GCSE, A-Level & IB Physics and Chemistry
For about seven years, scientists thought the atom was a ball of positive charge with electrons dotted through it like fruit in a pudding. Then three people in a Manchester laboratory fired particles at a sheet of gold foil, and the model fell apart in an afternoon.
This page covers what the plum pudding model said, why it was a reasonable idea at the time, and exactly how the gold foil experiment disproved it — including the three observations and the three conclusions that exam questions ask for again and again.
The model was put forward by J.J. Thomson in 1904. It describes the atom as:
Scroll the diagram sideways.
Where the name comes from
A plum pudding is a Christmas pudding with dried fruit set into it. The pudding is the positive charge and the fruit are the electrons. Thomson never used the phrase himself — it was attached to the model later — but it describes the picture well, which is why it stuck.
The one thing to be clear about
In this model there is no nucleus. That is the whole point of it, and it is what makes the model wrong. If you draw a plum pudding atom with a blob in the middle, you have drawn a different model.
It is easy to laugh at a wrong model, but Thomson’s was a genuinely good piece of reasoning. It is worth understanding why, because exam questions sometimes ask what evidence it was based on.
So it was not a wild guess. It explained everything known at the time: atoms contain electrons, atoms are neutral, atoms are roughly spherical. It was wrong, but it was wrong for a respectable reason — nobody had yet done an experiment that could see inside an atom.
This is the experiment that ended the plum pudding model. Get the names right, because examiners do: it was carried out by Hans Geiger and Ernest Marsden, working under Ernest Rutherford‘s direction at Manchester. Marsden was an undergraduate at the time. Rutherford did not do the experiment himself — he set the problem and later worked out what the results meant.
Three dates, often muddled into one
Scroll the diagram sideways.
| Part | Why it was there |
|---|---|
| Alpha source in a lead block | Alpha particles are positive and relatively heavy. The lead absorbs everything except a narrow beam through the slit. |
| Very thin gold foil | Gold can be hammered just a few atoms thick, so a particle meets almost no obstacles on its way through. Thin matters: thick foil would confuse the results. |
| Detector screen all around | It had to surround the foil, not just sit behind it. If the screen were only behind, nobody would ever have seen the particles that came back — and that is the whole discovery. |
| A vacuum | Alpha particles are absorbed by air, so the apparatus had to be evacuated. |
If the plum pudding model were right, the positive charge in the foil would be thinly spread. An alpha particle would feel only a weak push from any part of it, so every particle should sail through with at most a tiny deflection.
Scroll the diagrams sideways.
Rutherford’s reaction
He later described the result as being “as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” A 15-inch naval shell weighs about a tonne. Nothing in the plum pudding model could turn one round.
This is the part exam questions are built on, and the six-mark version follows exactly this shape: what the old model predicted, what was actually seen, and what that forces you to conclude. Learn it as three columns, not as a list of facts.
| Plum pudding predicted | What was seen | So we conclude |
|---|---|---|
| Every particle slowed slightly by passing through solid matter | Most passed straight through with no deflection at all | The atom is mostly empty space. |
| At most a tiny nudge, well under one degree, from thinly spread charge | Some were deflected through noticeable angles | The centre is positively charged, because like charges repel. |
| Nothing could ever send a particle backwards | A very few bounced back past ninety degrees | The centre is tiny and very dense, holding most of the atom’s mass. |
How to remember which is which
The mark students miss
The third observation is worth two conclusions, not one. Most answers say “there is a nucleus” and stop. Say that the nucleus is small and that it is dense, containing most of the mass, and explain that the rarity of the bounce-back is what shows it is small.
Rutherford published his interpretation in 1911: the atom has a tiny, dense, positively charged nucleus at its centre, with electrons somewhere outside it and empty space in between. Roughly one alpha particle in a few thousand came back at more than 90° — rare enough to tell him the nucleus is minute.
“Mostly empty space” is a phrase students repeat without feeling it. The numbers are worth a moment.
| Thing | Diameter |
|---|---|
| An atom | about 10−10 m |
| Its nucleus | about 10−15 m |
What that ratio means
The nucleus is roughly 100,000 times smaller across than the atom. Scale the atom up to the size of a football pitch, 100 m long, and the nucleus would be about 1 mm across — a grain of sand on the centre spot. Everything else is empty.
And because volume goes as the cube of the diameter, a ratio of 100,000 across means the nucleus occupies roughly one part in 1015 of the atom’s volume — a thousandth of a trillionth.
This is the answer to observation one
Students quote “mostly empty space” without connecting it to anything. The connection is this: if the nucleus fills only one part in a thousand trillion of the atom, then an alpha particle fired at a sheet of gold will almost always miss every nucleus in its path. Hitting one is like firing a bullet across a football pitch and clipping a single grain of sand. The scale is not a side fact — it is the reason most particles went straight through.
Rutherford’s model was a huge improvement, but it had a problem of its own, and examiners like asking about it.
The flaw in Rutherford’s atom
In Rutherford’s picture the electrons orbit the nucleus freely. But a charged particle moving in a circle is accelerating, and an accelerating charge continuously radiates energy. Losing energy, the electron should spiral inwards and crash into the nucleus. Work the classical physics through and an atom should collapse in about 10−11 seconds — a hundred-billionth of a second. Atoms plainly last longer than that, so something in the model was wrong.
That is the model you draw at GCSE and the one most A-level and IB questions assume: a nucleus of protons and neutrons, with electrons in shells around it.
| Course | What is asked |
|---|---|
| GCSE AQA, Edexcel, OCR | Describe each model, state the three observations and their conclusions, and explain why models change when new evidence appears. Physics and chemistry both. |
| A-level AQA, Edexcel, OCR | The same, plus the instability of Rutherford’s model, and calculating the distance of closest approach from the alpha particle’s energy. |
| IB Physics atomic structure | Less description, more interpretation: what the evidence shows, why each model was replaced, and the Bohr model applied to hydrogen. |
| IB Chemistry | The models as background to electron arrangement and emission spectra, rather than as history for its own sake. |
Scroll the diagrams sideways.
| Who and when | The model | What forced the change |
|---|---|---|
| Dalton early 1800s | Atoms are tiny solid spheres that cannot be divided. | Thomson found the electron, so atoms clearly do have parts. |
| Thomson 1904 | Plum pudding. Electrons embedded in a ball of positive charge. | The gold foil experiment. Nothing in this model could bounce an alpha particle back. |
| Rutherford 1911 | A tiny dense positive nucleus, electrons outside, mostly empty space. | Orbiting electrons should radiate energy and spiral into the nucleus. |
| Bohr 1913 | Electrons in fixed energy levels at set distances, which stops them spiralling in. | Atomic masses were too big for protons alone. |
| Chadwick 1932 | Adds the neutron to the nucleus. No change to the shells. | — |
The pattern worth noticing
| The mistake | What goes wrong | Do this instead |
|---|---|---|
| Saying Rutherford did the experiment | Geiger and Marsden did it in 1909. Rutherford directed the work and interpreted it in 1911. | Geiger and Marsden carried it out; Rutherford explained it. |
| Drawing a nucleus in the plum pudding model | That is not the model. The whole idea is that the positive charge is spread out. | No nucleus. Positive charge fills the sphere. |
| Only giving one conclusion for the bounce-back | It is worth two: the nucleus is small and it is dense. | Say both, and say the rarity is what shows it is small. |
| Saying the alpha particles were attracted | Alpha particles are positive and so is the nucleus, so they are repelled. | Repelled, because like charges repel. |
| Confusing Rutherford’s and Bohr’s models | Rutherford gave us the nucleus. Bohr added the fixed shells. | Nucleus = Rutherford. Shells = Bohr. |
| Thinking the model was wrong because Thomson was careless | It fitted every piece of evidence that existed in 1904. | Models change when new evidence arrives, which is the point of the question. |
| Forgetting why the foil had to be thin | Thick foil means many collisions, and the results could not be interpreted. | Thin foil, so each particle meets at most one nucleus. |
Answer each one before tapping to check.
A sphere of positive charge spread evenly through the whole atom, with negative electrons embedded inside it. The charges balance so the atom is neutral overall. There is no nucleus. Proposed by J.J. Thomson in 1904.
That the atom is mostly empty space. If atoms were solid all the way through, the particles could not have passed through a sheet of metal.
One: the centre of the atom is very dense and contains most of its mass, because only something much heavier than an alpha particle could reverse it. Two: the centre is very small, because so few particles hit it. Both marks are available; most students only give the first.
Because alpha particles are positively charged and so is the nucleus, and like charges repel. The closer a particle came, the stronger the repulsion and the bigger the deflection.
It predicted that every alpha particle would pass through with only a tiny deflection, because the positive charge was thought to be spread thinly through the atom, so no part of it could push hard. It was wrong because the positive charge is in fact concentrated in a tiny dense nucleus, which can repel an alpha particle strongly enough to turn it round.
Hans Geiger and Ernest Marsden, in 1909, at Manchester. They worked under Ernest Rutherford, who set the problem and published the interpretation — the nuclear model — in 1911. Saying “Rutherford did the experiment” is the commonest error on this topic.
Because in Rutherford’s model the orbiting electrons should continuously radiate energy, spiral inwards and collapse into the nucleus, which would mean no atom could exist for long. Bohr proposed that electrons occupy fixed energy levels and do not radiate while they stay in them.
Divide the diameters
10−10 ÷ 10−15 = 105
about 100,000 times smaller across
Scale the atom to a 100 m football pitch and the nucleus is about 1 mm wide.
Because it is a clean example of a model being replaced by new experimental evidence. The plum pudding model was accepted because it fitted everything known in 1904. When an experiment produced a result it could not explain, it was replaced — not because anyone disliked it, but because the evidence demanded it.
It is J.J. Thomson’s 1904 model of the atom: a sphere of positive charge spread evenly throughout, with negative electrons embedded inside it like fruit in a pudding. The charges cancel so the atom is neutral. It has no nucleus, which is why the gold foil experiment disproved it.
J.J. Thomson, in 1904, seven years after he discovered the electron in 1897. He never called it the plum pudding model himself; the nickname came later.
Because it looks like a Christmas pudding with dried fruit set into it. The pudding is the sphere of positive charge and the pieces of fruit are the electrons dotted through it.
The gold foil experiment, also called the alpha scattering or Geiger–Marsden experiment, carried out in 1909. Alpha particles were fired at a very thin sheet of gold. A small number bounced back, which the plum pudding model could not explain, because nothing in it was dense enough to reverse a heavy positive particle.
Hans Geiger and Ernest Marsden, in Rutherford’s laboratory at Manchester in 1909. Rutherford directed the work and published the interpretation in 1911. The experiment is often named after Rutherford, which is why so many students write that he performed it.
Most passed straight through, so the atom is mostly empty space. Some were deflected, so the centre is positively charged and repels them. A very few bounced back, so the centre is both tiny and very dense, holding most of the atom’s mass.
In the plum pudding model the positive charge is spread through the whole atom and there is no nucleus. In the nuclear model the positive charge and nearly all the mass are packed into a tiny nucleus at the centre, with the electrons outside it and empty space between. That difference is exactly what the gold foil experiment revealed.
So that each alpha particle met at most one nucleus. In thicker foil a particle would be deflected several times, and the scattering pattern could not be interpreted. Gold is used because it is the most malleable metal, so it can be beaten to only a few atoms thick.
Because a screen only behind the foil would have caught the particles that went through and missed the ones that came back — and the particles that came back are the entire discovery. Surrounding the apparatus is what made the result visible.
An electron orbiting a nucleus is a charged particle moving in a circle, and such a particle should radiate energy continuously. Losing energy, it would spiral inwards and hit the nucleus, so atoms could not be stable. Bohr solved this in 1913 by proposing fixed energy levels in which electrons do not radiate.
Almost all of it. An atom is about 10−10 m across and its nucleus about 10−15 m, so the nucleus is roughly 100,000 times smaller in diameter. If the atom were a 100 m football pitch, the nucleus would be about a millimetre across at the centre spot.
James Chadwick discovered the neutron in 1932, explaining why atoms weigh about twice what their protons alone would account for. Beyond that, the quantum model replaces fixed orbits with regions of probability called orbitals, but GCSE, A-level and IB questions on this topic almost always expect the Bohr picture.
Yes. It appears in GCSE physics and chemistry, at A-level, and in IB physics and chemistry as part of atomic structure. At the higher levels the focus shifts from describing the models towards explaining what the evidence showed and why each model was replaced.
Because new experimental evidence appears that the existing model cannot explain. The plum pudding model was not careless — it fitted everything known in 1904. It was replaced the moment an experiment produced a result it could not account for. That is how science is supposed to work, and it is usually what a question about “the development of the atomic model” is really asking.
Questions on atomic models are among the most predictable on the paper — the same three observations, the same three conclusions, year after year. The students who lose marks are not the ones who do not know it, but the ones who give one conclusion where two were wanted. Our tutors teach IB and A‑level physics and chemistry and work through your own past papers, so we fix the way you answer rather than reteaching what you know.
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