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Physics Factsheet September 2002

Number 40

Probing Matter This Factsheet will explain: • some of the techniques which scientists use to investigate matter; • some of the important discoveries which have been made using the techniques; • the importance of this kind of investigation as an illustration of good science practice.

Geiger and Marsden confirmed their results and realized that there was no way that the existing model could explain the results obtained.

Before studying this Factsheet you should ensure that you are familiar with the outline of the work of Rutherford from your GCSE course

The Nuclear Model of the Atom

Rutherford suggested a new model of the atom, which would explain the experimental results — the “Nuclear” model of the atom.

The alpha scattering experiments

Diffuse negative cloud

Before the scattering experiments, the accepted model of the atom was of a homogenous “blob” of positive material with negative “bits” distributed evenly throughout it, rather like a currant bun. Indeed, it was sometimes called the “Currant Bun” or “Plum Pudding” model of the atom.

Rutherford realized that the large scattering angles could not be explained as the sum of a large number of smaller scatterings, because the gold was too thin. In order to account for the large scattering angles, the atom must consist of a minute dense centre, in which almost all of the mass and all the positive charge is concentrated, so that the majority of the particles went through only slightly deviated by avoiding the central charge/mass concentration, but some which went closer to the centres would be highly deviated. He was able to calculate the relative size of the charge/mass concentration — which he called the “nucleus” — from the proportion of highly scattered particles. He concluded that the nucleus has a radius of the order of 10-15 m, compared to the atom with radius of the order of 10-10 m.

Geiger and Marsden carried out an investigation, in which they fired alpha particles (helium nuclei) at a very thin sheet of gold and measured the proportion of particles which were scattered through various angles.

Evacuated vessel

Nucleus containing most of the mass and all of the positive charge

Ring of detectors

Exam Hint: You may be asked to draw or complete paths like these. They are parabolas. The closer the approach to the nucleus, the sharper the bend. The neutron remained undetected at this time. Thin sheet of gold

The results of this experiment caused the existing model of the atom to be abandoned in favour of the nuclear model and completely altered our understanding of matter. It ranks among the most significant investigations in the history of science.

High energy beam of alpha particles

Deep inelastic scattering

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Alpha particles cannot be used to probe deeper into matter. If we wish to investigate the nature of each of the particles in the nucleus i.e. the proton and the neutron, we must use high energy electrons. Just as the non-regular scattering of alpha particles through the atom lead to our understanding of separate particles making up the atom, so the non-uniform scattering of electrons from hydrogen nuclei lead to an understanding that each of these particles is not a “blob”, but also made up of smaller particles.

Rutherford scattering probes the structure of the atom. It is alpha particles which are used in Rutherford Scattering

The result they expected, based on the model, was that the particles would go through the gold, but would be deviated through small angles due to electrostatic repulsions between the positive particles and the positive charges in the atoms. Most of the particles did behave as expected, but to their amazement, Geiger and Marsden found that some particles were deviated by large angles, and some almost came back the way they had entered. gold nuclei

Investigations like these led to the discovery of quarks, the particles which make up the proton and neutron. This type of scattering is called “inelastic” because interactions between the particles do not conserve kinetic energy. Exam Hint: You may need to know more about Fundamental Particles. On some specifications it is not required for the core syllabus, only as an optional topic.

paths of alpha particles

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Physics Factsheet

Probing Matter

• • •

Exam Workshop This is a typical weak student’s answer to an exam question. The comments explain what is wrong with the answers and how they can be improved. The examiner’s mark scheme is given below.

deep inelastic scattering is used to find out about the make up of protons and neutrons high energy electrons are used kinetic energy is not conserved

(a) The diagram shows a line of gold nuclei, such as might have been the target for an alpha scattering experiment.

........ ........

Typical Exam Question (a) Complete the following table, which compares two types of scattering experiment.

Add lines to the diagram to show the paths of the alpha particles. (3)

Deep inelastic scattering Incident particles Target

Electrons Gold atoms

1/3

(2) (b) Write a short paragraph describing how the results of the experiments changed scientific thinking.

Although the candidate has shown deviation, s/he has not shown an understanding that the closer the line of approach to the nucleus, the greater the deviation, nor is there any indication of a very large deviation. At least three paths, one showing a direct approach and a large deviation should have been shown.

(4)

Incident particles

Alpha scattering Alpha particles

Deep inelastic scattering Electrons

Target

Gold atoms

nucleons

(b) Explain why this experiment led to the conclusion that most of the mass and all of the positive charge of the atom was concentrated in a small space at the centre. (3)

Alpha particle scattering: Before the experiment, scientists believed that positive material was distributed throughout the atom. The results could only be explained by the positive charge being concentrated in a tiny volume at the centre – the nucleus.

The deviations could not have been caused by any other arrangement. The positively charged alpha particles were repelled by the positive nuclei. 1/3 The candidate appreciates that the results cannot be explained by the then accepted model, but does not really say why the nuclear explanation is the only sensible one.

Deep inelastic scattering: Before experiments, the proton, neutron and electron were believed to be fundamental particles. The results showed that these particles were made up of yet more fundamental particles.

Examiner’s Answers (a) (as shown on the first page) Three paths, parabolas, showing hardly any deviation for paths between the nuclei, larger deviation for closer approach, including one of about 1700.

Questions 1. Describe the alpha scattering experiments.

(b) Positive particles are deviated by the repulsion of the positive nucleus. Since particles were deviated through different angles, matter must be distributed unevenly. The large deviations must be caused by regions of dense mass and charge. Since only relatively few particles were deviated through large angles the dense regions must be small compared to the atom as a whole.

2. Why was it important that the vessel was evacuated? 3. (a) The nucleus of carbon-14 has a radius of 2.70 × 10-15 m. Calculate the volume of this nucleus. (b) The mass of the nucleus is 2.34 × 10-26 kg. Calculate the density of the nuclear material of the atom. (c) Compare this value with that of ordinary materials.

Answers 1. See the text. 2. It is important that the vessel was evacuated so that the alpha particles were deviated only by the gold atoms, not by extraneous air atoms or molecules.

Acknowledgements: This Physics Factsheet was researched and written by Janice Jones. The Curriculum Press,Unit 305B, The Big Peg,120 Vyse Street, Birmingham, B18 6NF.

3. (a) Volume = 4/3 × 3.142 × (2.70 × 10-15)3 = 8.24 × 10-44m3. (b) Density = mass/volume = 2.34 ×10-26/(8.24 × 10-44) = 2.84 ×1017kg/m3. (c) Although there is a large range of densities for ordinary materials, this value is many orders of magnitude greater than any of them, the highest of which is about 104kg/m3.

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