IB Physics SLTopic 5 — The Atomic & Nuclear WorldPaper 1 & 2α scattering~8 min read
Rutherford’s Gold-Foil Experiment
At the start of the 20th century everyone pictured the atom as a soft blob of positive “pudding” with electrons dotted through it. Then Rutherford’s team fired tiny positive bullets at a sheet of gold — and a handful bounced straight back. It was as astonishing as firing a shell at tissue paper and having it rebound at you, and it rewrote the atom overnight.
📘 What you need to know
Alpha particles (helium nuclei, positively charged) were fired at very thin gold foil, and a detector counted how many scattered to each angle
Most alpha particles passed straight through undeflected → the atom is mostly empty space
Some deflected through small angles → there is a positive nucleus at the centre (like charges repel)
A very few bounced straight back (angles > 90°) → the nucleus is tiny, dense, and holds almost all the atom’s mass and charge
Conclusion: an atom is a small, dense, positive nucleus surrounded by orbiting electrons — the nuclear model
The atom is about 100,000 times larger than its nucleus (roughly 10−10 m across versus 10−15 m)
Set-up details matter: a lead container makes a narrow beam, the foil is thin so particles get through, and the chamber is evacuated so air can’t stop the alphas
The Experiment
A source of alpha particles was sealed in a lead block. Alphas shoot out in all directions, but lead absorbs them — so a narrow channel let just a thin, straight collimated beam escape toward the target. That target was a sheet of gold, hammered to about 10−6 m thick (gold is used because it’s so malleable). A movable detector swung around the foil to count how many alpha particles arrived at each angle.
The whole thing sat inside an evacuated chamber. Alpha particles are strongly ionising and get stopped by just a few centimetres of air, so the vacuum made sure they reached the foil — and the detector — without colliding with air molecules first.
Lead collimates the beam, the thin gold foil is the target, and the detector swings around the vacuum chamber to count alpha particles at every scattering angle.
lead container
absorbs stray α →
thin, narrow beam
evacuated chamber
no air to stop α →
α reach the foil
What They Saw
Zoom in on the foil and here’s the story. Nearly every alpha particle sailed straight through as if nothing were there. A small fraction were nudged off course by a few degrees. And once in a great while, one came flying back the way it came. Three observations, three clues:
Far from the nucleus (A) the alpha sails past untouched — the atom is mostly empty. A closer pass (B) is repelled through a small angle. A near head-on approach (C) is thrown almost straight back — only a tiny, dense, positive nucleus could do that.
most: straight through
→
atom mostly empty space
some: small deflection
→
central positive nucleus
few: bounce back
→
tiny, dense nucleus holds the mass
What It Means
Put the three clues together and the pudding model collapses. The atom must be mostly empty space, with all its positive charge and nearly all its mass packed into a minuscule central nucleus, while the light electrons occupy the huge space around it. That nucleus is astonishingly small — the whole atom is about 100,000 times wider than the nucleus at its heart. Rutherford’s alpha particles had revealed the nuclear model of the atom.
Quick recap: alpha particles fired at thin gold foil mostly passed straight through (empty space), some deflected slightly (a central positive nucleus), and a rare few bounced back (a tiny, dense, massive nucleus). The atom is a small positive nucleus surrounded by electrons, around 100,000 times smaller than the atom itself.
🧭 Turning each observation into evidence
Straight through → most of the atom is empty space (nothing there to hit)
Deflected a little → there’s a concentrated positive charge repelling the positive alpha
Bounced back → that charge sits in a tiny, dense region carrying almost all the mass
Always name the pair: exam questions want the observation and the conclusion it supports, not just one
Remember the “why” of the kit: lead for a narrow beam, thin foil so alphas pass, vacuum so air doesn’t stop them
WE 1
In the gold-foil experiment: (a) most alpha particles passed straight through the foil. State what this suggests about the atom. (b) A very small number were deflected through angles greater than 90°. State two things this suggests about the nucleus.
Part (a) — straight through
The atom is mostly empty spacePart (b) — bounced back (any two)
• the nucleus is very small
• it is very dense, holding most of the atom’s mass
• it carries a concentrated positive charge (to repel the positive α)
Only a tiny, dense, highly charged region could turn a fast alpha particle right around.
WE 2
A gold atom is about 1 × 10⁻¹⁰ m across, while its nucleus is about 1 × 10⁻¹⁵ m across. (a) Show that the atom is roughly 100,000 times wider than the nucleus. (b) Explain how this links to the main observation of the experiment.
Part (a) — take the ratio of the widthsratio = (1 × 10⁻¹⁰) ÷ (1 × 10⁻¹⁵)= 10⁻¹⁰ ⁺ ¹⁵ = 10⁵= 100,000 times widerPart (b) — the link
The nucleus fills only a tiny fraction of the atom, so the atom is nearly all empty space
→ most alpha particles miss it and pass straight through
💡 Top tips
Learn the three observation–conclusion pairs (straight through → empty; small deflection → positive nucleus; bounce back → tiny, dense, massive nucleus) — they’re the most-asked part
It’s repulsion, not attraction. The positive alpha is pushed away by the positive nucleus — never say the electrons deflect it
Bouncing back is the killer clue. Small deflections alone don’t prove the nucleus is tiny; the rare large-angle scatter does
Know the kit “why”: lead → narrow beam, thin gold → alphas get through, vacuum → air can’t absorb the alphas first
⚠ Common mistakes
Saying the electrons deflect the alpha particles — it’s the concentrated positive nucleus repelling them
Muddling the conclusions — “straight through” shows empty space, “bounce back” shows a tiny dense nucleus; don’t swap them
Picturing the nucleus as a big chunk of the atom — it’s about 1/100,000 of the atom’s width
Giving only the observation or only the conclusion — exam marks need both, clearly linked
Up next: now that we know the atom has a nucleus, we’ll name what’s inside it. The next page covers protons, neutrons and electrons, and the tidy shorthand — the ᴬ𝓏X notation — physicists use to label any nucleus.
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