Science 7–10 · Year 9
Rutherford's gold-foil experiment as a rolling-ball model of the nucleus
Periodic table and atomic structure
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The idea
Most of an atom is empty space with a tiny, dense, positively charged nucleus, because most alpha particles pass straight through a foil and a very few bounce back.
What you need
- scattering hill (a smooth 1/r-shaped plastic or plaster hill) on a flat board, or a large sheet of paper with a hidden shape under it
- ball bearings or marbles, a launching ramp of fixed height, chalk or carbon paper to mark paths
- protractor and ruler
How to do it
- Launch a ball from the ramp aimed at the hill along a line offset from the centre by a measured distance (the impact parameter); mark where it rolls off the board.
- Repeat for offsets from 0 to 10 cm in 1 cm steps, three balls per offset, and record the deflection angle each time.
- Plot deflection angle against offset. Note how many launches are deflected by more than 90 degrees.
- Compare with Geiger and Marsden's 1909 finding, which Rutherford explained, that about 1 in 8000 alpha particles was turned back by metal foil, and explain what that says about the size of the nucleus compared with the atom.
What you should see
Balls aimed well off-centre roll past almost undeflected; as the offset shrinks the deflection grows, and only near-head-on launches turn back. The 1/r hill gives a force falling as 1/r squared, the same law as the electric repulsion between an alpha particle and a nucleus. The learner knows it worked when the angle-against-offset graph falls smoothly and large deflections occur only for the smallest offsets.
What changes
- What you change
- impact parameter (offset of the launch line from the hill centre)
- What you measure
- deflection angle
- What you keep the same
- ramp height (ball speed)
- same ball
- same hill
- board level
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Atoms are solid balls all the way through.
- Electrons deflect the alpha particles.
- The nucleus takes up most of the atom.
Safety card
Hazards
- rolling balls on the floor are a trip hazard
Controls
- work on a table or a taped-off floor area
Note
No chemicals or heat.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-PRT-01SC5-WS-06
- Australian Curriculum v9AC9S9U06AC9S9H01
Sources
The pages the author read to write this activity.