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Evacuated

vessel

Thin sheet of 

gold

Ring of 

detectors

High energy

beam of alpha

particles

Probing Matter

September 2002 Number 40

 F actsheetP hysics

1

 

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.

Before studying this Factsheet you should ensure that you are familiar

with the outline of the work of Rutherford from your GCSE course

The alpha scattering experimentsBefore 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.

Geiger and Marsden confirmed their results and realized that there was no

way that the existing model could explain the results obtained.

Rutherford suggested a new model of the atom, which would explain the

experimental results — the “Nuclear” model of the atom.

The Nuclear Model of the Atom

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.

 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.

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.

 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.

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.

Deep inelastic scattering

Alpha particles cannot be used to probe deeper into matter. If we wish toinvestigate 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.

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.

gold nuclei

paths of alpha particles

Nucleus containing most of the

mass and all of the positive chargeDiffuse negative

cloud

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Probing Matter   Physics Factsheet

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Typical Exam Question

(a) Complete the following table, which compares two types of 

scattering experiment.

Deep inelastic scattering

Incident particles Electrons

Target Gold atoms

  (2)

(b) Write a short paragraph describing how the results of the

  experiments changed scientific thinking. (4)

• 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 

 Alpha scattering Deep inelastic scattering

Incident particles  Alpha particles Electrons

Target Gold atoms nucleons

 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.

 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.

Questions

1. Describe the alpha scattering experiments.

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.

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.

(a) The diagram shows a line of gold nuclei, such as might have been

the target for an alpha scattering experiment.

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.

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.

 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.

Physics Factsheets may be copied free of charge by teaching staff or students,

 provided that their school is a registered subscriber.

 No part of these Factsheets may be reproduced, stored in a retrieval system,

or transmitted, in any other form or by any other means, without the prior 

 permission of the publisher. ISSN 1351-5136 

. . . . . . . .Add lines to the diagram to show the paths of the alpha particles.

(3)

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.

(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)

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 nuclearexplanation is the only sensible one.

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.

(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.

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