Outcomes

Pupils should be able to describe and compare the Plum Pudding model and the Rutherford model of the atom.
Pupils should recall that like charges repel and opposite charges attract.
Using the observations from the Geiger and Marsden’s experiment pupils should be able to make conclusions about the structure of the atom and how they support the Rutherford model.
Pupils should understand the factors (charge and speed) which affect the deflection of alpha particles by a nucleus.

Specification References

7.15 Describe the results of Geiger and Marsden’s experiments with gold foil and alpha particles

7.16 Describe Rutherford’s nuclear model of the atom and how it accounts for the results of Geiger and Marsden’s experiment and understand the factors (charge and speed) which affect the deflection of alpha particles by a nucleus.

Starter

Have students read outloud, and in turn, the dialogue on Radioactivity

Main Body of Lesson

Help students recall that like charges repel and unlike charges attract. Think about using the demo from identifying forces.
Teacher must state that the alpha particles are positively charged.
Introduce the plum pudding model
Introduce the Alpha scattering experiment, discuss the experimental set-up (the vacuum, alpha source, lead casing, phophorus screen etc..) and predict what would happen when alpha particles are fired at atoms based on the plum pudding model.
Use the 'Plum pudding model box' and the 'Rutherford model box' to simulate the experiment. Start with the ‘dilute positive sphere' in the 'Plum pudding model box', explaining that this is what they expected to see and then move onto the 'Rutherford model box'. These should be done undercover, the boxes should not be lifted. Students should only observe the results.
Emphasise the observations as a class and then split them into groups and using the observations and conclusions sheet they could try to draw a conclusion on the structure of the atom.

Plenary

Remove the box and review the conclusions made
Ask the pupils to predict what would happen to the path of deflection of the alpha particle if the speed of the alpha particle is changed. Using the ramp shoot 'fast alpha particles' and 'slow alpha particles' at the Potential Hill. Were their predictions correct? Discuss
Extension- How would the charge of the nucleus or the target particle affect the path of deflection? Two sized potential hills would be needed here to demonstate this.
There is a good opportunity to revise standard form, prefixes and conversions by discussing the sizes of the nucleus and atom. Firstly state that the diameter of an atom is 10^-10 and the nucleus is 10^-14. You could ask the pupils to write this in standard form and long form. Ask them how much larger the atom is compared to the nucleus (10^4). Proceed to pull a piece of paper or plasticine 1mm in diameter and say that if this 1mm piece of plasticine is the nucleus how big is the atom? They should hopefully multiply it by 10^4 and get 10,000mm. They can then convert to metres and 10m. What is the radius? Where is the edge of the atom then and pace it out in the class. Now if there is time you can replace the nucleus with a student. They should measure about 40 cm in diameter. That times 10^4 will give them 400,000cm or converting to metres is 4,000m. The radius is 2,000m or 2km. Pick a place about 2km from the school as the 'edge' of the atom. Now ask them where the next closest nucleus will be or the distance between adjacent nuclei!! Yes 4km!! That should get the point across and it really help them understand powers of ten from the TAS topic.

Homework

Write a dialogue between two people explaining the plum pudding and Rutherford models, the alpha scattering experiment and how their observations led to the Rutherford model of the atom.

Additional Information

Resources Required

Dialogue sheet on Radioactivity
Plum pudding sheet
Alpha scattering experiment sheet
One Ramp
Set of metal balls
One Rutherford model and one ‘dilute positive charge’ drawn on a piece of paper with two boxes to go over top. Boxes must have slits along base to allow ball to travel through.
Observations and conclusions sheet
Two sizes of potential hills or lycra sheet with middle pulled upward by different amounts?

Textbook References

Physics for You p.g. 94-95

Website References

Here is a nice simple step by step approach to Rutherford investigated.
Here is a nice animation of particle scattering

Skills Addressed

Safety/Hazards

Ball bearings should be picked up asap after use and should all be accounted for to help prevent trips.
The radioactive sources should be used in accordance with the Radiation local rules and risk assessment.
The dry ice should be handled with a spoon.

Notes

None

Forces & Matter Outline

Forces & Matter

3-FAM-03

Contents