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Everything around you — the chair you sit on, the water you drink, the air you breathe — is made of tiny particles far too small to see. The particle model pictures all matter as built from these particles, and it explains a huge amount of everyday behaviour: why a solid keeps its shape, why a liquid flows, why a gas fills any container. But those particles are themselves made of even smaller atoms, and the picture scientists hold of the atom today was pieced together over more than a century, as one experiment after another overturned the model that came before. This lesson opens Topic P1 (Matter) of OCR Gateway Combined Science A by setting out the particle model of solids, liquids and gases, and then tracing the development of the atomic model from Thomson's plum pudding to Rutherford's nuclear atom and Bohr's energy levels, along with the astonishingly small size of an atom.
By the end of this lesson you should be able to describe the arrangement, spacing and motion of the particles in solids, liquids and gases, describe how the atomic model developed (Thomson → Rutherford → Bohr), explain what the alpha-scattering experiment showed, and recall the typical order of magnitude of the size of an atom.
This lesson is largely AO1 (recall the particle arrangements and the sequence of atomic models), with AO2 where you apply the particle picture to explain a property such as why a gas fills its container, and AO3 when you evaluate what the alpha-scattering results imply about the structure of the atom.
Matter is usually found in one of three states: solid, liquid or gas. The particle model explains the differences between them entirely in terms of how the particles are arranged and how they move. The diagram below shows the three arrangements side by side.
In a solid, the particles are packed closely together in a regular, fixed pattern (a lattice), all touching. They cannot move from place to place — they only vibrate about fixed positions — and they have the least energy of the three states. Because strong forces of attraction lock the particles in place, a solid has a fixed shape and a fixed volume, and it cannot be compressed.
In a liquid, the particles are still close together and touching, but in a random, irregular arrangement. They can move around and slide past one another, so a liquid can flow and takes the shape of its container, yet it keeps a fixed volume and is almost incompressible. They have more energy than in a solid but less than in a gas.
In a gas, the particles are far apart with large empty spaces between them, moving quickly in all directions (constant random motion). They have the most energy of the three states. Because they are far apart and free to move anywhere, a gas has no fixed shape and no fixed volume — it spreads out to fill any container — and it can be compressed by pushing the particles closer together.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, fixed lattice | Random, close | Random, far apart |
| Spacing | Very close | Close | Very far apart |
| Motion | Vibrate in place | Slide past each other | Move fast in all directions |
| Energy | Least | Medium | Most |
| Shape | Fixed | Takes container's shape | Fills container |
| Volume | Fixed | Fixed | Not fixed |
| Compressible? | No | Almost no | Yes |
Exam Tip: Learn the columns as a block — arrangement, spacing, motion, energy — for each state. Almost every "explain the property" question is answered by quoting the right particle behaviour, for example "a gas can be compressed because there are large spaces between the particles".
Exam Tip: A common misconception is that "particles in a solid don't move at all". They do move — they vibrate about fixed positions; they simply cannot travel from one place to another the way liquid and gas particles can.
The power of the particle model is that each everyday property follows directly from how the particles behave. It is worth practising the because statements:
When a substance is heated, energy is transferred to its particles, making them move faster and weakening the forces holding them together; with enough energy a solid melts and a liquid boils. Cooling reverses these changes. (Changes of state are covered in full in a later lesson; here the key point is that the same particles are simply rearranged.)
Exam Tip: When you "explain" a property, name the particle feature and the consequence: "the particles are far apart (feature), so the gas can be compressed (consequence)". A bare statement of the feature without the consequence often loses the mark.
The particles in the model above are built from atoms — the tiny building blocks of every element. Atoms are almost unimaginably small: the radius of a typical atom is only about 1×10−10 m (a tenth of a nanometre), so around a hundred million atoms would fit across the width of a single human hair.
radius of a typical atom≈1×10−10 m
Yet the modern picture of the atom — a tiny, dense nucleus surrounded by electrons — did not arrive all at once. It was built up over more than a century as new experiments forced scientists to change their models. You should be able to put the main models in order and say what new evidence prompted each change. (The detailed make-up of the nucleus, isotopes and radioactive decay belong to Topic P4; here we focus on how the model of the whole atom developed and on the atom's size.)
graph LR
A["Dalton<br/>solid spheres"] --> B["Thomson<br/>plum pudding"]
B --> C["Rutherford<br/>nuclear atom"]
C --> D["Bohr<br/>energy levels"]
Dalton (early 1800s) — solid spheres. John Dalton proposed that each element is made of tiny, identical, indivisible solid spheres called atoms, which cannot be broken down or changed. This was a huge step, but it pictured the atom as a featureless ball with no internal parts.
Thomson (1897) — the plum-pudding model. J. J. Thomson discovered the electron — a tiny, negatively charged particle far smaller than an atom. Since atoms are electrically neutral, he reasoned the negative electrons must be balanced by positive charge, and proposed the plum-pudding model: a ball of positive charge (the "pudding") with negative electrons dotted through it (the "plums"). The atom was no longer indivisible — it had parts.
Rutherford (1909–1911) — the nuclear atom. Ernest Rutherford, with Hans Geiger and Ernest Marsden, carried out the alpha-scattering experiment (described below). Its surprising results could not be explained by the plum-pudding model and led Rutherford to propose the nuclear model: the atom has a tiny, dense, positively charged nucleus at its centre, with electrons around the outside and mostly empty space in between.
Bohr (1913) — electrons in energy levels. Niels Bohr refined the nuclear model. A problem with Rutherford's atom was that orbiting electrons should spiral into the nucleus, making the atom unstable. Bohr proposed that electrons orbit the nucleus only at fixed distances, in definite energy levels (shells), and do not spiral in. This explained why atoms are stable and matched the evidence from the light that atoms give out.
| Scientist | Model / discovery | Key idea added |
|---|---|---|
| Dalton | Solid spheres | Atoms are tiny, indivisible spheres |
| Thomson | Plum pudding | Atoms contain electrons; positive "dough" with negative "plums" |
| Rutherford | Nuclear atom | A tiny dense positive nucleus; mostly empty space |
| Bohr | Energy levels | Electrons orbit in fixed energy levels (shells) |
Exam Tip: A favourite exam task is to put these models in order and say why each changed. The trigger for the big jump from Thomson to Rutherford was the alpha-scattering experiment; Bohr's change explained why atoms are stable.
The single most important experiment in this story is alpha scattering, carried out under Rutherford's direction. In it, a beam of alpha particles (small, positively charged particles emitted by a radioactive source) was fired at an extremely thin sheet of gold foil. A detecting screen around the foil recorded where the alpha particles went after passing through — or bouncing off — it.
The three observations — and what each one told Rutherford — are the heart of the experiment:
| Observation | What was seen | What it showed |
|---|---|---|
| 1 | Most alpha particles passed straight through the foil, undeflected | The atom is mostly empty space |
| 2 | Some alpha particles were deflected through small angles | The centre of the atom is positively charged (it repelled the positive alphas) |
| 3 | A very few alpha particles bounced straight back (through more than 90°) | The positive charge and mass are concentrated in a tiny, dense nucleus |
The plum-pudding model predicted that the alpha particles should all pass straight through with at most very slight deflections, because the positive charge was spread thinly throughout the atom. The fact that a few alphas bounced almost straight back was, in Rutherford's words, almost as incredible as firing a shell at tissue paper and having it come back at you. The only way to explain it was that almost all the atom's mass and all its positive charge are packed into a minute central nucleus, with the rest of the atom being empty space through which most alphas sail unhindered. This is exactly the nuclear model.
In the alpha-scattering experiment, a very small number of alpha particles bounced almost straight back off the gold foil. What does this observation tell us about the atom, and why?
Step 1 — recall the property of an alpha particle: it is positively charged and has appreciable mass.
Step 2 — explain the bounce-back: to repel a fast, positive alpha particle straight back, it must meet something positively charged (like charges repel) that is also dense and massive enough not to be pushed aside.
Step 3 — locate that charge and mass: because only a very few alphas bounce back, this region must be extremely small — a tiny nucleus holding the positive charge and most of the mass.
Answer: the bounce-back shows the atom has a tiny, dense, positively charged nucleus, because only a concentrated positive charge could repel a positive alpha particle straight back, and its rarity shows the nucleus is very small.
Exam Tip: Link each observation to its conclusion: straight through → mostly empty space; deflected → positive nucleus; bounced back → tiny, dense, massive nucleus. Quoting "like charges repel" explains why the positive nucleus deflects the positive alpha particles.
| Misconception | The correct idea |
|---|---|
| "Particles in a solid don't move at all" | They vibrate about fixed positions — they just cannot move from place to place |
| "Liquids have much bigger gaps than solids" | Liquid particles are almost as close as in a solid; the big jump in spacing is to the gas |
| "When a gas is compressed the particles get smaller" | The particles stay the same size; they are simply pushed closer together into the spaces |
| "Rutherford discovered the electron" | Thomson discovered the electron; Rutherford discovered the nucleus |
| "Alpha scattering proved the plum-pudding model" | It disproved plum pudding and led to the nuclear model |
| "The nucleus fills most of the atom" | The nucleus is far smaller than the atom; the atom is mostly empty space |
Question (6 marks): Describe the plum-pudding model of the atom, and explain how the results of the alpha-scattering experiment led scientists to replace it with the nuclear model.
Mid-band response: "The plum-pudding model was a ball of positive charge with electrons in it. In the experiment they fired alpha particles at gold foil. Most went straight through, but some bounced back. This showed the atom has a nucleus, so the plum-pudding model was wrong."
Examiner-style commentary: The plum-pudding description and the headline conclusion are present, but the reasoning is thin. To climb a band, give all three observations, say what each one shows, and explain why the bounce-back means a tiny, dense, positive nucleus.
Stronger response: "In the plum-pudding model the atom was a ball of positive charge with negative electrons dotted through it. Alpha particles were fired at a thin gold foil and a screen detected where they went. Most passed straight through, showing the atom is mostly empty space. Some were deflected, showing there is a concentration of positive charge. A few bounced straight back, showing this positive charge is in a tiny, dense nucleus. The plum-pudding model could not explain the particles bouncing back, so it was replaced by the nuclear model."
Examiner-style commentary: A clear, correct answer giving the plum-pudding model, all three observations and their conclusions. To reach the top band, state that like charges repel (so a positive nucleus repels the positive alphas) and contrast this with what the plum-pudding model predicted.
Top-band response: "In the plum-pudding model (Thomson), the atom was a ball of positive charge with tiny negative electrons embedded throughout it, like plums in a pudding. To test it, a beam of alpha particles (small, fast, positively charged particles) was fired at a very thin gold foil, and a detector recorded the directions in which they emerged. There were three observations. (1) Most passed straight through undeflected — showing the atom is mostly empty space. (2) Some were deflected through small angles — showing the atom contains a concentration of positive charge, which repels the positive alphas because like charges repel. (3) A very few bounced almost straight back — showing that the positive charge and almost all the mass are packed into a tiny, dense nucleus, since only a small, massive, highly charged region could turn a fast alpha particle right around. The plum-pudding model predicted the positive charge was spread out, so all the alphas should pass through with only slight deflection; it could not explain the back-scattering. Rutherford therefore replaced it with the nuclear model — a minute, dense, positive nucleus surrounded by mostly empty space containing the electrons."
Examiner-style commentary: Full marks. It describes the plum-pudding model, gives all three observations with the correct conclusion for each, uses "like charges repel", contrasts the plum-pudding prediction with the result, and names the nuclear model — a complete account of how the evidence forced the change.
This content is aligned with OCR Gateway Combined Science A (J250), Topic P1 Matter. Refer to the official OCR specification for exact wording.