# Alpha particle scattering

##### Demonstration

Rutherford’s scattering experiment was an ingenious piece of design and interpretation. Whilst it is not possible to reproduce the experiment in a school laboratory, it is well worth demonstrating how it was carried out using photographs, pictures and analogies.

#### Apparatus and materials

Pictures of Rutherford scattering (for example the two further down the page)

#### Health & Safety and Technical notes

Originally the scintillations were counted by eye: trained observers counted for a short time in a darkened room. Rutherford is reputed to have sung Onward Christian Soldiers as he waited for the next flash of light! Today, modern detectors, such as photomuliplier tubes connected to a data logger, would be used.

Much later, a similar method (deep inelastic scattering of electrons) was used to probe inside neutrons and protons and determine their structure - they are made of three smaller particles called quarks.

#### Procedure

a It is helpful to show students a video of the alpha scattering experiment and to emphasize that this is not just one more measurement in atomic physics but rather one of the great turning points in physics. It changed scientists' picture of atoms permanently. (You may be able to get hold of a second-hand copy of the Nuffield A-Level version.)

b Show students a picture of the layout of a scattering experiment showing alpha particles being fired at a ‘solid’ gold foil.

c Most of the alpha particles go straight through the gold but a few bounce back. From this Rutherford deduced that the atom could not be solid (neither uniform nor a plum pudding [see guidance note Developing a model of the atom: the nuclear atom]) but is mainly hollow with a tiny nucleus.

#### Teaching notes

1 99.99% of alpha particles are undeflected. This implies that the atom is mainly hollow.

2 Some alpha particles bounce back. This implies that there is a single structure in the atom which is more massive than the alpha particle and (probably) repels it.

3 More careful studies carried out by Geiger and Marsden provided results that were consistent with a single nucleus carrying a positive charge +Ze where Z is the atomic number of the scattering atom.

4 Students need help with these ideas. Analogies such as the three mentioned below will help.

A haystack analogy
Suppose you wished to investigate the shape and size of a concealed object. Pretend you have a large mound or truss of hay and you suspect that there are some small but massive iron cannon balls hidden in it. Suppose you are not allowed to pull the hay aside. You might still investigate its secret store by firing a stream of machine-gun bullets into it. Let us make some predictions.

i If the hay is only a thin covering on a solid mound of cannon balls then all the bullets will bounce back or perhaps stop and never reappear.

ii If there is only hay, and no cannon balls, then the bullets will go completely through the bale of hay, moving almost as fast as they went in.

iii However if the mound was mostly hay with a few dense cannon balls scattered through it, spaced well apart, then many of the bullets will go straight through it and a few will bounce back. The proportion bouncing back indicates the spacing of the 'nuclei' in the hay. The more that bounce back, the closer together are the canon balls. To realistically model the nuclei in gold foil, 10 cm canon balls would need a distance of 10 km between them. This illustrates how tiny a nucleus is and how hollow an atom is.

Marbles analogy
Another model might include marbles rolling down a slightly sloping table with a few spikes sticking out such as in a pin-ball machine. What would happen to the marbles?

Experimental analogies
You can demonstrate magnetic, electrostatic and gravitational models in the experiments listed below. Rutherford himself used the magnetic model and analogy to explain his theory.

5 How science works extension: This experiment provides an excellent opportunity to discuss how scientific theories and principles develop over time and the issues that can arise that may prevent them from being accepted instantly because they contradicted existing views. See the case study Rutherford's alpha scattering experiment

See also the guidance note The great scattering experiments. This can help students understand how a nuclear model of the atom came to be. The data and analysis may be too complicated for some students, but there is great value in using ‘real’ results in terms of the narrative of scientific discovery.

There is supplementary information in the guidance note Developing a model of the atom: a nuclear atom

This experiment was safety-checked in December 2006