific conclusion about atomic structure.

Examiner Reports flag

Three recurring errors: writing "positive energy" (should be "charge"); claiming the plum pudding model has an "overall positive" charge (it's neutral); and writing generic names like "atomic model" (specific names — plum pudding / nuclear / Bohr — are required).

The Solid Sphere (Dalton)

Before 1897, atoms were thought to be tiny indivisible spheres. This is the model attributed to John Dalton — atoms were the smallest particle of matter, with no internal structure.

Solid sphere model

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.

What changed it

In 1897, J.J. Thomson discovered the electron — a particle smaller than the atom, carrying a negative charge. This proved atoms can be divided into smaller pieces. The solid-sphere model was no longer adequate.

Spec wording for this model

AQA's exact wording: "atoms were thought to be tiny spheres that could not be divided." Use this phrase if asked to describe the pre-electron model.

The Plum Pudding Model (Thomson, 1904)

The discovery of the electron led directly to a new model. Thomson proposed that atoms contained smaller particles arranged in a specific way:

Plum pudding model

The atom is a ball of positive charge with negative electrons embedded in it. The atom is neutral overall: the positive ball is balanced by the negative electrons.

Mark-scheme phrase

"A ball of positive charge with negative electrons embedded in it. Neutral overall."

The Alpha Scattering Experiment (1909)

Geiger and Marsden, working under Rutherford, fired positively charged alpha particles at a thin sheet of gold foil. They expected (based on plum pudding) that all particles would pass straight through with only minor deflections. Instead:

The three observations

1.Most alpha particles passed straight through. 2. A small number were deflected at angles. 3. A very few bounced almost straight back.

Decoding the Three Results

Each observation forced a specific conclusion about the structure of the atom. This is one of AQA's favourite question patterns: give one observation, ask for the conclusion.

ObservationConclusion about the atom

Most alpha particles passed straight throughThe atom is mostly empty space — there's nothing in the way of most particles.

A small number were deflectedThere must be a concentrated positive charge somewhere that repels the (positive) alpha particles.

A very few bounced almost straight backThe positive charge and almost all the mass are concentrated in a tiny, dense region — the nucleus — at the centre.

Why this killed the plum pudding

If charge were spread out (as plum pudding suggests), you'd expect small deflections from most particles — not the observed pattern of mostly nothing, then a few extreme deflections. The data didn't fit. The model had to change.

The Nuclear Model (Rutherford, 1911)

Rutherford proposed a new model that fit the alpha scattering data:

Nuclear model

The mass of the atom is concentrated at the centre in a tiny nucleus. The nucleus is positively charged. The rest of the atom is mostly empty space, with electrons somewhere outside the nucleus.

Mark-scheme essentials

Two phrases must appear: "mass concentrated at the centre / in the nucleus" and "nucleus is (positively) charged". A third mark is often available for "most of the atom is empty space".

Bohr's Adaptation (1913)

Rutherford's model said electrons existed somewhere outside the nucleus — but didn't say where. Niels Bohr refined it:

Bohr model

Electrons orbit the nucleus at specific distances — fixed energy levels (also called shells). Bohr's theoretical calculations agreed with experimental observations.

AQA accepts both

Mark schemes accept "shells" or "energy levels" interchangeably. The phrase that earns the mark is "specific distances" — Bohr's contribution was the idea that the orbits are fixed, not just anywhere.

Protons, Neutrons & How Science Progresses

Two more discoveries completed the modern picture of the nucleus. Spec scope is short here: AQA does not require the experimental detail of Chadwick's work — but it does test the broader point about how scientific models change.

Proton

Later experiments showed the positive charge of any nucleus could be split into a whole number of smaller particles, each with the same amount of positive charge. These were named protons.

Neutron — Chadwick (1932)

The experimental work of James Chadwick provided evidence for neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

How scientific models change

The 20-year gap is itself part of the lesson. New scientific models aren't accepted overnight — they go through peer review (other scientists check the evidence and challenge the conclusions), and they're only adopted once the new evidence convinces the wider scientific community. Chadwick's neutron was eventually accepted because his experimental evidence was strong and reproducible.

Spec scope note

"Details of experimental work supporting the Bohr model are not required. Details of Chadwick's experimental work are not required." You only need to know what was discovered and by whom, not how. Don't waste exam time describing Chadwick's beryllium experiments — focus on the timeline and the role of peer review.

The Timeline

Five stages, in order. AQA Foundation has asked questions where you place particles in order of discovery — this is the sequence:

Pre-1897

Solid sphere model (Dalton). Atoms = tiny indivisible spheres.

1897

Electron discovered (Thomson). First subatomic particle found.

1904

Plum pudding model (Thomson). Ball of positive charge with electrons embedded.

1909

Alpha scattering experiment (Geiger, Marsden, Rutherford). Most particles pass through; some deflect; very few bounce back.

1911

Nuclear model (Rutherford). Mass and positive charge concentrated in a tiny nucleus; mostly empty space.

1913

Bohr model. Electrons orbit at specific distances (energy levels). Calculations matched experiment.

1932

Neutron evidence (Chadwick). ~20 years after the nucleus became accepted.

Order of particle discovery

Earliest → Latest: electron (1897) → proton → neutron (1932)

Plum Pudding vs Nuclear

The spec explicitly requires you to be able to "describe the difference between the plum pudding model and the nuclear model". A direct comparison question.

FeaturePlum puddingNuclear

Positive chargeSpread out as a "ball" filling the whole atomConcentrated in a tiny nucleus at the centre

Mass distributionSpread evenly throughout the atomAlmost all in the nucleus

ElectronsEmbedded inside the positive ballOutside the nucleus, in the empty space around it

Empty spaceNone — the atom is "filled"Most of the atom is empty space

Easy 2-mark template

"In the plum pudding model the positive charge is spread out across the whole atom. In the nuclear model the positive charge is concentrated in a tiny nucleus at the centre, and most of the atom is empty space."

PEE Framework — Worked Chain

Q: Explain why the alpha scattering experiment led to the plum pudding model being replaced. 3 MARKS

P — PointThe plum pudding model could not explain the results of the alpha scattering experiment, so it was replaced by the nuclear model.

E — EvidenceMost alpha particles passed straight through (atom is mostly empty space); some were deflected (concentrated positive charge); a very few bounced back (mass concentrated in a tiny dense region).

E — ExplainPlum pudding predicted small deflections everywhere — not the observed pattern. New experimental evidence forced the model to change to one with a tiny, dense, positively charged nucleus.

How marks are scored

Mark 1: an observation (e.g. "most pass through"). Mark 2: a corresponding conclusion ("atom is mostly empty space"). Mark 3: explicit statement that the new model places positive charge / mass in a tiny nucleus. Quoting the experiment alone is not enough — link each result to a structural conclusion.

Examiner Alert

Three errors flagged across P1 Jun 2023 + P1 Jun 2024

P1 Jun 2023 Q01.1: students wrote "ball of positive energy" instead of "positive charge" — mark lost. Energy and charge are different concepts.

P1 Jun 2024 Q03.2: students said the plum pudding model has an "overall positive charge" — wrong. The atom is neutral overall: the positive ball is balanced by the embedded negative electrons.

P1 Jun 2024 Q03.1: students named the model as "atomic model" or "modern model" — both lose the mark. Specific names required: plum pudding, nuclear, or Bohr.

Weak student

"The plum pudding model is a ball of positive energy with negative electrons. The atom has an overall positive charge."

Two errors: "energy" → should be charge; "overall positive" → should be neutral overall.

Strong student

"The plum pudding model is a ball of positive charge with negative electrons embedded in it. The atom is neutral overall."

Hits both marks: "charge" + neutral overall.

Common Errors

Students often writeCorrect versionWhy it matters

"Ball of positive energy ""Ball of positive charge "Energy and charge are different physical quantities

"Overall positive charge" (plum pudding)" Neutral overall — positive ball balanced by negative electrons"All atomic models describe a neutral atom

" Atomic model " or " modern model "Plum pudding / nuclear / BohrGeneric names not accepted by mark schemes

"Bohr proved electrons orbit the nucleus"Bohr proposed electrons orbit at specific distances ; calculations matched experimentRutherford had electrons outside the nucleus already; Bohr added the specific distances idea

"The experiment fired electrons at gold foil"Alpha particles fired at gold foilAlpha particles are positively charged — that's why deflection reveals the positive nucleus

Practice Question 1

Describe the plum pudding model of the atom.2 MARKS

(Pattern from AQA 8462 Paper 1, June 2023 Q01.1)

1

The atom is a ball of positive charge (not "energy" — this loses the mark). [1 mark]

2

With negative electrons embedded in it, making the atom neutral overall. [1 mark]

Final 2-mark answer: "The plum pudding model has the atom as a ball of positive charge with negative electrons embedded in it. It is neutral overall."

Practice Question 2

In the alpha particle scattering experiment, the following three observations were made. Give the conclusion that can be drawn from each.3 MARKS

1

Most alpha particles passed straight through the gold foil.

2

A small number of alpha particles were deflected.

3

A very small number of alpha particles bounced almost straight back.

1

Most pass through → the atom is mostly empty space. [1 mark]

2

Small number deflected → there is a concentrated positive charge in the atom that repels the (positively charged) alpha particles. [1 mark]

3

Very few bounce back → the positive charge and almost all the mass are concentrated in a tiny, dense region (the nucleus) at the centre of the atom. [1 mark]

Examiner note: each conclusion must be linked clearly to its observation. "The atom has a nucleus" alone is not enough — you need the size/mass detail. Watch out for "atom is hollow" — this is too vague; "mostly empty space" is the mark-scheme phrase.

Practice Question 3

(a) Name the model of the atom that came before the nuclear model. 1 MARK

(b) Explain how the alpha particle scattering experiment led to that earlier model being replaced. Use ideas about both the observations and the structure of the atom. 3 MARKS

(a)

1

The plum pudding model.[1 mark — specific name required; "atomic model" loses the mark]

(b)

P

The plum pudding model could not explain the alpha scattering results. [1 mark]

E

Plum pudding has the positive charge spread evenly across the whole atom — it predicts only small deflections. But the experiment showed most alpha particles passed straight through, and a very small number bounced back at large angles. [1 mark]

E

These results are only explained if the positive charge and mass are concentrated in a tiny, dense nucleus, with most of the atom being empty space — the nuclear model. [1 mark]

Summary

The Five-Stage Sequence

Solid sphere (pre-1897): tiny spheres that could not be divided

Plum pudding (Thomson, 1904): ball of positive charge with negative electrons embedded; neutral overall

Nuclear (Rutherford, 1911): mass and positive charge concentrated in a tiny nucleus; mostly empty space

Bohr (1913): electrons orbit at specific distances (energy levels / shells); calculations matched experiment

Neutron evidence (Chadwick, 1932): ~20 years after the nucleus was accepted

Alpha Scattering — Three Observations, Three Conclusions

Most pass through → atom is mostly empty space

Some deflected → there is a concentrated positive charge

Very few bounce back → mass is in a tiny, dense nucleus at the centre

Exam Tip — three things to never write

(1) "Positive energy" — write "positive charge". (2) "Overall positive" — write "neutral overall". (3) "Atomic model" / "modern model" — write the specific name (plum pudding, nuclear, Bohr).

Up Next — Lesson 1.04: Subatomic Particles & Relative Atomic Mass

The three subatomic particles (proton, neutron, electron) — their charges, masses, and locations. The atomic number, mass number, and how to calculate relative atomic mass from isotope abundances.