Rutherford Scattering — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

Section 1 of 9The experiment

1

A narrow beam of alpha particles (positively charged) was fired at a very thin sheet of gold foil, in a vacuum.

2

A fluorescent screen around the foil detected the alpha particles by tiny flashes of light.

3

The scientists, Geiger and Marsden working under Rutherford, counted how many particles arrived at each angle.

4

The results were compared with what the plum pudding model predicted.

Section 2 of 9Observations and what they show

Observation	Conclusion
Most alpha particles passed straight through	The atom is mostly empty space
Some were deflected through small angles	Passing near a positive charge repels them
A very few (about 1 in 10 000) bounced back	A tiny, dense, positively charged nucleus holds nearly all the mass
Deflected particles were positive alpha particles	The nucleus must be positive, since like charges repel

Section 3 of 9Result against the old model

Plum pudding prediction

• Plum pudding prediction With charge spread out, alpha particles would only be deflected by small angles, and none would rebound.

Actual result

• Actual result Large-angle scattering occurred, which could not be explained without a concentrated positive charge.

Nuclear model

• Nuclear model Rutherford concluded that the positive charge and most of the mass are in a very small nucleus with electrons orbiting at a distance.

Section 4 of 9Why each design choice mattered

Experimental design

Alpha particles

• Alpha particles They are heavy, fast and positively charged, so they are strongly deflected by any concentrated positive charge they approach.

Thin gold foil

• Thin gold foil Gold can be beaten into foil only a few atoms thick, so each alpha particle is likely to meet at most one nucleus.

Vacuum

• Vacuum Air particles would absorb or scatter the alpha particles, which have a short range in air, and spoil the results.

Moveable detector

• Moveable detector Counting at many angles gave a pattern that could be compared with predictions from each model.

Section 5 of 9From scattering to the modern atom

1Nuclear model

• Nuclear model A small, dense nucleus surrounded by empty space and electrons.

2Bohr's improvement

• Bohr's improvement Niels Bohr suggested that electrons orbit in fixed energy levels at set distances from the nucleus.

3Protons found

• Protons found Later experiments showed the positive charge is carried by particles called protons.

4Neutrons found

• Neutrons found James Chadwick discovered the neutron about 20 years later, explaining the mass of the nucleus.

Section 6 of 9Linking path to structure

Path of alpha particle	Where it passes	Reason
Straight through, no change	Far from any nucleus	Negligible force acts on it because atoms are mostly empty space
Small deflection	Fairly close to a nucleus	Repulsion between the positive alpha and the positive nucleus
Large deflection or rebound	Almost head-on to a nucleus	Very strong repulsion from a tiny, dense, concentrated charge

Section 7 of 9Interpreting the numbers

Why so few rebounded

• Why so few rebounded The nucleus is about 10 000 times smaller than the atom, so only a particle heading almost directly at a nucleus is turned back.

Key exam wording

• Key exam wording Say the nucleus is positively charged because the positive alpha particles were repelled,
  • and that it is small because so few were deflected by large angles.

Section 8 of 9Key terms

Term	Meaning
Alpha particle	A positively charged particle made of two protons and two neutrons.
Scattering	The change in direction of particles when they interact with matter.
Nucleus	The tiny central part of the atom containing protons and neutrons.
Gold foil	The very thin metal sheet used as the target in the experiment.
Fluorescent screen	A screen that gives a flash of light when struck by a particle.
Electrostatic repulsion	The force pushing two like charges apart.

Section 9 of 9Exam technique

1

Saying the alpha particles were attracted to the nucleus; they were repelled because both are positive.

2

Concluding that the nucleus is large — the results showed it is extremely small.

3

Forgetting that most particles went straight through and that this is evidence of empty space.

4

Mixing up the experiment's conclusions with the plum pudding predictions.