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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Basic Guide Dentistry Series
- Sprache: Englisch
Basic Guide to Dental Radiography provides an essential introduction to radiography in the dental practice. Illustrated throughout, this guide outlines and explains each topic in a clear and accessible style.
- Comprehensive coverage includes general physics, principles of image formation, digital image recording, equipment, biological effects of x-rays and legislation
- Suitable for the whole dental team
- Illustrated in full colour throughout
- Ideal for those completing mandatory CPD in radiography
- Useful study guide for the NEBDN Certificate in Dental Radiography, the National Certificate in Radiography or the Level 3 Diploma in Dental Nursing
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Table of Contents
Cover
Title Page
Acknowledgements
Chapter 1: General physics
ATOMS AND MOLECULES
ENERGY
ELECTRICAL ENERGY
ELECTRIC CURRENT
Chapter 2: X‐ray production
X‐RAY TUBE FILAMENT
TRANSFORMERS
X‐RAY TUBE ANODE
VOLTAGE RECTIFICATION
HALF‐WAVE AND FULL‐WAVE RECTIFICATION
THE PROCESS OF X‐RAY PRODUCTION
BREMSSTRAHLUNG (CONTINUOUS) SPECTRA
PRODUCTION OF LINE SPECTRA
FACTORS AFFECTING THE NUMBER OF X‐RAY QUANTA PRODUCED
THE PROPERTIES OF X‐RAYS
Chapter 3: X‐ray interaction with matter
FACTORS AFFECTING ATTENUATION
Chapter 4: Principles of image formation
IMAGING GEOMETRY
THE IDEAL IMAGING SYSTEM
EFFECT OF NONPARALLEL RELATIONSHIP
DIFFERENTIAL ABSORPTION
Chapter 5: Imaging with dental X‐ray film
DENTAL X‐RAY FILM
THE CONSTRUCTION OF DENTAL FILM
FILM EMULSION
FILM CHARACTERISTICS
FILM SPEED
FILM CONTRAST
FILM LATITUDE
CONTRAST AND SPEED
FILM PROCESSING
DEVELOPER
FIXER
WASHING
DRYING THE FILM
Chapter 6: Digital imaging recording
TYPES OF DIGITAL IMAGE RECORDER
DIGITAL IMAGE RECORDING ADVANTAGES
DIGITAL ANALOGUE DETAIL COMPARISON
DIGITAL IMAGE DENSITY MANIPULATION
CONTRAST MANIPULATION
Chapter 7: X‐ray equipment
INTRA‐ORAL
DENTAL INTRA‐ORAL TUBE HEAD
ORTHOPANTOMOGRAPHY
Chapter 8: Radiation doses and dose measurement
UK POPULATION RADIATION DOSE
DOSE MEASUREMENT
TYPES OF DOSE METERS
THIMBLE IONISATION DOSE METER PICTURE
READING A TLD RECORD
DOSE QUANTITIES
Chapter 9: Biological effects of X-rays
INDIRECT EFFECTS OF RADIATION DOSES
DIRECT BIOLOGICAL EFFECTS OF RADIATION DOSES
STOCHASTIC EFFECTS OF RADIATION EXPOSURE
DETERMINISTIC OR NON‐STOCHASTIC EFFECTS
WHO WILL BE AFFECTED
DETRIMENT WITH AGE AT EXPOSURE
DOSE REDUCTION FOR PATIENTS
DOSE REDUCTION FOR STAFF
Chapter 10: Legislation: Ionising Radiations Regulations 1999 (IRR 1999)
IONISING RADIATIONS REGULATIONS 1999 (STATUTORY INSTRUMENT 3232)
Chapter 11: Legislation: Ionising Radiation (Medical Exposures) Regulations 2000 (IR(ME)R 2000), Statutory Instrument 1059
DUTIES OF THE EMPLOYER
DUTIES OF THE PRACTITIONER, OPERATOR AND REFERRER
JUSTIFICATION OF MEDICAL EXPOSURES
EXPERT ADVICE
TRAINING
Chapter 12: Quality assurance
STAFF TRAINING
FILM STORAGE
FILM HOLDERS
CASSETTES
FILM PROCESSORS
CHEMICAL LEVELS
DARKROOM TESTING
X‐RAY EQUIPMENT QUALITY ASSURANCE
Chapter 13: Dental intra‐oral paralleling techniques
PERFORMING THE EXAMINATION
INTRA‐ORAL PARALLELING TECHNIQUES
THE FILM HOLDERS
EXAMPLES OF FILM HOLDERS
ASSEMBLING THE FILM HOLDERS
THE FILM
THE CENTRAL RAY
PLACING THE HOLDERS INTO THE PATIENT’S MOUTH
UPPER ANTERIOR FILM HOLDER PLACEMENT
APPEARANCE OF AN IMAGE OF THE UPPER CENTRAL INCISORS
LOWER ANTERIOR FILM HOLDER PLACEMENT
UPPER POSTERIOR HOLDER PLACEMENT
LOWER POSTERIOR HOLDER PLACEMENT
ASSESSMENT OF THE IMAGE
RADIOGRAPHIC ASSESSMENT
ASSESSMENT OF THE VERTICAL ANGLE
THE IMAGE
ASSESSING THE MESIODISTAL (LATERAL) ANGLE
DENSITY ASSESSMENT
CONTRAST ASSESSMENT
SHARPNESS ASSESSMENT
ARTEFACTS
BITEWING RADIOGRAPHS
BITEWING RADIOGRAPHS: PLACING THE HOLDER INTO THE MOUTH
CENTAL LONG‐AXIS POSITION FOR BITEWING RADIOGRAPHS
ASSESSING THE BITEWING IMAGE
Chapter 14: Orthopantomography
PERFORMING THE EXAMINATION
OPG IMAGE ASSESSMENT
FRANKFORT PLANE ASSESSMENT
MEDIAN SAGITTAL PLANE ASSESSMENT
FOCAL TROUGH ASSESSMENT
Chapter 15: Other dental radiographic techniques
BISECTING ANGLE PERIAPICAL RADIOGRAPHS
OCCLUSAL RADIOGRAPHS
CEPHALOMETRIC IMAGING
RARELY USED TECHNIQUES
Appendix A: Adequate training
FURTHER READING
Appendix B: Image quality troubleshooting
Index
End User License Agreement
List of Illustrations
Chapter 01
Figure 1.1 Classic basic model for the structure of the atom
Figure 1.2 Redistribution of electrons to the atoms’ lowest energy state
Figure 1.3 (a) Atomic bonding (ionic bond). (b) Atomic bonding (covalent bond)
Figure 1.4 Explanation of expansion due to heating
Figure 1.5 (a, b and c) Electric forces
Figure 1.6 Charge induction through electron orbit distortion
Figure 1.7 Valence and conduction bands in conductors and insulators
Figure 1.8 Open and closed circuits
Figure 1.9 Direct current flow
Figure 1.10 Alternating current flow
Chapter 02
Figure 2.1 X‐ray tube filament with electron cloud
Figure 2.2 Step‐down transformer windings
Figure 2.3 Step‐up transformer windings
Figure 2.4 Inside the X‐ray tube (tube insert)
Figure 2.5 Tube current, milliamps (mA)
Figure 2.6 Half‐wave rectified voltage waveform
Figure 2.7 Full wave rectified voltage waveform
Figure 2.8 Medium‐ or high‐frequency voltage waveform
Figure 2.9 X‐ray target face
Figure 2.10 Tungsten target detail (interactions with filament electrons)
Figure 2.11 Individual X‐ray quanta forming a beam
Figure 2.12 Distribution of continuous and line spectra
Figure 2.13 Production of characteristic (line) spectra
Figure 2.14 Electromagnetic radiations (configuration of electric and magnetic waves)
Figure 2.15 Electromagnetic induction
Figure 2.16 X‐rays travelling in straight lines
Chapter 03
Figure 3.1 Intensity of a beam of X‐rays
Figure 3.2 Attenuation of an X‐ray beam (75% attenuation)
Figure 3.3 Attenuation of an X‐ray beam (50% attenuation)
Figure 3.4 Attenuation of an X‐ray beam (thickness of the attenuator)
Figure 3.5 Attenuation of an X‐ray beam (density of the attenuator): (a) 2 inch thick sponge and (b) four sponges making up 2 inches of sponge
Figure 3.6 Attenuation of an X‐ray beam: (a) low beam energy and (b) high beam energy
Figure 3.7 Electron energy in successive electron shells
Figure 3.8 Binding energy in successive electron shells
Figure 3.9 Photoelectric absorption
Figure 3.10 Compton scattering
Chapter 04
Figure 4.1 Apparent and actual (line) focus
Figure 4.2 Production of small apparent focus from a large actual focus
Figure 4.3 Effect of focus size on fine detail
Figure 4.4 Ideal imaging geometry
Figure 4.5 Effect of focus on blurring of the image
Figure 4.6 Old closed cone X‐ray unit
Figure 4.7 Comparison of image blurring at different focus‐to‐skin distances
Figure 4.8 (a) Image with film and tooth parallel. (b) Image with film and tooth not parallel
Figure 4.9 Image formation (differential absorption)
Figure 4.10 Image formation (effect of mA on contrast)
Figure 4.11 Image formation (effect of kV on image contrast)
Chapter 05
Figure 5.1 Components of the dental film pack
Figure 5.2 Function of the film pack lead foil
Figure 5.3 Construction of dental X‐ray film
Figure 5.4 Silver bromide reaction to X‐ray exposure
Figure 5.5 Effects on the film emulsion of different levels of radiation exposure
Figure 5.6 Effect of duplitising the emulsion
Figure 5.7 Film characteristic curve features
Figure 5.8 Characteristic curve position and film speed
Figure 5.9 Characteristic curve shape and film contrast
Figure 5.10 Characteristic curve effects on film and exposure latitude
Figure 5.11 Independence of speed and contrast characteristics
Figure 5.12 (a) Action of developer on exposed and unexposed crystals. (b) Developer action; electrons attracting positive silver ions in the emulsion
Figure 5.13 Developing time and effect of low developer levels
Figure 5.14 Effect of development on film density and contrast
Figure 5.15 Function of the fixer on exposed emulsions
Chapter 06
Figure 6.1 Fluorescence (excited electrons fall back into place emitting light)
Figure 6.2 Phosphorescence (electrons held at increased energy levels for a time)
Figure 6.3 Image formation in phosphor plates
Figure 6.4 Charge‐coupled devices (imaging sequence)
Figure 6.5 Comparison of digital/analogue image
Figure 6.6 Manipulation of density in digital images
Figure 6.7 Manipulation of contrast in digital imaging
Chapter 07
Figure 7.1 Intra‐oral X‐ray tube housing
Figure 7.2 X‐ray tube exit port detail
Figure 7.3 Mandibular imaging difficulties
Figure 7.4 Tomographic imaging effect
Figure 7.5 Mandibular tomography difficulties
Figure 7.6 Pan‐oral tomographic pivot points
Figure 7.7 X‐ray tube and film movement during pan‐oral tomography
Chapter 08
Figure 8.1 An example of a thimble ionisation dose meter (immediate readout)
Figure 8.2 Dose recording on a TLD
Figure 8.3 Thermoluminescent material, dose reading
Chapter 09
Figure 9.1 Radiation detriment, variations with age at exposure
Chapter 10
Figure 10.1 Bespoke risk assessment for radiological installations
Figure 10.2 Internal scatter and lead rubber protection
Figure 10.3 (a) Designated controlled areas. (b) Determination of controlled areas
Figure 10.4 Designated controlled areas and advice on operator position
Chapter 12
Figure 12.1 (a) Correct film storage position of boxes side by side. (b) Incorrect stacking of boxes
Figure 12.2 Film rotated in film holder
Figure 12.3 Film not properly supported in film holder
Figure 12.4 Correct positioning of film wedge and cone for a stepped‐wedge test
Figure 12.5 In a standard test: when the chemicals are new, there will be a completely black area surrounding the image of the wedge. This is because the wedge is smaller than a standard‐sized film or phosphor plate, and X‐rays will hit the film or plate directly all around the wedge, and it takes little exposure to make a film totally black when processed properly
Figure 12.6 A series of four wedge tests. It can be clearly seen that the first three tests, comparing each step with the same one in the neighbouring test show a consistent result. However in the fourth test, each step appears lighter than the corresponding test in each of the preceding three tests. The developer is underperforming because it has become exhausted
Figure 12.7 Safelight coin test preparation (A) exposed for 6 minutes (F) for 1 minute
Figure 12.8 (a) Acceptable coin test result. (b) Unacceptable coin test result
Chapter 13
Figure 13.1 Correct position for teeth on bite block
Figure 13.2 (a) Dentsply RINN posterior holder. (b) Kerr Super‐Bite posterior holder.
Figure 13.3 (a) Kerr Super‐Bite anterior holder. (b) Dentsply RINN anterior holder.
Figure 13.4 (a) Blip position upper teeth. (b) Blip position lower teeth
Figure 13.5 (a) Starting position, (b) mid insertion, (c) final position.
Figure 13.6 (a and b) Film placement for upper central incisors.
Figure 13.7 Image of upper central incisors.
Figure 13.8 (a) Starting position, (b) mid insertion, (c) final position.
Figure 13.9 Film placement for lower central incisors.
Figure 13.10 Image of lower central incisors
Figure 13.11 (a) Starting position, (b) final position.
Figure 13.12 (a) Film placement (PA upper right 5, 6, 7), (b) PA upper right 5, 6, 7 and (c) shows the typical of radiograph produced.
Figure 13.13 (a) Film placement (PA upper right 5, 6), (b) PA upper right 5, 6.
Figure 13.14 (a) Starting position, (b) mid insertion, (c) final position
Figure 13.15 Film placement relative to lower molars/premolars.
Figure 13.16 Image of PA lower 5, 6, 7
Figure 13.17 Demonstration of vertical angle
Figure 13.18 Demonstration of mesiodistal angle.
Figure 13.19 (a) Correct vertical angle. (b) Incorrect vertical angle
Figure 13.20 (a) Image correct vertical angle. (b) Image incorrect vertical angle
Figure 13.21 (a) Correct vertical and incisors. (b) Incorrect vertical angle and incisors
Figure 13.22 (a) Diagnosis, vertical angle correct. (b) Diagnosis, vertical angle incorrect
Figure 13.23 Demonstration of mesiodistal angle
Figure 13.24 (a) Diagnosis lateral angle correct. (b) Diagnosis lateral angle incorrect
Figure 13.25 Demonstration of radiographic contrast
Figure 13.26 The lamina dura is a dense white line marked (A), the canal for the ligament is a dark line marked (B), and the bony trabeculae is a fine honeycomb appearance between the teeth. The bony trabecular pattern is probably the least reliable of the assessments as it is so variable.
Figure 13.27 (a) Kerr Kwik‐Bite ring and (b) Kerr Kwik‐Bite index. (c) Dentsply RINN bitewing holder.
Figure 13.28 Central axis for bitewing radiograph
Figure 13.29 Inserting bitewing films.
Figure 13.30 (a) Final film position for bitewing. (b) Film position for bitewing.
Figure 13.31 Kerr Kwik‐Bite index, potential positions for the edge of the X‐ray tube cone.
Figure 13.32 Typical appearance of a bitewing radiograph.
Chapter 14
Figure 14.1 Positioning lines for an OPG
Figure 14.2 (a) Correct hard palate appearance. (b) Incorrect hard palate appearance
Figure 14.3 Asymmetric hard palate appearance
Figure 14.4 Assessment for tilting of the head in OPGs
Figure 14.5 Assessing rotation of the head on an OPG
Figure 14.6 OPG assessment of the focal trough
Figure 14.7 Frankfort plane and the focal trough
Figure 14.8 OPG, tongue not in the roof of the mouth
Chapter 15
Figure 15.1 Relative film/tooth position for bisecting angle technique
Figure 15.2 Incorrect central ray positioning in bisecting angle techniques
Figure 15.3 Correct central ray position for bisecting angle
Figure 15.4 Central ray directions for bisecting angle techniques
Figure 15.5 Demonstration of molar cusps in bisecting angle technique
Figure 15.6 Incisal edge position in bisecting angle technique
Figure 15.7 Elongation and foreshortening in bisecting angle techniques
Figure 15.8 (a) Occlusal film positions for full cover of the maxillary arch. (b) Alternative film position for oblique images
Figure 15.9 Beam alignment for upper occlusal radiography
Figure 15.10 The general appearance of an upper standard occlusal radiograph. Though it is usual to see approximately 1 to 1.5 cm of unexposed film anteriorly with a slightly elliptical shape to the unexposed area, errors in the vertical angle will cause large‐scale elongation or foreshortening of the teeth and distortion of the image of the palate. It is very important that the image quality and potential distortion are assessed prior to any diagnostic conclusions being drawn.
Figure 15.11 Upper oblique occlusal X‐ray tube positioning
Figure 15.12 (a) Film positions to complete full. (b) Film position for imaging of the standard lower occlusal
Figure 15.13 (a) Lower occlusal positioning. (b) Typical appearance of a lower central occlusal radiograph.
Figure 15.14 (a) Infero‐superior. (b) Typical appearance of an infero‐superior occlusal radiograph
Figure 15.15 Positioning lines for cephalometry
Figure 15.16 (a) Superimposed clinoid. (b) Clinoid processes not superimposed
Figure 15.17 (a) No tilting of the head. (b) Appearance with the head tilted
Figure 15.18 (a) Postero‐anterior chest. (b) Lateral chest
Figure 15.19 (a) Standard radiographic image. (b) Parallax imaging techniques
Figure 15.20 Positioning for lateral oblique
Figure 15.21 The standard centring point 2.5 cm below and in front of the angle of the mandible is shown by the red circle on the jaw. The idea is for the central ray to pass approximately through the body of the mandible in contact with the film cassette
Figure 15.22 Oblique mandible image.
Appendix B
Figure B.1
Figure B.2
Figure B.3
Figure B.4
Figure B.5
Figure B.6
Figure B.7
Figure B.8
Figure B.9
Figure B.10
Figure B.11
Figure B.12
Figure B.13
Guide
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BASIC GUIDE TO DENTAL RADIOGRAPHY
Tim Reynolds
Education Consultant in Dental Radiology
This edition first published 2016© 2016 by John Wiley & Sons, Ltd
Registered OfficeJohn Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK
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All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.
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The contents of this work are intended to further general scientific research, understanding, and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method, diagnosis, or treatment by health science practitioners for any particular patient. The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of medicines, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine, equipment, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. Readers should consult with a specialist where appropriate. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom.
Library of Congress Cataloging‐in‐Publication Data
Names: Reynolds, Tim (Lecturer in radiography), author.Title: Basic guide to dental radiography / Tim Reynolds.Description: Chichester, West Sussex ; Ames, Iowa : John Wiley & Sons Ltd., 2016. | Includes bibliographical references and index.Identifiers: LCCN 2016024804 | ISBN 9780470673126 (pbk.) | ISBN 9781118916261 (Adobe PDF) | ISBN 9781118916278 (epub)Subjects: MESH: Radiography, Dental–methodsClassification: LCC RK309 | NLM WN 230 | DDC 617.6/07572–dc23
LC record available at https://lccn.loc.gov/2016024804
A catalogue record for this book is available from the British Library.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Cover image: ©gmutlu/gettyimages
I dedicate this text to R. F. Farr, Chacket, John Ball, Brian Murphy and to my father who passed away on 27 September 2013 and will never see this text published though he asked me about it often during its preparation.
Acknowledgements
A massive thank you is due to my wife and two young children for putting up with the hours I have spent, effectively, an absent husband and father while putting together this text.
They have missed me and put up with it in good grace; the advantage of being able to quietly get on with the work has been a blessing although there were times that I would have welcomed an interruption.
Also many additional thanks to my wife for allowing me to take and to publish a number of photographs of her in less than flattering circumstances (film holders in position).
From my past I must thank Mr R. F. Farr and Dr Chacket, both formerly of the Queen Elizabeth Hospital Birmingham.
Also Mr John Ball, sadly no longer with us; he was formerly Deputy Principal of the Dudley Road School of Radiography.
The three of them together must form what is probably the greatest physics teaching team ever. Whatever understanding I have of the physics, geometry and theories of imaging in radiography comes largely from their teaching.
Lastly, Mr Brian Murphy, formerly Principal of the Dudley Road School of Radiography; he was possessed of the greatest breadth of knowledge of radiography that I have ever encountered.
Like John Ball, Brian is no longer with us but will always be remembered; he was a great teacher, mentor and in later years a very good friend. The career that I built was through his example and his guidance; I owe him much and he will not be forgotten.
These four together have provided the vast bulk of the information contained within these pages.
Chapter 1General physics
ATOMS AND MOLECULES
Whenever setting out on a project of this type, it is difficult to know what to use as your starting point.
Let us start by looking at what makes up the world as we know it.
We look around and see lakes, mountains, fields, etc., but what if we could look into these things and see what makes them what they are?
We would see atoms and molecules.
There can’t be many people who have not heard of these, but what are they?
Atoms and molecules are linked to elements and compounds (here is the problem – almost every time we mention anything, it will lead us straight to something else we need to know).
Elements are single chemical substances such as oxygen, hydrogen, sulphur, etc. We can take a large amount of an element and keep cutting it down to make it smaller and smaller, but there is a limit to how small we can make it.
We come to a point where all that we have is a single atom of the substance; if we then cut it to an even smaller size, we will be breaking down the atom, and it will no longer be that particular substance.
Atoms are the smallest particle of an element that can exist and still behave as that element.
Breaking down an atom eventually produces just a collection of the bits that make up the atom.
Here we go again! What is smaller than an atom? Or what are atoms made of?
There are many so‐called fundamental particles that make up the atoms that provide the basic building blocks for all of the things that we see, touch and know of. Some of these fundamental particles are only now being discovered.
For the purposes of fulfilling the basic guide brief, we will concentrate on only three types of particle: protons, neutrons and electrons.
Protons and neutrons are large (that’s relative; remember we would need very powerful microscope to see even these particles), and electrons are small.
To represent the difference in these particles in a way you can visualise, think of placing a single grape pip on the ground and then standing a person 6 ft tall next to it.
The grape pip represents the size of an electron, and the 6‐ft‐tall person the size of a proton or a neutron. Protons and neutrons are slightly different in size, but for our purposes they can be considered to be the same, but electrons are 1840 times smaller than either of the other two particles.
The protons and electrons each have an electrical charge and these charges are of opposite poles (like the two ends of a battery). The protons have a positive charge (+ve), and the electrons a negative charge (−ve).
Despite the relative size difference of the particles, the two charges, although opposite poles (or signs), are of equal size or strength.
So the positive charge on one large proton is completely cancelled out by the negative charge on one tiny electron.
Neutrons have no charge at all (they are neutral).
How do these particles fit together to make an atom?
Figure 1.1 shows what has become an accepted idea of the appearance of an atom.
Figure 1.1 Classic basic model for the structure of the atom
There is a large central nucleus, containing protons and neutrons with the electrons circling in a number of orbits at different distances from the nucleus. These orbits have traditionally been called electron shells or energy shells.
This model will be adequate for our understanding, but do remember that the electron orbits are not all in the same plane. The atom is three‐dimensional, and the electron orbits taken all together would make a pattern much more like looking at a football.
This makes sense if you think of the electron orbits as actual shells; they completely surround the nucleus much like the layers of an onion. This is difficult to demonstrate on a flat page, and we have become used to the picture as shown (Figure 1.1) with lots of circles having the same centre.
The number of protons in the nucleus tells us what sort of atom it is. A nucleus containing 6 protons would be a carbon nucleus, 11 protons sodium, and 82 protons lead. The number of protons present is the atomic number of the element and of course of the atom; the number of protons in fact tells us what chemical substance the atom is.
The protons in the nucleus all have a positive charge, and the tendency for positive charges is to push each other apart just like two magnetic north or two south poles would. They need something to keep them from pushing each other away; this function is performed by the neutrons. The neutrons don’t do this job alone, but for the purposes of this particular text, we need look no further into nuclear forces. At very low atomic numbers, there will be equal numbers of protons and neutrons; however as atomic number increases, the higher concentration of positively charged protons needs a higher number of neutrons to overcome the forces of repulsion between them.
The number of electrons in each orbit is specific and is determined by the following formula:
where E is the number of electrons and n is the number of the electron shell.
So, the closest shell to the nucleus is number 1. In that shell, you can have 2 × 12 electrons.
12 is 1 × 1 so that 1 multiplied by 2 tells us we can have two electrons in the first shell.
In the second shell we can have 2 × 22. So 2 × 2 (n2) = 4 multiplied by 2 gives 8.
In the third shell 2 × 32 gives 2 × 9. So 18 electrons would be allowed in shell 3.
No electrons can be positioned in shell 2 if shell 1 is not full and none in shell 3 if shell 2 is not full. That is to say that all inner shells must be filled before outer shells can contain any electrons. If an electron were removed from an inner shell, then one would move down from an outer shell to fill the gap. (This becomes important when we consider the effects of exposure to radiation.)
The process works like this because atoms always exist in their lowest energy state (ground state) and inner shell electrons are the low energy ones. So if we take out a low‐energy inner shell electron, the atom is at a higher level of energy than it could be, so an electron from a shell further out falls to fill the gap and in the process gives up some of its energy.
The electron filling the gap will give up some energy because it can only be in the lower shell if it has the correct level of energy. This process will continue until the exchange takes place at the outermost shell of the atom. There will then be an electron space free in the outer shell of the atom (the one that is the greatest distance from the nucleus) (Figure 1.2).
Figure 1.2 Redistribution of electrons to the atoms’ lowest energy state
From the previous descriptions, it is clear that most of the mass of an atom (it’s easier to think of this as weight or just the solid material) is in the nucleus of the atom.
A carbon atom with 6 protons and 6 neutrons (there are forms of carbon with a different number of neutrons, but we are not concerned with isotopes in this text) will have 6 electrons circulating in two discrete orbits (2 in shell 1 and 4 in shell 2). So in terms of the sheer bulk of material in relative terms, the electrons account for six times one, and the nucleus for 12 times 1840.
This means the solid matter that makes up an atom is mostly contained in the nucleus (where the big particles are). However if we look at the overall size of the atom (from one side of the outer electron shell to the other), most of it is not made up of material at all but of empty space. Even taking into account the relatively large particles in the nucleus, all elements including things like lead have atoms that are almost entirely free space. This is sort of like a large fishing net on a trawler; if the net was 50 yd by 50 yd, the overall size is massive, but if you just put the material making the net together, it would be tiny by comparison; the overall measurements of the net are made up mostly of the gaps between the materials.
To round off our investigation of atoms, the following is presented.
The electron shells are not called 1, 2 and 3 but are denoted by letters, number 1 is K, number 2 L, number 3 M, and so on; this form of atomic structure will be found in any basic science or physics book though not in advanced texts on the topic. The shell numbers simply allow us to calculate the number of electrons allowed to be in the particular orbit or shell.
On page 1 when we started talking about atoms, we also mentioned molecules, so we now need to bring those back into our thinking.
When we introduced molecules we said the atoms and molecules were linked to elements and compounds.
We have discussed elements, so what are compounds?
A compound is a combination of two or more elements; a combination that everyone knows is H2O (water).
The formula indicates that there are two hydrogen atoms and one of oxygen. The collection of three atoms shown is a molecule of water. If we try to cut this down to make an even smaller amount, we end up with something that is no longer water. Take the oxygen out and we simply have two atoms of hydrogen; if we take away an hydrogen atom, we have an hydroxide or an hydroxyl radical.
A molecule is the smallest particle of a compound that can exist and still behave chemically as that compound.
NB: Molecules are not always made up of atoms from different elements; a molecule is a collection of two or more atoms; they could be two atoms of oxygen or any other element.
Why do the atoms of different elements join together to make molecules of compounds? We could get into a big discussion on chemistry here, but we don’t actually need to.
The simplest way of thinking about the reason for these combinations is that all atoms would like their outer electron shell to be full.
If it isn’t then they may combine with another material by taking an electron from it; the materials are then held together by a tug of war over the electron because both atoms want it in their outer shell. This type of joining is called an ionic bond (Figure 1.3a).
Another way in which atoms join is that they may share electrons, so that electrons in their outer shells effectively take part in the outer orbit of both atoms; this is a bit more like walking hand in hand down the road with your partner. This is a covalent (sharing) bond (Figure 1.3b).
Figure 1.3 (a) Atomic bonding (ionic bond). (b) Atomic bonding (covalent bond)
We have just learned that the outer shell electrons of an atom give the atom its chemical properties; they are what make the atoms of one element bond with atoms of other elements to make the molecules of a compound. We can consider outer shell electrons to be ‘chemical glue’.
Remember previously we said the electrons could not be put into outer shells if inner ones were not filled and that if an electron were taken out of an inner shell, its place would be taken by an electron from a shell further out; this process continues until all inner shell vacancies are filled. The result is that following the final movement, there will be a vacancy on the electron shell most remote from the nucleus (the outside shell). This shell gives the atom its chemical properties, so removing electrons from atoms changes their chemical properties. This is an important concept for understanding the biological effects of X‐rays.
ENERGY
The previous section looked at the material (stuff) that makes up the world that we live in. Next thing to consider is what makes things work.
As always we should look for clues in what we do or say every day. If you have had a tough couple of days, worked hard and not slept, you come to a point where you say I have had it, I can’t go on, and I’ve got no energy.
When we want to measure either amount of energy stored or used in a system, we might use a different word to describe it in different situations. There is however a general measure of energy that can be used in any circumstances; it is the Joule.
If you have ever been on a diet and watching food labels, you will have seen a statement telling you the number of joules (usually kilojoules (kJ)), so you can work out how many chocolate bars you need to just get you through the day.
So there it is, energy is what enables us to do work (or to play); some physics books actually define energy as the ability to do work.
There are lists indicating many sorts of energy, but if you look carefully at each, you can pretty well fit them all into one of the following two categories: kinetic energy (KE) and potential energy (PE).
Kinetic energy is the energy of movement, and potential energy is stored energy.
In classical physics we have the conservation of energy that says energy can be neither created nor destroyed but merely changed from one form to another.
In a very simple example of energy conversion, we could consider lifting a weight from the floor to a shelf six feet high. The work that you have done in lifting that weight is now stored as potential energy. If the weight is then pushed off the shelf, it falls; during the fall it has kinetic energy (energy of movement). On hitting the floor there will be a loud bang (sound energy), and a little heat will be produced.
The potential energy has been through two changes: potential energy to kinetic and kinetic energy to sound and heat.
Other types of energy that you might see are electrical and chemical; there are others but we do not need to produce a full list to examine the basic principles.
What type of energy is electrical energy? The answer is, ‘it depends what it’s doing’.
Electrical Energy in a battery is potential energy; it’s there but it is doing nothing, but it does have the potential to make a small light bulb or a small electric motor work. When we turn on the switch to make the battery work, the electrical energy travels along wires or another form of connection to the item we want to work. As the electrical energy travels to the object, it is kinetic in nature (energy of movement). When the electrical energy arrives at its destination, it may be converted again. In a light bulb, it will produce light and heat energy. In a small electric motor, it will produce mechanical kinetic energy (it makes the motor parts move).
Heat energy is kinetic energy. We think of things as hot or cold, and we can feel the difference what we can’t see is happening to the molecules in a hot or cold object.
When objects are cool the molecules do not move very much; as we increase the temperature, they move around more and much more quickly. We can see some evidence of this because we all know that as things warm up, they get bigger (expand). They do this because the molecules are moving about more. You can do an experiment to show this with a group of friends. Get them together and stand as still as you can and as close together. You will be able to fit into quite a small space. Now start moving around as if you might be dancing, and see how much more space is required by the group as a whole. This is exactly what happens when an object is heated (Figure 1.4).
Figure 1.4 Explanation of expansion due to heating
When the bar is cold, the individual molecules (represented by the circles) are packed close together. After heating each molecule will move about; let’s say they move backwards and forwards between the lines I have set at each side of them. Look at how much longer the bar would have to be to allow this movement.
When we give objects heat energy, we change their temperature. Heat energy can be transferred (moved) in three ways: conduction, convection and radiation.
Conduction of heat energy is through simple physical contact. The heat energy is passed from one molecule or one body to another. If you were standing in that closely packed group of friends and you were the only one that wanted to dance, the person next to you would soon be forced to move and then the one next to them and so on. If you were standing next to another group, they would also soon be forced to start moving.
Convection is how heat is generally transferred in liquids and gases; the warm molecules actually move from one area to another. If you have a bath with the tap set at one end and you fill it with cold water and then put hot water in the tap end, you can make the other end warm by swishing the water round with your hand. Eventually all of the water will have the same temperature because hot mixes with cold and the hot water molecules pass some of their heat to the cooler ones through conduction. So there will always be some conduction along with convection simply because the molecules are in contact with each other.
Radiation is the most difficult to understand as it does not pass through particles by movement or contact; in fact it does not pass through particles at all as it can move through a perfect vacuum (i.e. an area containing not even a single fundamental particle). Radiation is how we can feel the heat of the sun as it passes through millions of miles of space and gives kinetic energy to the molecules of our skin.
Large amounts of heat energy are produced when an X‐ray machine is working, and we have to be able to move it away to stop it from damaging the machine, so these heat transfer methods are important during the production of X‐rays.
ELECTRICAL ENERGY
The basis of electrical energy comes from the existence of the two types of electric charge that we have mentioned already, the positive charge on a proton and the negative charge on an electron. Electric charge is measured by a unit called the coulomb (C). This is a relatively large unit of charge, and for there to be 1 C of negative charge, we would need to collect 6 × 1018 electrons (that’s 6 followed by 18 noughts), or the same number of protons will produce 1 C of positive charge.
These charges have an influence on each other; forces will exist between them. If the charges are alike (two positives or two negatives), they will push each other away (Figure 1.5a and 1.5b). If they are unlike charges, they will attract each other (Figure 1.5c). This force is always present, it’s strength will depend the size of the charges and a number of other factors.
Figure 1.5 (a, b and c) Electric forces
The force of attraction can also be seen in objects that have no charge if a large enough external charge is brought close to it. This happens because the electron orbits around an atom can be distorted (have their shape changed) (Figure 1.6).
Figure 1.6 Charge induction through electron orbit distortion
The electron orbits have been distorted by the close positive charge; the electrons have been attracted to it, and the orbits have become elliptical. The positive charges of the nucleus are effectively further from the large external positive charge than the electrons are, and the two objects are attracted towards each other because the atom behaves as if it is negatively charged. If the external electric charge had been a negative one, the electrons would have been pushed away, and there would effectively be a force of repulsion between the external charge and the atom.
This effect has been induced by the external charge.
