At the start of the 20th century, nobody knew what was inside an atom. The popular picture was the “plum pudding” — a soft blob of positive charge with electrons dotted through it. Then Ernest Rutherford fired tiny positive bullets at a sheet of gold and watched where they went. Almost all sailed straight through, but a rare few bounced straight back — as shocking, he said, as a shell rebounding off tissue paper. That one observation destroyed the plum pudding and revealed the nuclear atom.
📘 What you need to know
Rutherford fired alpha particles (positive, the nucleus of helium) at a very thin gold foil
A movable detector recorded the angle of deflection and the number of particles at each angle
The apparatus needed a collimated beam (lead container), a thin foil, a movable detector, and a vacuum
Observation A: most alpha particles passed straight through undeflected → the atom is mostly empty space
Observation B: some deflected through small angles (<10°) → there is a positive nucleus (like charges repel)
Observation C: a very few bounced straight back (>90°) → the nucleus is tiny and holds most of the mass and charge
Conclusion: the atom is a small, dense, positively charged nucleus surrounded by orbiting electrons
The atom is about 100,000× larger than its nucleus (atom ~10−10 m, nucleus ~10−15 m)
The apparatus
Every part of Rutherford’s set-up was there for a reason. Get the “why” behind each and you’ve answered half the exam questions on this topic.
A collimated beam from the lead-housed source strikes the thin gold foil; the detector swings round the circular track to count alpha particles at each angle θ, all inside a vacuum.
Part
Why it’s needed
Lead container for the source
Alpha particles are emitted in all directions; lead absorbs them, and a narrow hole lets out a collimated (narrow, parallel) beam
Thin gold foil (~10−6 m)
Thin enough that alphas can pass through; gold is malleable, so it hammers into extremely thin sheets
Movable detector
Records the number of alpha particles arriving at each angle of deflection
Evacuated chamber
Alphas are strongly ionising and stopped by air within a few cm; a vacuum stops them colliding with air molecules on the way
Why gold, of all things? Two reasons, and examiners want both. First, gold is the most malleable metal, so it can be beaten into a foil just a few thousand atoms thick — thin enough to let alpha particles through. Second, being a single dense element it gives a clean, repeatable target. If the foil were any thicker the alphas would simply stop, and you’d learn nothing.
The three observations
An alpha particle is the nucleus of a helium atom, so it carries a positive charge. When Rutherford fired a beam of them at the foil, three distinct things happened — and each one tells us something about the atom’s structure.
Most alphas (A) miss every nucleus and go straight through empty space. Some (B) pass near a positive nucleus and are nudged aside. A rare one (C) heads dead-on and is flung straight back.
Observation
What it means
A. Most alphas pass straight through undeflected
The atom is mostly empty space
B. Some deflect through small angles (<10°)
There is a positive nucleus at the centre (two positives repel)
C. A very few bounce straight back (>90°)
The nucleus is extremely small and holds nearly all the mass and charge
Straight through
so atom is
Mostly empty space
rare bounce-back
Tiny dense nucleus
The conclusion: the nuclear atom
Putting the three observations together, Rutherford concluded that an atom is a small, dense, positively charged nucleus that carries almost all the atom’s mass, surrounded by negatively charged electrons in the vast empty space around it.
A tiny positive nucleus (~10−15 m) holds nearly all the mass; electrons occupy the huge space around it (~10−10 m). The atom is around 100,000× wider than its nucleus.
Let that scale sink in. If the nucleus were a marble on the centre spot of a football stadium, the electrons would be tiny specks up in the back row of the stands, and everything between would be empty. That’s why nearly every alpha particle missed everything and flew straight through — and why the rare head-on bounce was such a bombshell.
WE 1
In the gold foil experiment, most alpha particles passed straight through the foil, but a very small fraction were deflected through large angles. (a) State what the large fraction passing straight through tells us about the atom. (b) State what the tiny fraction bouncing back tells us about the nucleus. (c) Explain why the foil had to be extremely thin.
(a) most pass straight throughthe atom is mostly empty space(b) a few bounce straight back
The nucleus is very small and very dense, holding almost all the mass and charge.
the nucleus is tiny and concentrates the mass and (positive) charge(c) why so thin?
A thick foil would absorb the alpha particles or cause multiple deflections.
so alphas can pass through and each scatters off at most one nucleusKeep the two “fractions” straight: the huge fraction passing through is about empty space; the tiny fraction bouncing back is about the small, dense nucleus. Examiners award those as two separate marks.
WE 2
A gold atom has a diameter of about 1 × 10−10 m and its nucleus a diameter of about 1 × 10−15 m. (a) Calculate how many times wider the atom is than the nucleus. (b) Using this ratio, explain qualitatively why only a tiny fraction of alpha particles are deflected back.
(a) ratio of diametersratio = (1 × 10⁻¹⁰) / (1 × 10⁻¹⁵)= 1 × 10⁵ = 100,000 times wider(b) why so few bounce back?
The nucleus is a minuscule target within a mostly empty atom.
The chance of a near head-on hit is therefore extremely small.
almost all alphas miss the tiny nucleus, so only a tiny fraction are repelled backBecause area scales as the square of the size, the nucleus takes up only about one part in 10¹⁰ of the atom’s cross-section — a vanishingly small bullseye. That’s the whole reason back-scattering is so rare.
WE 3
Explain how each of the following features of Rutherford’s apparatus was essential: (a) the source housed in a lead block, (b) the evacuated chamber.
(a) lead block around the source
Alphas are emitted in all directions; lead absorbs them.
A narrow channel lets out a collimated beam.
to produce a narrow, parallel beam of alpha particles(b) evacuated chamber
Alphas are highly ionising and are stopped by air within a few centimetres.
so the alphas aren’t absorbed or scattered by air before reaching the foilBoth marks hinge on the special properties of alpha particles: they spray out in every direction (hence the lead collimator) and they’re stopped by just a few cm of air (hence the vacuum). Tie each apparatus feature back to a property of the alpha.
🔬 Answering a gold-foil question
Most pass straight through → the atom is mostly empty space.
Some deflect a little → there’s a positive nucleus (like charges repel).
A few bounce back → the nucleus is tiny, dense, and holds the mass and charge.
Apparatus “why”? Tie each part to a property of the alpha (all-directions → lead; stopped by air → vacuum).
Why gold? Malleable, so it beats into an ultra-thin foil.
Scale: atom ~10−10 m, nucleus ~10−15 m — a factor of ~100,000.
💡 Top tips
Learn the three observations and their three conclusions as matched pairs.
Alpha particle = helium nucleus = positive. That’s why the nucleus repels it.
Tie every apparatus part to a property of the alpha particle.
Gold is chosen for its malleability — it makes ultra-thin foil.
The atom is ~100,000× bigger than the nucleus — know this order of magnitude.
Saying the deflections show the nucleus is negative — both alpha and nucleus are positive, so they repel
Confusing the conclusions: “straight through” is about empty space, not the nucleus size
Forgetting the chamber is evacuated — air would absorb the alphas
Thinking a thicker foil would give better results — it would stop the alphas
Claiming most alphas are deflected — the vast majority pass straight through
Saying the electrons deflect the alphas — electrons are far too light; it’s the nucleus
Quick recap: Rutherford fired positive alpha particles at a thin gold foil and counted them with a movable detector in a vacuum. Most passed straight through (the atom is mostly empty space), some deflected slightly (a positive nucleus repels them), and a rare few bounced straight back (the nucleus is tiny, dense, and holds the mass and charge). The result was the nuclear atom: a minuscule positive nucleus surrounded by electrons, with the atom about 100,000× wider than its nucleus.
Rutherford’s experiment told us the atom has a tiny central nucleus — but not how to describe what’s inside it. To count the protons and neutrons in any nucleus and write it down properly, we use a compact shorthand with two numbers stacked before the element symbol. Next page: Nuclear Notation.
Gold foil experiment not sinking in?
Book a free meeting and we’ll lock down the three observations, their conclusions, and the “why” behind every part of the apparatus.
