MrChemCoach

Rutherford’s Gold Foil Experiment

IGCSE 0620 · Atomic structure · 3D lab
All labs
Am-241 · 5.5 MeV Gold · 400 nm Drag to turn · scroll to zoom · shift-drag to pan
Detections Ready
Alpha particles detected0
Passed straight throughDEFLECTION UNDER 1° 0-
Slightly deflected1° TO 10° 0-
Strongly deflected10° TO 90° 0-
BackscatteredOVER 90° · BACK TOWARDS THE SOURCE 0-

Press Start. Watch the zinc sulfide screen: almost every flash lands straight ahead, which is the observation that needed explaining.

ApparatusSet up, then run

1 · Alpha emission rate

600 / s

A real school source emits far more than this. The rate here sets how fast the statistics build up - the proportions never change, because they come from the physics, not the rate.

2 · Gold foil thickness

400 nm

3 · Show

4 · Analysis

5 · The numbers behind it

Scattering angle against number detected No data yet
Counted in this run Rutherford prediction

Why the plum pudding model could not survive this

Thomson · plum pudding

Positive charge spread thinly through the whole atom, with electrons dotted inside it. The repulsion on an alpha particle is weak everywhere, because the charge it meets is never concentrated. Predicted result: everything goes almost straight through, deflections of a fraction of a degree at most. Nothing should ever come back.

Rutherford · nuclear

All the positive charge packed into a nucleus about 20,000 times smaller than the atom, with the rest empty space. Almost every alpha particle misses it completely and carries on. The rare one that comes close meets an enormous concentrated charge and is thrown through a large angle - or straight back.

What did Rutherford discover?
The atom is mostly empty space Almost every alpha particle went straight through a sheet of gold thousands of atoms thick. There was nothing there to stop them.
The positive charge is in a tiny nucleus Only a concentrated positive charge could repel a fast, positive alpha particle hard enough to turn it through 90° or more.
Nearly all the mass is in the nucleus To bounce an alpha particle backwards, the thing it hit had to be much heavier than the alpha particle itself.
The nucleus is minute A gold nucleus is about 7 fm across; the whole atom is about 144 pm. That is a ratio of roughly 1 to 20,000 in width, and about a million-millionth by volume.
Quick check
0 / 5

5 / 5

What this experiment shows

In 1909 Hans Geiger and Ernest Marsden fired alpha particles from a radioactive source at a sheet of gold foil only a few hundred nanometres thick, and looked for the tiny flashes each one made on a zinc sulfide screen. Almost every alpha particle went straight through as if the foil were not there. A few were deflected through small angles. And roughly one in every thirty thousand came back towards the source. Thomson’s plum pudding model, with the positive charge spread thinly through the whole atom, could not deflect a fast alpha particle by more than a fraction of a degree, so it could not survive the result. Rutherford’s explanation was that all the positive charge and nearly all the mass sit in a nucleus perhaps twenty thousand times smaller than the atom, and that the rest is empty space.

most pass straight through · a few deflect · about 1 in 30,000 comes back

What you need

  • A sealed alpha source, americium-241 in the simulation, inside a lead castle
  • A collimator to narrow the emission into one beam
  • A gold foil a few hundred nanometres thick, held in a frame
  • A zinc sulfide screen, which flashes where an alpha particle lands
  • A microscope on a rotating arm, to count the flashes at any angle
  • An evacuated chamber, so the alpha particles are not stopped by air

How it is done

  1. Evacuate the chamber, so nothing but the foil is in the beam’s way.
  2. Let the collimated alpha beam strike the gold foil at the centre.
  3. Set the microscope to one scattering angle and count the flashes for a fixed time.
  4. Move the microscope round to the next angle and count again.
  5. Repeat all the way round, including angles greater than 90 degrees.

What you should see

  • Well over 99 per cent of the alpha particles are deflected by less than one degree.
  • A small proportion are deflected through angles between 1 and 10 degrees.
  • A far smaller proportion are deflected through more than 10 degrees.
  • About one alpha particle in thirty thousand is turned through more than 90 degrees.
  • The count at a fixed angle is proportional to sinθ / sin⁴(θ/2), which is what Coulomb repulsion from a point charge predicts.

Where marks are lost

  • Saying the alpha particle hits the nucleus. It never touches it - at 5.5 MeV the closest approach is about 41 fm and a gold nucleus is only about 7 fm across. The deflection is electrostatic repulsion between two positive charges.
  • Saying the electrons deflect the alpha particles. An electron is about 7,300 times lighter than an alpha particle and barely disturbs it.
  • Saying most alpha particles are deflected. Almost all of them are not, and that is half the evidence: the atom is mostly empty space.
  • Treating backscattering as common. It is the rarity of the large angles, together with the fact that they happen at all, that fixes how small the nucleus must be.
  • Saying the experiment proved the existence of neutrons or of electron shells. It did neither. It located the positive charge and the mass.

Questions students ask

Why do most alpha particles pass straight through the gold foil?
A gold foil a few hundred nanometres thick is over a thousand atoms deep, but the nucleus of each atom occupies only about a million-millionth of the atom’s volume. Nearly every alpha particle therefore passes far from every nucleus it meets and is hardly deflected at all. That is the evidence that an atom is mostly empty space.
Why were a few alpha particles deflected straight back?
Because now and then one is aimed almost exactly at a nucleus. Approaching a charge of +79e head on, a 5.5 MeV alpha particle is stopped and turned round by electrostatic repulsion before it gets within about 41 femtometres. Only a charge concentrated into something that small could repel it that hard.
Why could the plum pudding model not explain the result?
In Thomson’s model the positive charge is spread through the whole atom, so an alpha particle inside it feels only part of that charge pulling it in different directions at once. The largest deflection such a model can produce is a fraction of a degree. It cannot produce a single alpha particle coming back.
What did the experiment actually establish?
That the positive charge and nearly all the mass of an atom are concentrated in a very small nucleus, and that the rest of the atom is empty space. It did not establish the neutron, which James Chadwick identified in 1932, nor the arrangement of the electrons.
Why was the apparatus evacuated?
Alpha particles are stopped by a few centimetres of air. Without a vacuum the beam would be scattered and absorbed before it ever reached the foil, and the counts would mean nothing.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.

1