Rutherford's experiment

Portrait of Ernest Rutherford — Ernest Rutherford discovered the proton in 1909.

At that time it was known that certain (radioactive) atoms could emit particles called alpha particles, which are positively charged.

When a very thin gold foil is bombarded with these particles, a surprising result is obtained: many of them are deflected, and some even bounce back. A zinc sulfide screen reveals the result with a flash of light.

Rutherford's experiment: alpha particles pass through the gold foil and a few are deflected or bounce back

Rutherford and his students were as surprised as if, on firing a cannon at a distant sheet of paper, the ball had bounced back towards the cannon.

This experiment showed that atoms are practically empty space, except for a small, positively charged central nucleus, which is what makes some of the positive alpha particles deflect or bounce back.

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