The Gold-Foil Ambush
One bounce in ten thousand revealed the nucleus
What this lesson covers
Why it matters
In 1911, Rutherford aimed a stream of α-particles — doubly charged helium ions, He²⁺ — at a gold sheet one-millionth of a centimetre thin. He expected every single one to punch straight through. The atom had other plans.
Predict first
Rutherford could have used any metal foil. Why did he choose gold?
He wanted as thin a layer of atoms as possible. Gold's unmatched malleability gave a foil one-millionth of a centimetre thick — a wall only a few thousand atoms deep for the α-particles to interrogate.
- Gold is the most malleable metal — it hammers into the thinnest possible foil — correct
- Gold is the heaviest element, so it blocks radiation best
- Gold reacts with alpha particles and makes them visible
What you do
Relive the most surprising experiment in chemistry: match each gold-foil observation to the conclusion it forced.
Check yourself
Why were α-particles the perfect probe for peering inside the atom?
An α-particle is a doubly charged helium ion, He²⁺. Massive and positive, it barrels through empty space and diffuse charge — only the dense, positive nucleus can deflect or rebound it.
Which picture of the atom did Rutherford's experiment demolish?
A uniform sphere of positive charge would deflect α-particles only slightly. The 1-in-10,000 rebounds demanded all the positive charge and mass be squeezed into one tiny nucleus.
Rutherford's own model had a fatal flaw. What was it?
By classical physics, a revolving charged particle must continuously lose energy, so Rutherford's planet-like electrons should collapse into the nucleus. Explaining the atom's stability had to wait for Bohr.
- They are heavy, fast and positively charged, so only a concentrated positive charge can turn them — correct
- They are negatively charged, so atoms attract them inward
- They are massless rays, like light
- Thomson's plum pudding — charge spread evenly could never bounce an α-particle back — correct
- Bohr's model of electrons in fixed energy orbits
- Dalton's idea that matter is made of atoms
- An orbiting electron should radiate energy, spiral inward and crash — yet atoms are stable — correct
- It could not explain why most α-particles passed straight through
- It wrongly placed the electrons inside the nucleus