Physics 11–12 · Year 12

Rolling with Rutherford: finding a hidden target's size from marble scattering

Module 8: From the Universe to the Atom (Structure of the Atom)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Firing probes at an unseen target and counting deflections reveals the target's size, the reasoning that turned alpha-particle scattering into the nuclear atom.

What you need

QuarkNet Rolling with Rutherford kit: a cardboard shield hiding a row of target marbles, probe marbles, a marked launch line and recording sheet (the NSW Department of Education Module 8 guide suggested investigation)
Ruler, spreadsheet for the class histogram

How to do it

Hide the target row under the shield; roll 100 probe marbles from evenly spaced points along the launch line and record each as a hit (deflected) or a miss.
Estimate the target diameter from the hit fraction: hits / rolls = N (D + d) / W for N targets of diameter D, probes of diameter d and launch width W.
Lift the shield and measure the true diameter; compute the percentage difference.
Pool the class results into a histogram and discuss how the estimate improves with trials.
Simulation: fire alpha particles at gold and read the distance of closest approach for a head-on collision.

What you should see

With targets of 16 mm diameter and 16 mm probes, the effective hit width per target is 32 mm; five targets across a 400 mm line give 40 hits in 100 rolls, and with 100 rolls the hit count varies by about 5 (the binomial spread), so one group's diameter estimate is uncertain by about 3.9 mm (25 per cent), while pooling ten groups' 1000 rolls cuts that to about 1.2 mm (8 per cent). In the simulation a 5.0 MeV alpha particle heading straight at a gold nucleus stops 4.55e-14 m from its centre, thousands of times smaller than the atom, which is why most alphas pass straight through.

What changes

What you change: number of probe rolls
What you measure: estimated target diameter and its error
What you keep the same: same targets and probes
rolls spread evenly across the line

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Most alphas bounce back from gold foil; nearly all pass through and only a tiny fraction is turned back through more than 90 degrees.
The atom is a solid ball; almost all of its volume is empty space around a tiny nucleus.

Safety card

Low riskLearners carry it out

Hazards

marbles on the floor

Controls

collect marbles after each run

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15 PH11/12-5 PH11/12-6
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-04 PY-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities