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💡 The big test
How do you look inside something you can never see? You fire tiny bullets at it and watch how they bounce. That is exactly what Geiger and Marsden did with gold foil.
In 1911 , Geiger and Marsden (under Rutherford ) fired a narrow beam of alpha (α) particles — tiny, positively charged particles — at an extremely thin sheet of gold foil. This deflection of particles from their straight path is called scattering , so it is also called the α-ray scattering experiment .
What did the gold foil experiment observe?
Gold Foil Observations
Most alpha particles passed straight through Some deflected at large angles A few bounced straight back Also called the alpha ray scattering experiment
- Most α-particles passed straight through undeflected.
- Some were sharply deflected at large angles.
- A few even bounced straight back .
Thomson’s model could not explain why some α-particles were deflected through large angles while most passed straight through.
NCERT Question 1 — Choose the correct options and
Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford’s gold foil experiment: (i) The experiment clearly showed the existence of neutrons in the nucleus. (ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom. (iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre. (iv) The way alpha particles were deflected showed that electrons move around the nucleus.
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Important Definitions
- Scattering — the deflection of alpha particles from their straight path; hence the gold foil experiment is also called the alpha-ray scattering experiment.
