The Free High School Science Texts: Textbooks for High School Students Chemistry

The Free High School Science Texts: Textbooks for High School Students Chemistry
FHSST Authors
The Free High School Science Texts:
Textbooks for High School Students
Studying the Sciences
Chemistry
Grades 10 - 12
Version 0
November 9, 2008
ii
Copyright 2007 “Free High School Science Texts”
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FHSST Core Team
Mark Horner ; Samuel Halliday ; Sarah Blyth ; Rory Adams ; Spencer Wheaton
FHSST Editors
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Whitfield
FHSST Contributors
Rory Adams ; Prashant Arora ; Richard Baxter ; Dr. Sarah Blyth ; Sebastian Bodenstein ;
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Daniels ; Sean Dobbs ; Fernando Durrell ; Dr. Dan Dwyer ; Frans van Eeden ; Giovanni
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Andrew Kubik ; Dr. Marco van Leeuwen ; Dr. Anton Machacek ; Dr. Komal Maheshwari ;
Kosma von Maltitz ; Nicole Masureik ; John Mathew ; JoEllen McBride ; Nikolai Meures ;
Riana Meyer ; Jenny Miller ; Abdul Mirza ; Asogan Moodaly ; Jothi Moodley ; Nolene Naidu ;
Tyrone Negus ; Thomas O’Donnell ; Dr. Markus Oldenburg ; Dr. Jaynie Padayachee ;
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iii
iv
Contents
I
II
Introduction
1
Matter and Materials
3
1 Classification of Matter - Grade 10
1.1
1.2
5
Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.1.1
Heterogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.2
Homogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.3
Separating mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Pure Substances: Elements and Compounds . . . . . . . . . . . . . . . . . . . .
9
1.2.1
Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.2.2
Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.3
Giving names and formulae to substances . . . . . . . . . . . . . . . . . . . . . 10
1.4
Metals, Semi-metals and Non-metals . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.1
Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.2
Non-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.3
Semi-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5
Electrical conductors, semi-conductors and insulators . . . . . . . . . . . . . . . 14
1.6
Thermal Conductors and Insulators . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.7
Magnetic and Non-magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . 17
1.8
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 What are the objects around us made of? - Grade 10
21
2.1
Introduction: The atom as the building block of matter . . . . . . . . . . . . . . 21
2.2
Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1
Representing molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3
Intramolecular and intermolecular forces . . . . . . . . . . . . . . . . . . . . . . 25
2.4
The Kinetic Theory of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5
The Properties of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3 The Atom - Grade 10
3.1
35
Models of the Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.1
The Plum Pudding Model . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.2
Rutherford’s model of the atom
v
. . . . . . . . . . . . . . . . . . . . . . 36
CONTENTS
3.1.3
3.2
3.3
CONTENTS
The Bohr Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1
How heavy is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.2
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Atomic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.1
The Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.2
The Nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4
Atomic number and atomic mass number . . . . . . . . . . . . . . . . . . . . . 40
3.5
Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.6
3.7
3.8
3.9
3.5.1
What is an isotope? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.5.2
Relative atomic mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Energy quantisation and electron configuration . . . . . . . . . . . . . . . . . . 46
3.6.1
The energy of electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.6.2
Energy quantisation and line emission spectra . . . . . . . . . . . . . . . 47
3.6.3
Electron configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.6.4
Core and valence electrons . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6.5
The importance of understanding electron configuration . . . . . . . . . 51
Ionisation Energy and the Periodic Table . . . . . . . . . . . . . . . . . . . . . . 53
3.7.1
Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.7.2
Ionisation Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
The Arrangement of Atoms in the Periodic Table . . . . . . . . . . . . . . . . . 56
3.8.1
Groups in the periodic table
. . . . . . . . . . . . . . . . . . . . . . . . 56
3.8.2
Periods in the periodic table . . . . . . . . . . . . . . . . . . . . . . . . 58
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4 Atomic Combinations - Grade 11
63
4.1
Why do atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2
Energy and bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3
What happens when atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4
Covalent Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.1
The nature of the covalent bond . . . . . . . . . . . . . . . . . . . . . . 65
4.5
Lewis notation and molecular structure . . . . . . . . . . . . . . . . . . . . . . . 69
4.6
Electronegativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7
4.8
4.6.1
Non-polar and polar covalent bonds . . . . . . . . . . . . . . . . . . . . 73
4.6.2
Polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Ionic Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.1
The nature of the ionic bond . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.2
The crystal lattice structure of ionic compounds . . . . . . . . . . . . . . 76
4.7.3
Properties of Ionic Compounds . . . . . . . . . . . . . . . . . . . . . . . 76
Metallic bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.8.1
The nature of the metallic bond . . . . . . . . . . . . . . . . . . . . . . 76
4.8.2
The properties of metals . . . . . . . . . . . . . . . . . . . . . . . . . . 77
vi
CONTENTS
4.9
CONTENTS
Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.9.1
The formulae of covalent compounds . . . . . . . . . . . . . . . . . . . . 78
4.9.2
The formulae of ionic compounds . . . . . . . . . . . . . . . . . . . . . 80
4.10 The Shape of Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.10.1 Valence Shell Electron Pair Repulsion (VSEPR) theory . . . . . . . . . . 82
4.10.2 Determining the shape of a molecule . . . . . . . . . . . . . . . . . . . . 82
4.11 Oxidation numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5 Intermolecular Forces - Grade 11
91
5.1
Types of Intermolecular Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2
Understanding intermolecular forces . . . . . . . . . . . . . . . . . . . . . . . . 94
5.3
Intermolecular forces in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6 Solutions and solubility - Grade 11
101
6.1
Types of solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.2
Forces and solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.3
Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7 Atomic Nuclei - Grade 11
107
7.1
Nuclear structure and stability . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2
The Discovery of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3
Radioactivity and Types of Radiation . . . . . . . . . . . . . . . . . . . . . . . . 108
7.4
7.3.1
Alpha (α) particles and alpha decay . . . . . . . . . . . . . . . . . . . . 109
7.3.2
Beta (β) particles and beta decay . . . . . . . . . . . . . . . . . . . . . 109
7.3.3
Gamma (γ) rays and gamma decay . . . . . . . . . . . . . . . . . . . . . 110
Sources of radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.1
Natural background radiation . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.2
Man-made sources of radiation . . . . . . . . . . . . . . . . . . . . . . . 113
7.5
The ’half-life’ of an element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.6
The Dangers of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.7
The Uses of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.8
Nuclear Fission
7.9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.8.1
The Atomic bomb - an abuse of nuclear fission . . . . . . . . . . . . . . 119
7.8.2
Nuclear power - harnessing energy . . . . . . . . . . . . . . . . . . . . . 120
Nuclear Fusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.10 Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.10.1 Age of Nucleosynthesis (225 s - 103 s) . . . . . . . . . . . . . . . . . . . 121
7.10.2 Age of Ions (103 s - 1013 s) . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.3 Age of Atoms (1013 s - 1015 s) . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.4 Age of Stars and Galaxies (the universe today) . . . . . . . . . . . . . . 122
7.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
vii
CONTENTS
CONTENTS
8 Thermal Properties and Ideal Gases - Grade 11
125
8.1
A review of the kinetic theory of matter . . . . . . . . . . . . . . . . . . . . . . 125
8.2
Boyle’s Law: Pressure and volume of an enclosed gas . . . . . . . . . . . . . . . 126
8.3
Charles’s Law: Volume and Temperature of an enclosed gas . . . . . . . . . . . 132
8.4
The relationship between temperature and pressure . . . . . . . . . . . . . . . . 136
8.5
The general gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.6
The ideal gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.7
Molar volume of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.8
Ideal gases and non-ideal gas behaviour . . . . . . . . . . . . . . . . . . . . . . 146
8.9
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
9 Organic Molecules - Grade 12
151
9.1
What is organic chemistry? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.2
Sources of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.3
Unique properties of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4
Representing organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.1
Molecular formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.2
Structural formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
9.4.3
Condensed structural formula . . . . . . . . . . . . . . . . . . . . . . . . 153
9.5
Isomerism in organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 154
9.6
Functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7
The Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7.1
The Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
9.7.2
Naming the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9.7.3
Properties of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.4
Reactions of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.5
The alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.6
Naming the alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.7
The properties of the alkenes . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.8
Reactions of the alkenes
9.7.9
The Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
. . . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.10 Naming the alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
9.8
9.9
The Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
9.8.1
Naming the alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
9.8.2
Physical and chemical properties of the alcohols . . . . . . . . . . . . . . 175
Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
9.9.1
Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
9.9.2
Derivatives of carboxylic acids: The esters . . . . . . . . . . . . . . . . . 178
9.10 The Amino Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.11 The Carbonyl Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
viii
CONTENTS
CONTENTS
10 Organic Macromolecules - Grade 12
185
10.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
10.2 How do polymers form? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.1 Addition polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.2 Condensation polymerisation . . . . . . . . . . . . . . . . . . . . . . . . 188
10.3 The chemical properties of polymers . . . . . . . . . . . . . . . . . . . . . . . . 190
10.4 Types of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5 Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5.1 The uses of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
10.5.2 Thermoplastics and thermosetting plastics . . . . . . . . . . . . . . . . . 194
10.5.3 Plastics and the environment . . . . . . . . . . . . . . . . . . . . . . . . 195
10.6 Biological Macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
10.6.1 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
10.6.2 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.6.3 Nucleic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
10.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
III
Chemical Change
209
11 Physical and Chemical Change - Grade 10
211
11.1 Physical changes in matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
11.2 Chemical Changes in Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
11.2.1 Decomposition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 213
11.2.2 Synthesis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
11.3 Energy changes in chemical reactions . . . . . . . . . . . . . . . . . . . . . . . . 217
11.4 Conservation of atoms and mass in reactions . . . . . . . . . . . . . . . . . . . . 217
11.5 Law of constant composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.6 Volume relationships in gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
12 Representing Chemical Change - Grade 10
223
12.1 Chemical symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
12.2 Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3 Balancing chemical equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3.1 The law of conservation of mass . . . . . . . . . . . . . . . . . . . . . . 224
12.3.2 Steps to balance a chemical equation
. . . . . . . . . . . . . . . . . . . 226
12.4 State symbols and other information . . . . . . . . . . . . . . . . . . . . . . . . 230
12.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
13 Quantitative Aspects of Chemical Change - Grade 11
233
13.1 The Mole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
13.2 Molar Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
13.3 An equation to calculate moles and mass in chemical reactions . . . . . . . . . . 237
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13.4 Molecules and compounds
CONTENTS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
13.5 The Composition of Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
13.6 Molar Volumes of Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
13.7 Molar concentrations in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
13.8 Stoichiometric calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
13.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
14 Energy Changes In Chemical Reactions - Grade 11
255
14.1 What causes the energy changes in chemical reactions? . . . . . . . . . . . . . . 255
14.2 Exothermic and endothermic reactions . . . . . . . . . . . . . . . . . . . . . . . 255
14.3 The heat of reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
14.4 Examples of endothermic and exothermic reactions . . . . . . . . . . . . . . . . 259
14.5 Spontaneous and non-spontaneous reactions . . . . . . . . . . . . . . . . . . . . 260
14.6 Activation energy and the activated complex . . . . . . . . . . . . . . . . . . . . 261
14.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
15 Types of Reactions - Grade 11
267
15.1 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.1 What are acids and bases? . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.2 Defining acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.3 Conjugate acid-base pairs . . . . . . . . . . . . . . . . . . . . . . . . . . 269
15.1.4 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
15.1.5 Acid-carbonate reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 274
15.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
15.2.1 Oxidation and reduction
. . . . . . . . . . . . . . . . . . . . . . . . . . 277
15.2.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
15.3 Addition, substitution and elimination reactions . . . . . . . . . . . . . . . . . . 280
15.3.1 Addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
15.3.2 Elimination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
15.3.3 Substitution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
15.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
16 Reaction Rates - Grade 12
287
16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
16.2 Factors affecting reaction rates . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
16.3 Reaction rates and collision theory . . . . . . . . . . . . . . . . . . . . . . . . . 293
16.4 Measuring Rates of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
16.5 Mechanism of reaction and catalysis . . . . . . . . . . . . . . . . . . . . . . . . 297
16.6 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
16.6.1 Open and closed systems . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.2 Reversible reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.3 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
16.7 The equilibrium constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
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16.7.1 Calculating the equilibrium constant . . . . . . . . . . . . . . . . . . . . 305
16.7.2 The meaning of kc values . . . . . . . . . . . . . . . . . . . . . . . . . . 306
16.8 Le Chatelier’s principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
16.8.1 The effect of concentration on equilibrium . . . . . . . . . . . . . . . . . 310
16.8.2 The effect of temperature on equilibrium . . . . . . . . . . . . . . . . . . 310
16.8.3 The effect of pressure on equilibrium . . . . . . . . . . . . . . . . . . . . 312
16.9 Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
16.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
17 Electrochemical Reactions - Grade 12
319
17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
17.2 The Galvanic Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
17.2.1 Half-cell reactions in the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . 321
17.2.2 Components of the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . . . . 322
17.2.3 The Galvanic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
17.2.4 Uses and applications of the galvanic cell . . . . . . . . . . . . . . . . . 324
17.3 The Electrolytic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
17.3.1 The electrolysis of copper sulphate . . . . . . . . . . . . . . . . . . . . . 326
17.3.2 The electrolysis of water . . . . . . . . . . . . . . . . . . . . . . . . . . 327
17.3.3 A comparison of galvanic and electrolytic cells . . . . . . . . . . . . . . . 328
17.4 Standard Electrode Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
17.4.1 The different reactivities of metals . . . . . . . . . . . . . . . . . . . . . 329
17.4.2 Equilibrium reactions in half cells . . . . . . . . . . . . . . . . . . . . . . 329
17.4.3 Measuring electrode potential . . . . . . . . . . . . . . . . . . . . . . . . 330
17.4.4 The standard hydrogen electrode . . . . . . . . . . . . . . . . . . . . . . 330
17.4.5 Standard electrode potentials . . . . . . . . . . . . . . . . . . . . . . . . 333
17.4.6 Combining half cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
17.4.7 Uses of standard electrode potential . . . . . . . . . . . . . . . . . . . . 338
17.5 Balancing redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
17.6 Applications of electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.1 Electroplating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.2 The production of chlorine . . . . . . . . . . . . . . . . . . . . . . . . . 348
17.6.3 Extraction of aluminium
. . . . . . . . . . . . . . . . . . . . . . . . . . 349
17.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
IV
Chemical Systems
353
18 The Water Cycle - Grade 10
355
18.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.2 The importance of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.3 The movement of water through the water cycle . . . . . . . . . . . . . . . . . . 356
18.4 The microscopic structure of water . . . . . . . . . . . . . . . . . . . . . . . . . 359
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18.4.1 The polar nature of water . . . . . . . . . . . . . . . . . . . . . . . . . . 359
18.4.2 Hydrogen bonding in water molecules . . . . . . . . . . . . . . . . . . . 359
18.5 The unique properties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
18.6 Water conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
18.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
19 Global Cycles: The Nitrogen Cycle - Grade 10
369
19.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.2 Nitrogen fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.3 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
19.4 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
19.5 Human Influences on the Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . . . 372
19.6 The industrial fixation of nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . 373
19.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
20 The Hydrosphere - Grade 10
377
20.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.2 Interactions of the hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.3 Exploring the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
20.4 The Importance of the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5 Ions in aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5.1 Dissociation in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
20.5.2 Ions and water hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.3 The pH scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.4 Acid rain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
20.6 Electrolytes, ionisation and conductivity . . . . . . . . . . . . . . . . . . . . . . 386
20.6.1 Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
20.6.2 Non-electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
20.6.3 Factors that affect the conductivity of water . . . . . . . . . . . . . . . . 387
20.7 Precipitation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
20.8 Testing for common anions in solution . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.1 Test for a chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.2 Test for a sulphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.3 Test for a carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
20.8.4 Test for bromides and iodides . . . . . . . . . . . . . . . . . . . . . . . . 392
20.9 Threats to the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
20.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
21 The Lithosphere - Grade 11
397
21.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
21.2 The chemistry of the earth’s crust . . . . . . . . . . . . . . . . . . . . . . . . . 398
21.3 A brief history of mineral use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
21.4 Energy resources and their uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 400
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21.5 Mining and Mineral Processing: Gold . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.2 Mining the Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.3 Processing the gold ore . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.4 Characteristics and uses of gold . . . . . . . . . . . . . . . . . . . . . . . 402
21.5.5 Environmental impacts of gold mining . . . . . . . . . . . . . . . . . . . 404
21.6 Mining and mineral processing: Iron . . . . . . . . . . . . . . . . . . . . . . . . 406
21.6.1 Iron mining and iron ore processing . . . . . . . . . . . . . . . . . . . . . 406
21.6.2 Types of iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
21.6.3 Iron in South Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
21.7 Mining and mineral processing: Phosphates . . . . . . . . . . . . . . . . . . . . 409
21.7.1 Mining phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.7.2 Uses of phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.8 Energy resources and their uses: Coal . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.1 The formation of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.2 How coal is removed from the ground . . . . . . . . . . . . . . . . . . . 411
21.8.3 The uses of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
21.8.4 Coal and the South African economy . . . . . . . . . . . . . . . . . . . . 412
21.8.5 The environmental impacts of coal mining . . . . . . . . . . . . . . . . . 413
21.9 Energy resources and their uses: Oil . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.1 How oil is formed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.2 Extracting oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.3 Other oil products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.9.4 The environmental impacts of oil extraction and use . . . . . . . . . . . 415
21.10Alternative energy resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.11Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
22 The Atmosphere - Grade 11
421
22.1 The composition of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . 421
22.2 The structure of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.1 The troposphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.2 The stratosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.3 The mesosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.2.4 The thermosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.3 Greenhouse gases and global warming . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.1 The heating of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.2 The greenhouse gases and global warming . . . . . . . . . . . . . . . . . 426
22.3.3 The consequences of global warming . . . . . . . . . . . . . . . . . . . . 429
22.3.4 Taking action to combat global warming . . . . . . . . . . . . . . . . . . 430
22.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
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23 The Chemical Industry - Grade 12
435
23.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2 Sasol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2.1 Sasol today: Technology and production . . . . . . . . . . . . . . . . . . 436
23.2.2 Sasol and the environment . . . . . . . . . . . . . . . . . . . . . . . . . 440
23.3 The Chloralkali Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
23.3.1 The Industrial Production of Chlorine and Sodium Hydroxide . . . . . . . 442
23.3.2 Soaps and Detergents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446
23.4 The Fertiliser Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.1 The value of nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.2 The Role of fertilisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.3 The Industrial Production of Fertilisers . . . . . . . . . . . . . . . . . . . 451
23.4.4 Fertilisers and the Environment: Eutrophication . . . . . . . . . . . . . . 454
23.5 Electrochemistry and batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.1 How batteries work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.2 Battery capacity and energy . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.3 Lead-acid batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.4 The zinc-carbon dry cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
23.5.5 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . 460
23.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
A GNU Free Documentation License
467
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Chapter 7
Atomic Nuclei - Grade 11
Nuclear physics is the branch of physics which deals with the nucleus of the atom. Within
this field, some scientists focus their attention on looking at the particles inside the nucleus
and understanding how they interact, while others classify and interpret the properties of nuclei.
This detailed knowledge of the nucleus makes it possible for technological advances to be made.
In this next chapter, we are going to touch on each of these different areas within the field of
nuclear physics.
7.1
Nuclear structure and stability
You will remember from an earlier chapter that an atom is made up of different types of particles:
protons (positive charge) neutrons (neutral) and electrons (negative charge). The nucleus is the
part of the atom that contains the protons and the neutrons, while the electrons are found in
energy orbitals around the nucleus. The protons and neutrons together are called nucleons. It
is the nucleus that makes up most of an atom’s atomic mass, because an electron has a very
small mass when compared with a proton or a neutron.
Within the nucleus, there are different forces which act between the particles. The strong
nuclear force is the force between two or more nucleons, and this force binds protons and
neutrons together inside the nucleus. This force is most powerful when the nucleus is small,
and the nucleons are close together. The electromagnetic force causes the repulsion between
like-charged (positive) protons. In a way then, these forces are trying to produce opposite effects
in the nucleus. The strong nuclear force acts to hold all the protons and neutrons close together,
while the electromagnetic force acts to push protons further apart. In atoms where the nuclei are
small, the strong nuclear force overpowers the electromagnetic force. However, as the nucleus
gets bigger (in elements with a higher number of nucleons), the electromagnetic force becomes
greater than the strong nuclear force. In these nuclei, it becomes possible for particles and energy
to be ejected from the nucleus. These nuclei are called unstable. The particles and energy that
a nucleus releases are referred to as radiation, and the atom is said to be radioactive. We are
going to look at these concepts in more detail in the next few sections.
7.2
The Discovery of Radiation
Radioactivity was first discovered in 1896 by a French scientist called Henri Becquerel while he
was working on phosphorescent materials. He happened to place some uranium crystals on black
paper that he had used to cover a piece of film. When he looked more carefully, he noticed that
the film had lots of patches on it, and that this did not happen when other elements were placed
on the paper. He eventually concluded that some rays must be coming out of the uranium
crystals to produce this effect.
His observations were taken further by the Polish scientist Marie Curie and her husband Pierre,
who increased our knowledge of radioactive elements. In 1903, Henri, Marie and Pierre were
107
7.3
CHAPTER 7. ATOMIC NUCLEI - GRADE 11
awarded the Nobel Prize in Physics for their work on radioactive elements. This award made
Marie the first woman ever to receive a Nobel Prize. Marie Curie and her husband went on to
discover two new elements, which they named polonium (Po) after Marie’s home country, and
radium (Ra) after its highly radioactive characteristics. For these dicoveries, Marie was awarded a
Nobel Prize in Chemistry in 1911, making her one of very few people to receive two Nobel Prizes.
teresting Marie Curie died in 1934 from aplastic anemia, which was almost certainly partly
Interesting
Fact
Fact
due to her massive exposure to radiation during her lifetime. Most of her work
was carried out in a shed without safety measures, and she was known to carry
test tubes full of radioactive isotopes in her pockets and to store them in her
desk drawers. By the end of her life, not only was she very ill, but her hands
had become badly deformed due to their constant exposure to radiation. Unfortunately it was only later in her life that the full dangers of radiation were
realised.
7.3
Radioactivity and Types of Radiation
In section 7.1, we discussed that when a nucleus is unstable it can emit particles and energy.
This is called radioactive decay.
Definition: Radioactive decay
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting
particles or electromagnetic waves. Radiation is the name for the emitted particles or
electromagnetic waves.
When a nucleus undergoes radioactive decay, it emits radiation and the nucleus is called radioactive. We are exposed to small amounts of radiation all the time. Even the rocks around us emit
radiation! However some elements are far more radioactive than others. Isotopes tend to be
less stable because they contain a larger number of nucleons than ’non-isotopes’ of the same
element. These radioactive isotopes are called radioisotopes.
Radiation can be emitted in different forms. There are three main types of radiation: alpha,
beta and gamma radiation. These are shown in figure 7.1, and are described below.
alpha (α)
beta (β)
gamma (γ)
paper
aluminium
Figure 7.1: Types of radiation
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
7.3.1
7.3
Alpha (α) particles and alpha decay
An alpha particle is made up of two protons and two neutrons bound together. This type of
radiation has a positive charge. An alpha particle is sometimes represented using the chemical
symbol He2+ , because it has the same structure as a Helium atom (two neutrons and two
protons) which is missing its two electrons, hence the overall charge of +2. Alpha particles
have very low penetration power. Penetration power describes how easily the particles can pass
through another material. Because alpha particles have a low penetration power, it means that
even something as thin as a piece of paper or the outside layer of the human skin, will absorb
these particles so that they can’t go any further.
Alpha decay occurs because the nucleus has too many protons, and this causes a lot of repulsion
between these like charges. To try to reduce this repulsion, the nucleus emits an α particle. This
can be seen in the decay of Americium (Am) to Neptunium (Np).
Example:
241
95 Am
→237
93 Np + αparticle
Let’s take a closer look at what has happened during this reaction. Americium (Z = 95; A =
241) undergoes α decay and releases one alpha particle (i.e. 2 protons and 2 neutrons). The
atom now has only 93 protons (Z = 93). On the periodic table, the element which has 93 protons
(Z = 93) is called Neptunium. Therefore, the Americium atom has become a Neptunium atom.
The atomic mass of the neptunium atom is 237 (A = 237) because 4 nucleons (2 protons and
2 neutrons) were emitted from the atom of Americium.
7.3.2
Beta (β) particles and beta decay
In certain types of radioactive nuclei that have too many neutrons, a neutron may be converted
into a proton, an electron and another particle (called a neutrino). The high energy electrons
that are released in this way are called beta particles. Beta particles have a higher penetration
power than alpha particles and are able to pass through thicker materials such as paper.
The diagram below shows what happens during β decay:
electron (β particle)
One of the neutrons from H-3
is converted to
a proton
Atomic nucleus
neutrino (ν̄)
Hydrogen-3
= one proton
Helium-3
An electron and a
neutrino are released
= one neutron
Figure 7.2: β decay in a hydrogen atom
During beta decay, the number of neutrons in the atom decreases by one, and the number of
protons increases by one. Since the number of protons before and after the decay is different,
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the atom has changed into a different element. In figure 7.2, Hydrogen has become Helium.
The beta decay of the Hydrogen-3 atom can be represented as follows:
3
1H
→32 He + βparticle + ν̄
teresting When scientists added up all the energy from the neutrons, protons and electrons
Interesting
Fact
Fact
involved in β-decays, they noticed that there was always some energy missing.
We know that energy is always conserved, which led Wolfgang Pauli in 1930 to
come up with the idea that another particle, which was not detected yet, also
had to be involved in the decay. He called this particle the neutrino (Italian
for ”little neutral one”), because he knew it had to be neutral, have little or no
mass, and interact only very weakly, making it very hard to find experimentally!
The neutrino was finally identified experimentally about 25 years after Pauli first
thought of it.
Due to the radioactive processes inside the sun, each 1 cm2 patch of the earth
receives 70 billion (70×109) neutrinos each second! Luckily neutrinos only interact very weakly so they do not harm our bodies when billions of them pass
through us every second.
7.3.3
Gamma (γ) rays and gamma decay
When particles inside the nucleus collide during radioactive decay, energy is released. This energy can leave the nucleus in the form of waves of electromagnetic energy called gamma rays.
Gamma radiation is part of the electromagnetic spectrum, just like visible light. However, unlike
visible light, humans cannot see gamma rays because they are at a higher frequency and a higher
energy. Gamma radiation has no mass or charge. This type of radiation is able to penetrate
most common substances, including metals. The only substances that can absorb this radiation
are thick lead and concrete.
Gamma decay occurs if the nucleus is at too high an energy level. Since gamma rays are part
of the electromagnetic spectrum, they can be thought of as waves or particles. Therefore in
gamma decay, we can think of a ray or a particle (called a photon) being released. The atomic
number and atomic mass remain unchanged.
Table 7.1 summarises and compares the three types of radioactive decay that have been discussed.
Table 7.1: A comparison of alpha, beta and gamma decay
Type of decay Particle/ray released
Change in
element
Alpha (α)
α particle (2 protons and 2 neutrons) Yes
Beta (β)
β particle (electron)
Yes
Gamma (γ)
γ ray (electromagnetic energy)
No
Penetration
power
Low
Medium
High
Worked Example 23: Radioactive decay
Question: The isotope
particles.
241
95 Pb
undergoes radioactive decay and loses two alpha
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7.3
photon (γ particle)
Helium-3
Helium-3
Figure 7.3: γ decay in a helium atom
1. Write the chemical formula of the element that is produced as a result of the
decay.
2. Write an equation for this decay process.
Answer
Step 1 : Work out the number of protons and/or neutrons that the radioisotope loses during radioactive decay
One α particle consists of two protons and two neutrons. Since two α particles are
released, the total number of protons lost is four and the total number of neutrons
lost is also four.
Step 2 : Calculate the atomic number (Z) and atomic mass number (A) of
the element that is formed.
Z = 95 − 4 = 91
A = 241 − 4 = 237
Step 3 : Refer to the periodic table to see which element has the atomic
number that you have calculated.
The element that has Z = 91 is Protactinium (Pa).
Step 4 : Write the symbol for the element that has formed as a result of
radioactive decay.
237
91 Pa
Step 5 : Write an equation for the decay process.
241
95 P b
→237
91 P a + 2 protons + 2 neutrons
Activity :: Discussion : Radiation
In groups of 3-4, discuss the following questions:
• Which of the three types of radiation is most dangerous to living creatures
(including humans!)
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• What can happen to people if they are exposed to high levels of radiation?
• What can be done to protect yourself from radiation (Hint: Think of what the
radiologist does when you go for an X-ray)?
Exercise: Radiation and radioactive elements
1. There are two main forces inside an atomic nucleus:
(a) Name these two forces.
(b) Explain why atoms that contain a greater number of nucleons are more
likely to be radioactive.
2. The isotope 241
95 Pb undergoes radioactive decay and loses three alpha particles.
(a) Write the chemical formula of the element that is produced as a result of
the decay.
(b) How many nucleons does this element contain?
3. Complete the following equation:
210
82 Am
→ (alpha decay)
4. Radium-228 decays by emitting a beta particle. Write an equation for this
decay process.
5. Describe how gamma decay differs from alpha and beta decay.
7.4
Sources of radiation
The sources of radiation can be either natural or man-made.
7.4.1
Natural background radiation
• Cosmic radiation
The Earth, and all living things on it, are constantly bombarded by radiation from space.
Charged particles from the sun and stars interact with the Earth’s atmosphere and magnetic
field to produce a shower of radiation, mostly beta and gamma radiation. The amount of
cosmic radiation varies in different parts of the world because of differences in elevation
and also the effects of the Earth’s magnetic field.
• Terrestrial Radiation
Radioactive material is found throughout nature. It occurs naturally in the soil, water, and
vegetation. The major isotopes that are of concern are uranium and the decay products of
uranium, such as thorium, radium, and radon. Low levels of uranium, thorium, and their
decay products are found everywhere. Some of these materials are ingested (taken in)
with food and water, while others are breathed in. The dose of radiation from terrestrial
sources varies in different parts of the world.
teresting Cosmic and terrestrial radiation are not the only natural sources. All people
Interesting
Fact
Fact
have radioactive potassium-40, carbon-14, lead-210 and other isotopes inside
their bodies from birth.
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7.4.2
7.5
Man-made sources of radiation
Although all living things are exposed to natural background radiation, there are other sources
of radiation. Some of these will affect most members of the public, while others will only affect
those people who are exposed to radiation through their work.
• Members of the Public
Man-made radiation sources that affect members of the public include televisions, tobacco (polonium-210), combustible fuels, smoke detectors (americium), luminous watches
(tritium) and building materials. By far, the most significant source of man-made radiation exposure to the public is from medical procedures, such as diagnostic x-rays, nuclear
medicine, and radiation therapy. Some of the major isotopes involved are I-131, Tc-99m,
Co-60, Ir-192, and Cs-137. The production of nuclear fuel using uranium is also a source
of radiation for the public, as is fallout from nuclear weapons testing or use.
• Individuals who are exposed through their work
Any people who work in the following environments are exposed to radiation at some time:
radiology (X-ray) departments, nuclear power plants, nuclear medicine departments and
radiation oncology (the study of cancer) departments. Some of the isotopes that are of
concern are cobalt-60, cesium-137, americium-241, and others.
teresting Radiation therapy (or radiotherapy) uses ionising radiation as part of cancer
Interesting
Fact
Fact
treatment to control malignant cells. In cancer, a malignant cell is one that
divides very rapidly to produce many more cells. These groups of dividing cells
can form a growth or tumour. The malignant cells in the tumour can take
nutrition away from other healthy body cells, causing them to die, or can increase the pressure in parts of the body because of the space that they take up.
Radiation therapy uses radiation to try to target these malignant cells and kill
them. However, the radiation can also damage other, healthy cells in the body.
To stop this from happening, shaped radiation beams are aimed from several
angles to intersect at the tumour, so that the radiation dose here is much higher
than in the surrounding, healthy tissue. But even doing this doesn’t protect all
the healthy cells, and that is why people have side-effects to this treatment.
Note that radiation therapy is different from chemotherapy, which uses chemicals,
rather than radiation, to destroy malignant cells. Generally, the side effects of
chemotherapy are greater because the treatment is not as localised as it is with
radiation therapy. The chemicals travel throughout the body, affecting many
healthy cells.
7.5
The ’half-life’ of an element
Definition: Half-life
The half-life of an element is the time it takes for half the atoms of a radioisotope to decay
into other atoms.
Table 7.2 gives some examples of the half-lives of different elements.
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Table 7.2: Table showing the half-life of
Radioisotope Chemical symbol
Polonium-212 Po-212
Sodium-24
Na-24
Strontium-90
Sr-90
Cobalt-60
Co-60
Caesium-137
Cs-137
Carbon-14
C-14
Calcium
Ca
Beryllium
Be
Uranium-235
U-235
a number of elements
Half-life
0.16 seconds
15 hours
28 days
5.3 years
30 years
5 760 years
100 000 years
2 700 000 years
7.1 billion years
So, in the case of Sr-90, it will take 28 days for half of the atoms to decay into other atoms. It
will take another 28 days for half of the remaining atoms to decay. Let’s assume that we have
a sample of strontium that weighs 8g. After the first 28 days there will be:
1/2 x 8 = 4 g Sr-90 left
After 56 days, there will be:
1/2 x 4 g = 2 g Sr-90 left
After 84 days, there will be:
1/2 x 2 g = 1 g Sr-90 left
If we convert these amounts to a fraction of the original sample, then after 28 days 1/2 of the
sample remains undecayed. After 56 days 1/4 is undecayed and after 84 days, 1/8 and so on.
Activity :: Group work : Understanding half-life
Work in groups of 4-5
You will need:
16 sheets of A4 paper per group, scissors, 2 boxes per group, a marking pen and
timer/stopwatch.
What to do:
• Your group should have two boxes. Label one ’decayed’ and the other ’radioactive’.
• Take the A4 pages and cut each into 4 pieces of the same size. You should now
have 64 pieces of paper. Stack these neatly and place them in the ’radioactive’
box. The paper is going to represent some radioactive material.
• Set the timer for one minute. After one minute, remove half the sheets of
paper from the radioactive box and put them in the ’decayed’ box.
• Set the timer for another minute and repeat the previous step, again removing
half the pieces of paper that are left in the radioactive box and putting them
in the decayed box.
• Repeat this process until 8 minutes have passed. You may need to start cutting
your pieces of paper into even smaller pieces as you progress.
Questions:
1. How many pages were left in the radioactive box after...
(a) 1 minute
(b) 3 minutes
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
(c) 5 minutes
2. What percentage (%) of the pages were left in the radioactive box after...
(a) 2 minutes
(b) 4 minutes
3. After how many minutes is there 1/128 of radioactive material remaining?
4. What is the half-life of the ’radioactive’ material in this exercise?
Worked Example 24: Half-life 1
Question: A 100 g sample of Cs-137 is allowed to decay. Calculate the mass of
Cs-137 that will be left after 90 years
Answer
Step 1 : You need to know the half-life of Cs-137
The half-life of Cs-137 is 30 years.
Step 2 : Determine how many times the quantity of sample will be halved in
90 years.
If the half-life of Cs-137 is 30 years, and the sample is left to decay for 90 years,
then the number of times the quantity of sample will be halved is 90/30 = 3.
Step 3 : Calculate the quantity that will be left by halving the mass of Cs-137
three times
1. After 30 years, the mass left is 100 g × 1/2 = 50 g
2. After 60 years, the mass left is 50 g × 1/2 = 25 g
3. After 90 years, the mass left is 25 g × 1/2 = 12.5 g
Note that a quicker way to do this calculation is as follows:
Mass left after 90 years = (1/2)3 × 100 g = 12.5 g (The exponent is the number
of times the quantity is halved)
Worked Example 25: Half-life 2
Question: An 80 g sample of Po-212 decays until only 10 g is left. How long did it
take for this decay to take place?
Answer
Step 1 : Calculate the fraction of the original sample that is left after decay
Fraction remaining = 10 g/80 g = 1/8
Step 2 : Calculate how many half-life periods of decay (x) must have taken
place for 1/8 of the original sample to be left
1
1
( )x =
2
8
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7.5
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
Therefore, x = 3
Step 3 : Use the half-life of Po-212 to calculate how long the sample was
left to decay
The half-life of Po-212 is 0.16 seconds. Therefore if there were three periods of
decay, then the total time is 0.16 × 3. The time that the sample was left to decay
is 0.48 seconds.
Exercise: Looking at half life
1. Imagine that you have 100 g of Na-24.
(a) What is the half life of Na-24?
(b) How much of this isotope will be left after 45 hours?
(c) What percentage of the original sample will be left after 60 hours?
2. A sample of Sr-90 is allowed to decay. After 84 days, 10 g of the sample
remains.
(a) What is the half life of Sr-90?
(b) How much Sr-90 was in the original sample?
(c) How much Sr-90 will be left after 112 days?
7.6
The Dangers of Radiation
Natural radiation comes from a variety of sources such as the rocks, sun and from space. However, when we are exposed to large amounts of radiation, this can cause damage to cells. γ
radiation is particularly dangerous because it is able to penetrate the body, unlike α and β
particles whose penetration power is less. Some of the dangers of radiation are listed below:
• Damage to cells
Radiation is able to penetrate the body, and also to penetrate the membranes of the
cells within our bodies, causing massive damage. Radiation poisoning occurs when a
person is exposed to large amounts of this type of radiation. Radiation poisoning damages
tissues within the body, causing symptoms such as diarrhoea, vomiting, loss of hair and
convulsions.
• Genetic abnormalities
When radiation penetrates cell membranes, it can damage chromosomes within the nucleus
of the cell. The chromosomes contain all the genetic information for that person. If the
chromosomes are changed, this may lead to genetic abnormalities in any children that are
born to the person who has been exposed to radiation. Long after the nuclear disaster
of Chernobyl in Russia in 1986, babies were born with defects such as missing limbs and
abnormal growths.
• Cancer
Small amounts of radiation can cause cancers such as leukemia (cancer of the blood)
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7.7
7.7
The Uses of Radiation
However, despite the many dangers of radiation, it does have many powerful uses, some of which
are listed below:
• Medical Field
Radioactive chemical tracers emitting γ rays can give information about a person’s internal
anatomy and the functioning of specific organs. The radioactive material may be injected
into the patient, from where it will target specific areas such as bones or tumours. As
the material decays and releases radiation, this can be seen using a special type of camera
or other instrument. The radioactive material that is used for this purpose must have a
short half-life so that the radiation can be detected quickly and also so that the material
is quickly removed from the patient’s body. Using radioactive materials for this purpose
can mean that a tumour or cancer may be diagnosed long before these would have been
detected using other methods such as X-rays.
Radiation may also be used to sterilise medical equipment.
Activity :: Research Project : The medical uses of radioisotopes
Carry out your own research to find out more about the radioisotopes that
are used to diagnose diseases in the following parts of the body:
– thyroid gland
– kidneys
– brain
In each case, try to find out...
1. which radioisotope is used
2. what the sources of this radioisotope are
3. how the radioisotope enters the patient’s body and how it is monitored
• Biochemistry and Genetics
Radioisotopes may be used as tracers to label molecules so that chemical processes such
as DNA replication or amino acid transport can be traced.
• Food preservation
Irradiation of food can stop vegetables or plants from sprouting after they have been
harvested. It also kills bacteria and parasites, and controls the ripening of fruits.
• Environment
Radioisotopes can be used to trace and analyse pollutants.
• Archaeology and Carbon dating
Natural radioisotopes such as C-14 can be used to determine the age of organic remains.
All living organisms (e.g. trees, humans) contain carbon. Carbon is taken in by plants
and trees through the process of photosynthesis in the form of carbon dioxide and is then
converted into organic molecules. When animals feed on plants, they also obtain carbon
through these organic compounds. Some of the carbon in carbon dioxide is the radioactive
C-14, while the rest is a non-radioactive form of carbon. When an organism dies, no more
carbon is taken in and there is a limited amount of C-14 in the body. From this point
onwards, C-14 begins its radioactive decay. When scientists uncover remains, they are able
to estimate the age of the remains by seeing how much C-14 is left in the body relative
to the amount of non-radioactive carbon. The less C-14 there is, the older the remains
because radioactive decay must have been taking place for a long time. Because scientists
know the exact rate of decay of C-14, they can calculate a very accurate estimate of the
age of the remains. Carbon dating has been a very important tool in building up accurate
historical records.
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
Activity :: Case Study : Using radiocarbon dating
Radiocarbon dating has played an important role in uncovering many aspects of South Africa’s history. Read the following extract from an article that
appeared in Afrol news on 10th February 2007 and then answer the questions
that follow.
The world famous rock art in South Africa’s uKhahlamba-Drakensberg,
a World Heritage Site, is three times older than previously thought, archaeologists conclude in a new study. The more than 40,000 paintings
were made by the San people some 3000 years ago, a new analysis had
shown.
Previous work on the age of the rock art in uKhahlamba-Drakensberg
concluded it is less than 1,000 years old. But the new study - headed
by a South African archaeologist leading a team from the University
of Newcastle upon Tyne (UK) and Australian National University in
Canberra - estimates the panels were created up to 3,000 years ago.
They used the latest radio-carbon dating technology.
The findings, published in the current edition of the academic journal
’South African Humanities’, have ”major implications for our understanding of how the rock artists lived and the social changes that were
taking place over the last three millennia,” according to a press release
from the British university.
Questions:
1. What is the half-life of carbon-14?
2. In the news article, what role did radiocarbon dating play in increasing our
knowledge of South Africa’s history?
3. Radiocarbon dating can also be used to analyse the remains of once-living
organisms. Imagine that a set of bones are found between layers of sediment
and rock in a remote area. A group of archaeologists carries out a series
of tests to try to estimate the age of the bones. They calculate that the
bones are approximately 23 040 years old.
What percentage of the original carbon-14 must have been left in the bones
for them to arrive at this estimate?
7.8
Nuclear Fission
Nuclear fission is a process where the nucleus of an atom is split into two or more smaller
nuclei, known as fission products. The fission of heavy elements is an exothermic reaction and
huge amounts of energy are released in the process. This energy can be used to produce nuclear
power or to make nuclear weapons, both of which we will discuss a little later.
Definition: Nuclear fission
The splitting of an atomic nucleus
Below is a diagram showing the nuclear fission of Uranium-235. An atom of Uranium-235 is
bombarded with a neutron to initiate the fission process. This neutron is absorbed by Uranium235, to become Uranium-236. Uranium-236 is highly unstable and breaks down into a number
of lighter elements, releasing energy in the process. Free neutrons are also produced during this
process, and these are then available to bombard other fissionable elements. This process is
known as a fission chain reaction, and occurs when one nuclear reaction starts off another,
which then also starts off another one so that there is a rapid increase in the number of nuclear
reactions that are taking place.
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7.8
Massive release of energy during nuclear
fission
Neutron is absorbed
by the nucleus of the
U-235 atom to form
U-236
U-235
U-236
b
neutron
The elements and
number of neutrons
produced
in
the
process, is random.
U-236
splits
into
lighter elements called
fission products and
free neutrons
7.8.1
The Atomic bomb - an abuse of nuclear fission
A nuclear chain reaction can happen very quickly, releasing vast amounts of energy in the process.
In 1939, it was discovered that Uranium could undergo nuclear fission. In fact, it was uranium
that was used in the first atomic bomb. The bomb contained huge amounts of Uranium-235,
enough to start a runaway nuclear fission chain reaction. Because the process was uncontrolled,
the energy from the fission reactions was released in a matter of seconds, resulting in the massive
explosion of that first bomb. Since then, more atomic bombs have been dropped, causing massive
destruction and loss of life.
Activity :: Discussion : Nuclear weapons testing - an ongoing issue
Read the article below which has been adapted from one that appeared in ’The
Globe’ in Washington on 10th October 2006, and then answer the questions that
follow.
US officials and arms control specialists warned yesterday that North Korea’s test of a small nuclear device could start an arms race in the region
and threaten the landmark global treaty designed nearly four decades ago
to halt the spread of nuclear weapons. US officials expressed concern
that North Korea’s neighbors, including Japan, Taiwan, and South Korea,
could eventually decide to develop weapons of their own. They also fear
that North Korea’s moves could embolden Iran, and that this in turn could
encourage Saudi Arabia or other neighbours in the volatile Middle East to
one day seek nuclear deterrents, analysts say.
North Korea is the first country to conduct a nuclear test after pulling
out of the Nuclear Nonproliferation Treaty. The treaty, which was created
in 1968, now includes 185 nations (nearly every country in the world).
Under the treaty, the five declared nuclear powers at the time (United
States, the Soviet Union, France, China, and Great Britain) agreed to
reduce their supplies of nuclear weapons. The treaty has also helped to
limit the number of new nuclear weapons nations.
But there have also been serious setbacks. India and Pakistan, which
never signed the treaty, became new nuclear powers, shocking the world
with test explosions in 1998. The current issue of nuclear weapons testing
in North Korea, is another such setback and a blow to the treaty.
Group discussion questions:
1. Discuss what is meant by an ’arms race’ and a ’treaty’.
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2. Do you think it is important to have such treaties in place to control the testing
and use of nuclear weapons? Explain your answer.
3. Discuss some of the reasons why countries might not agree to be part of a
nuclear weapons treaty.
4. How would you feel if South Africa decided to develop its own nuclear weapons?
7.8.2
Nuclear power - harnessing energy
However, nuclear fission can also be carried out in a controlled way in a nuclear reactor. A nuclear
reactor is a piece of equpiment where nuclear chain reactions can be started in a controlled and
sustained way. This is different from a nuclear explosion where the chain reaction occurs in
seconds. The most important use of nuclear reactors at the moment is to produce electrical
power, and most of these nuclear reactors use nuclear fission. A nuclear fuel is a chemical
isotope that can keep a fission chain reaction going. The most common isotopes that are used
are Uranium-235 and Plutonium-239. The amount of free energy that is in nuclear fuels is far
greater than the energy in a similar amount of other fuels such as gasoline. In many countries,
nuclear power is seen as a relatively environmentally friendly alternative to fossil fuels, which
release large amounts of greenhouse gases, and are also non-renewable resources. However,
one of the concerns around the use of nuclear power, is the production of nuclear waste which
contains radioactive chemical elements.
Activity :: Debate : Nuclear Power
The use of nuclear power as a source of energy has been a subject of much
debate. There are many advantages of nuclear power over other energy sources.
These include the large amount of energy that can be produced at a small plant,
little atmospheric pollution and the small quantity of waste. However there are also
disadvantages. These include the expense of maintaining nuclear power stations, the
huge impact that an accident could have as well as the disposal of dangerous nuclear
waste.
Use these ideas as a starting point for a class debate.
Nuclear power - An energy alternative or environmental hazard?
Your teacher will divide the class into teams. Some of the teams will be ’pro’
nuclear power while the others will be ’anti’ nuclear power.
7.9
Nuclear Fusion
Nuclear fusion is the joining together of the nuclei of two atoms to form a heavier nucleus.
If the atoms involved are small, this process is accompanied by the release of energy. It is the
nuclear fusion of elements that causes stars to shine and hydrogen bombs to explode. As with
nuclear fission then, there are both positive and negative uses of nuclear fusion.
Definition: Nuclear fusion
The joining together of the nuclei of two atoms.
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
7.10
You will remember that nuclei naturally repel one another because of the electrostatic force between their positively charged protons. So, in order to bring two nuclei together, a lot of energy
must be supplied if fusion is to take place. If two nuclei can be brought close enough together
however, the electrostatic force is overwhelmed by the more powerful strong nuclear force which
only operates over short distances. If this happens, nuclear fusion can take place. Inside the
cores of stars, the temperature is high enough for hydrogen fusion to take place but scientists
have so far been unsuccessful in making it work in the laboratory. One of the huge advantages
of nuclear fusion, if it could be made to happen, is that it is a relatively environmentally friendly
source of energy. The helium that is produced is not radioactive or poisonous and does not carry
the dangers of nuclear fission.
7.10
Nucleosynthesis
An astronomer named Edwin Hubble discovered in the 1920’s that the universe is expanding. He
measured that far-away galaxies are moving away from the earth at great speed, and the further
away they are, the faster they are moving.
Extension: What are galaxies?
Galaxies are huge clusters of stars and matter in the universe. The earth is part
of the Milky Way galaxy which is shaped like a very large spiral. Astronomers can
measure the light coming from distant galaxies using telescopes. Edwin Hubble was
also able to measure the velocities of galaxies.
These observations led people to see that the universe is expanding. It also led to the Big Bang
hypothesis. The ’Big Bang’ hypothesis is an idea about how the universe may have started.
According to this theory, the universe started off at the beginning of time as a point which then
exploded and expanded into the universe we live in today. This happened between 10 and 14
billion years ago.
Just after the Big Bang, when the universe was only 10−43 s old, it was very hot and was
made up of quarks and leptons (an example of a lepton is the electron). As the universe
expanded, (∼ 10−2 s) and cooled, the quarks started binding together to form protons and
neutrons (together called nucleons).
7.10.1
Age of Nucleosynthesis (225 s - 103 s)
About 225 s after the Big Bang, the protons and neutrons started binding together to form simple
nuclei. The process of forming nuclei is called nucleosynthesis. When a proton and a neutron
bind together, they form the deuteron. The deuteron is like a hydrogen nucleus (which is just a
proton) with a neutron added to it so it can be written as 2 H. Using protons and neutrons as
building blocks, more nuclei can be formed as shown below. For example, the Helium-4 nucleus
(also called an alpha particle) can be formed in the following ways:
2
H + n → 3H
deuteron + neutron → triton
then:
3
H+p →
4
He
triton + proton → Helium4 (alpha particle)
or
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7.11
CHAPTER 7. ATOMIC NUCLEI - GRADE 11
2
H+p →
3
He
deuteron + proton → Helium3
then:
3
He + n →
4
He
Helium3 + neutron → Helium4 (alpha particle)
Some 7 Li nuclei could also have been formed by the fusion of 4 He and 3 H.
7.10.2
Age of Ions (103 s - 1013 s)
However, at this time the universe was still very hot and the electrons still had too much energy
to become bound to the alpha particles to form helium atoms. Also, the nuclei with mass
numbers greater than 4 (i.e. greater than 4 He) are very short-lived and would have decayed
almost immediately after being formed. Therefore, the universe moved through a stage called
the Age of Ions when it consisted of free positively charged H+ ions and 4 He ions, and negatively
charged electrons not yet bound into atoms.
7.10.3
Age of Atoms (1013 s - 1015 s)
As the universe expanded further, it cooled down until the electrons were able to bind to the
hydrogen and helium nuclei to form hydrogen and helium atoms. Earlier, during the Age of Ions,
both the hydrogen and helium ions were positively charged which meant that they repelled each
other (electrostatically). During the Age of Atoms, the hydrogen and helium along with the
electrons, were in the form of atoms which are electrically neutral and so they no longer repelled
each other and instead pulled together under gravity to form clouds of gas, which evetually
formed stars.
7.10.4
Age of Stars and Galaxies (the universe today)
Inside the core of stars, the densities and temperatures are high enough for fusion reactions to
occur. Most of the heavier nuclei that exist today were formed inside stars from thermonuclear reactions! (It’s interesting to think that the atoms that we are made of were actually
manufactured inside stars!). Since stars are mostly composed of hydrogen, the first stage of
thermonuclear reactions inside stars involves hydrogen and is called hydrogen burning. The
process has three steps and results in four hydrogen atoms being formed into a helium atom
with (among other things) two photons (light!) being released.
The next stage is helium burning which results in the formation of carbon. All these reactions
release a large amount of energy and heat the star which causes heavier and heavier nuclei to
fuse into nuclei with higher and higher atomic numbers. The process stops with the formation
of 56 Fe, which is the most strongly bound nucleus. To make heavier nuclei, even higher energies
are needed than is possible inside normal stars. These nuclei are most likely formed when huge
amounts of energy are released, for example when stars explode (an exploding star is called a
supernova). This is also how all the nuclei formed inside stars get ”recycled” in the universe to
become part of new stars and planets.
7.11
Summary
• Nuclear physics is the branch of physics that deals with the nucleus of an atom.
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CHAPTER 7. ATOMIC NUCLEI - GRADE 11
7.11
• There are two forces between the particles of the nucleus. The strong nuclear force is
an attractive force between the neutrons and the electromagnetic force is the repulsive
force between like-charged protons.
• In atoms with large nuclei, the electromagnetic force becomes greater than the strong
nuclear force and particles or energy may be released from the nucleus.
• Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting
particles or electromagnetic waves.
• The particles and energy released are called radiation and the atom is said to be radioactive.
• Radioactive isotopes are called radioisotopes.
• Radioactivity was first discovered by Marie Curie and her husband Pierre.
• There are three types of radiation from radioactive decay: alpha (α), beta (β) and
gamma (γ) radiation.
• During alpha decay, an alpha particle is released. An alpha particle consists of two protons
and two neutrons bound together. Alpha radiation has low penetration power.
• During beta decay, a beta particle is released. During beta decay, a neutron is converted
to a proton, an electron and a neutrino. A beta particle is the electron that is released.
Beta radiation has greater penetration power than alpha radiation.
• During gamma decay, electromagnetic energy is released as gamma rays. Gamma radiation has the highest penetration power of the three radiation types.
• There are many sources of radiation. Some of natural and others are man-made.
• Natural sources of radiation include cosmic and terrestrial radiation.
• Man-made sources of radiation include televisions, smoke detectors, X-rays and radiation
therapy.
• The half-life of an element is the time it takes for half the atoms of a radioisotope to
decay into other atoms.
• Radiation can be very damaging. Some of the negative impacts of radiation exposure
include damage to cells, genetic abnormalities and cancer.
• However, radiation can also have many positive uses. These include use in the medical
field (e.g. chemical tracers), biochemistry and genetics, use in food preservation, the
environment and in archaeology.
• Nuclear fission is the splitting of an atomic nucleus into smaller fission products. Nuclear
fission produces large amounts of energy, which can be used to produce nuclear power,
and to make nuclear weapons.
• Nuclear fusion is the joining together of the nuclei of two atoms to form a heavier nucleus.
In stars, fusion reactions involve the joining of hydrogen atoms to form helium atoms.
• Nucleosynthesis is the process of forming nuclei. This was very important in helping to
form the universe as we know it.
Exercise: Summary exercise
1. Explain each of the following terms:
(a) electromagnetic force
(b) radioactive decay
(c) radiocarbon dating
123
7.11
CHAPTER 7. ATOMIC NUCLEI - GRADE 11
2. For each of the following questions, choose the one correct answer:
(a) The part of the atom that undergoes radioactive decay is the...
i. neutrons
ii. nucleus
iii. electrons
iv. entire atom
(b) The radio-isotope Po-212 undergoes alpha decay. Which of the following
statements is true?
i. The number of protons in the element remains unchanged.
ii. The number of nucleons after decay is 212.
iii. The number of protons in the element after decay is 82.
iv. The end product after decay is Po-208.
3. 20 g of sodium-24 undergoes radoactive decay. Calculate the percentage of the
original sample that remains after 60 hours.
4. Nuclear physics can be controversial. Many people argue that studying the
nucleus has led to devastation and huge loss of life. Others would argue that
the benefits of nuclear physics far outweigh the negative things that have come
from it.
(a) Outline some of the ways in which nuclear physics has been used in negative
ways.
(b) Outline some of the benefits that have come from nuclear physics.
124
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