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Lesson · Grade 8 · Science

The Gold Foil Experiment: Discovering the Nucleus

  • Grade 8
  • Science
  • 1 min 49 s
  • British Columbia
  • BC8-SC-24

Alpha particles are fired at very thin gold foil inside a detecting screen. The plum pudding model predicts that all of them pass straight through, but in fact most go straight through, a few are deflected and about 1 in 8,000 bounce back. Rutherford's 1911 model explains this: an atom is mostly empty space with a tiny, dense, positive nucleus, far smaller than it can be drawn, like a pea in a stadium.

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In one sentence

The question this lesson answers

What is inside an atom? A famous experiment with gold foil helped find out.

The old plum pudding model pictured an atom as a ball of spread-out positive charge, with tiny negative electrons dotted through it.

Transcript

What the lesson says

What is inside an atom? A famous experiment with gold foil helped find out.

The old plum pudding model pictured an atom as a ball of spread-out positive charge, with tiny negative electrons dotted through it.

In 1909, Rutherford's team used a radioactive source to fire alpha particles: tiny, fast particles with a positive charge.

They aimed them at very thin gold foil. A screen around it flashed wherever a particle hit.

In the plum pudding model, the positive charge is spread too thin to turn them much, so all should pass straight through.

Most did go straight through, but a few were deflected, meaning pushed off at an angle.

And about 1 in 8,000 bounced back toward the source.

Rutherford was amazed: the plum pudding model could not explain this.

In 1911, Rutherford proposed a new model. An atom is mostly empty space, so most particles pass straight through.

At its centre is a tiny, positive nucleus. It is dense: almost all the atom's mass is packed into it.

Both are positive, and like charges repel, so a particle passing close to the nucleus is pushed off course.

One heading straight at it is pushed back, which is rare because the nucleus is so small.

Even this dot is far too big. If an atom were as wide as a stadium, its nucleus would be about the size of a pea.

So an atom is mostly empty space, with a tiny, dense, positive nucleus at its centre.

Curriculum

Where this fits

British Columbia, Grade 8

Science, Chemistry: atomic theory and modelsBC8-SC-24

Previous: How the model of the atom changed: Dalton to BohrNext: Brownian motion: evidence that molecules exist

The outcomes and strands follow the British Columbia curriculum.

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