2013 | ISBN-10: 0123851106 | 848 pages | PDF | 32 MB This textbook provides essential information for students of inorganic chemistry or for chemists pursuing self-study. The presentation of topics is made with an effort to be clear and concise so that the book is portable and user friendly.
Trang 3Inorganic Chemistry
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Trang 5AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD
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Library of Congress Cataloging-in-Publication Data
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Library of Congress Cataloging-in-Publication Data
House, J E
Inorganic chemistry / James House e 2nd ed
p cm
Includes bibliographical references and index
1 Chemistry, Inorganic e Textbooks I Title
QD151.5.H68 2013
546–dc23
2012017867ISBN: 978-0-12-385110-9
For information on all Elsevier publications
visit our website atwww.store.elsevier.com
Printed and bound in China
12 13 11 10 9 8 7 6 5 4 3 2
Trang 7PREFACE TO THE SECOND EDITION xi
PREFACE TO THE FIRST EDITION xii
PART 1 Structure of Atoms and Molecules 1
CHAPTER 1 Light, Electrons, and Nuclei 3
1.1 Some Early Experiments in Atomic Physics 3
1.2 The Nature of Light 7
1.3 The Bohr Model 11
1.4 ParticleeWave Duality 14
1.5 Electronic Properties of Atoms 16
1.6 Nuclear Binding Energy 21
1.7 Nuclear Stability 23
1.8 Types of Nuclear Decay 25
1.9 Predicting Decay Modes 28
CHAPTER 2 Basic Quantum Mechanics and Atomic Structure 33
2.1 The Postulates 33
2.2 The Hydrogen Atom 40
2.3 The Helium Atom 45
2.4 Slater Wave Functions 47
2.5 Electron Configurations 49
2.6 Spectroscopic States 54
CHAPTER 3 Covalent Bonding in Diatomic Molecules 61
3.1 The Basic Ideas of Molecular Orbital Methods 61
3.2 The Hþ2 and H2Molecules 69
3.3 Diatomic Molecules of Second-Row Elements 71
3.4 Photoelectron Spectroscopy 77
3.5 Heteronuclear Diatomic Molecules 79
3.6 Electronegativity 82
3.7 Spectroscopic States for Molecules 85
v
Trang 8CHAPTER 4 A Survey of Inorganic Structures and Bonding 89
4.1 Structures of Molecules Having Single Bonds 89
4.2 Resonance and Formal Charge 99
4.3 Complex Structures: A Preview of Coming Attractions 111
4.4 Electron-Deficient Molecules 119
4.5 Structures Having Unsaturated Rings 121
4.6 Bond Energies 123
CHAPTER 5 Symmetry and Molecular Orbitals 129
5.1 Symmetry Elements 129
5.2 Orbital Symmetry 137
5.3 A Brief Look at Group Theory 139
5.4 Construction of Molecular Orbitals 144
5.5 Orbitals and Angles 150
5.6 Simple Calculations Using the Hückel Method 152
PART 2 Condensed Phases 167
CHAPTER 6 Dipole Moments and Intermolecular Interactions 169
6.1 Dipole Moments 169
6.2 DipoleeDipole Forces 174
6.3 Dipole-Induced Dipole Forces 176
6.4 London (Dispersion) Forces 177
6.5 The van der Waals Equation 181
6.6 Hydrogen Bonding 183
6.7 Cohesion Energy and Solubility Parameters 192
6.8 Solvatochromism 196
CHAPTER 7 Ionic Bonding and Structures of Solids 201
7.1 Energetics of Crystal Formation 201
7.2 Madelung Constants 205
7.3 The Kapustinskii Equation 209
7.4 Ionic Sizes and Crystal Environments 210
7.5 Crystal Structures 213
7.6 Solubility of Ionic Compounds 219
7.7 Proton and Electron Affinities 224
7.8 Structures of Metals 227
7.9 Defects in Crystals 230
7.10 Phase Transitions in Solids 233
7.11 Heat Capacity 234
7.12 Hardness of Solids 237
vi Contents
Trang 9CHAPTER 8 Dynamic Processes in Inorganic Solids 243
8.1 Characteristics of Solid-State Reactions 243
8.2 Kinetic Models for Reactions in Solids 245
8.3 Thermal Methods of Analysis 253
8.4 Effects of Pressure 254
8.5 Reactions in Some Solid Inorganic Compounds 256
8.6 Phase Transitions 258
8.7 Reactions at Interfaces 262
8.8 Diffusion in Solids 263
8.9 Sintering 265
8.10 Drift and Conductivity 267
PART 3 Acids, Bases, and Solvents 271
CHAPTER 9 AcideBase Chemistry 273
9.1 Arrhenius Theory 273
9.2 BrønstedeLowry Theory 276
9.3 Factors Affecting the Strength of Acids and Bases 279
9.4 AcideBase Character of Oxides 284
9.5 Proton Affinities 286
9.6 Lewis Theory 288
9.7 Catalytic Behavior of Acids and Bases 292
9.8 The HardeSoft Interaction Principle (HSIP) 296
9.9 Electronic Polarizabilities 305
9.10 The Drago Four-Parameter Equation 306
CHAPTER 10 Chemistry in Nonaqueous Solvents 313
10.1 Some Common Nonaqueous Solvents 313
10.2 The Solvent Concept 314
10.3 Amphoteric Behavior 316
10.4 The Coordination Model 317
10.5 Chemistry in Liquid Ammonia 318
10.6 Liquid Hydrogen Fluoride 324
10.7 Liquid Sulfur Dioxide 326
10.8 Superacids 330
PART 4 Chemistry of the Elements 335
CHAPTER 11 Chemistry of Metallic Elements 337
11.1 The Metallic Elements 337
11.2 Band Theory 338
vii Contents
Trang 1011.3 Groups IA and IIA Metals 341
11.4 Zintl Phases 349
11.5 Aluminum and Beryllium 351
11.6 The First-Row Transition Metals 353
11.7 Second- and Third-Row Transition Metals 355
11.8 Alloys 357
11.9 Chemistry of Transition Metals 360
11.10 The Lanthanides 367
CHAPTER 12 Organometallic Compounds of the Main Group Elements 375
12.1 Preparation of Organometallic Compounds 376
12.2 Organometallic Compounds of Group IA Metals 378
12.3 Organometallic Compounds of Group IIA Metals 380
12.4 Organometallic Compounds of Group IIIA Metals 383
12.5 Organometallic Compounds of Group IVA Metals 387
12.6 Organometallic Compounds of Group VA Elements 388
12.7 Organometallic Compounds of Zn, Cd, and Hg 389
CHAPTER 13 Chemistry of Nonmetallic Elements I Hydrogen, Boron, Oxygen, and Carbon 393
13.1 Hydrogen 393
13.2 Boron 400
13.3 Oxygen 411
13.4 Carbon 420
CHAPTER 14 Chemistry of Nonmetallic Elements II Groups IVA and VA 439
14.1 The Group IVA Elements 439
14.2 Nitrogen 456
14.3 Phosphorus, Arsenic, Antimony, and Bismuth 471
CHAPTER 15 Chemistry of Nonmetallic Elements III Groups VIAeVIIIA 499
15.1 Sulfur, Selenium, and Tellurium 499
15.2 The Halogens 520
15.3 The Noble Gases 538
PART 5 Chemistry of Coordination Compounds 551
CHAPTER 16 Introduction to Coordination Chemistry 553
16.1 Structures of Coordination Compounds 553
16.2 MetaleLigand Bonds 557
16.3 Naming Coordination Compounds 559
16.4 Isomerism 561
16.5 A Simple Valence Bond Description of Coordinate Bonds 568
viii Contents
Trang 1116.6 Magnetism 572
16.7 A Survey of Complexes of First-Row Metals 574
16.8 Complexes of Second- and Third-Row Metals 576
16.9 The 18-Electron Rule 576
16.10 Back Donation 580
16.11 Complexes of Dinitrogen, Dioxygen, and Dihydrogen 585
CHAPTER 17 Ligand Fields and Molecular Orbitals 591
17.1 Splitting of d Orbital Energies in Octahedral Fields 591
17.2 Splitting of d Orbital Energies in Fields of Other Symmetry 594
17.3 Factors AffectingD 598
17.4 Consequences of Crystal Field Splitting 600
17.5 JahneTeller Distortion 603
17.6 Spectral Bands 604
17.7 Molecular Orbitals in Complexes 606
CHAPTER 18 Interpretation of Spectra 617
18.1 Splitting of Spectroscopic States 617
18.2 Orgel Diagrams 621
18.3 Racah Parameters and Quantitative Methods 624
18.4 The Nephelauxetic Effect 627
18.5 TanabeeSugano Diagrams 629
18.6 The Lever Method 633
18.7 Jørgensen’s Method 636
18.8 Charge Transfer Absorption 637
18.9 Solvatochromism 639
CHAPTER 19 Composition and Stability of Complexes 643
19.1 Composition of Complexes in Solution 643
19.2 Job’s Method of Continuous Variations 645
19.3 Equilibria Involving Complexes 647
19.4 Distribution Diagrams 652
19.5 Factors Affecting the Stability of Complexes 655
CHAPTER 20 Synthesis and Reactions of Coordination Compounds 665
20.1 Synthesis of Coordination Compounds 665
20.2 Substitution Reactions in Octahedral Complexes 671
20.3 Ligand Field Effects 678
20.4 Acid-Catalyzed Reactions of Complexes 681
20.5 Base-Catalyzed Reactions of Complexes 682
20.6 The Compensation Effect 684
20.7 Linkage Isomerization 685
ix Contents
Trang 1220.8 Substitution in Square Planar Complexes 687
20.9 The Trans Effect 690
20.10 Electron Transfer Reactions 694
20.11 Reactions in Solid Coordination Compounds 697
CHAPTER 21 Complexes Containing MetaleCarbon and MetaleMetal Bonds 707
21.1 Binary Metal Carbonyls 707
21.2 Structures of Metal Carbonyls 711
21.3 Bonding of Carbon Monoxide to Metals 712
21.4 Preparation of Metal Carbonyls 715
21.5 Reactions of Metal Carbonyls 716
21.6 Structure and Bonding in MetaleAlkene Complexes 721
21.7 Preparation of MetaleAlkene Complexes 727
21.8 Chemistry of Cyclopentadienyl and Related Complexes 728
21.9 Bonding in Ferrocene 732
21.10 Reactions of Ferrocene and other Metallocenes 735
21.11 Complexes of Benzene and Related Aromatics 738
21.12 Compounds Containing MetaleMetal Bonds 741
CHAPTER 22 Coordination Compounds in Catalysis 747
22.1 Elementary Steps in Catalytic Processes 748
22.2 Homogeneous Catalysis 761
CHAPTER 23 Bioinorganic Chemistry 773
23.1 What Metals Do in Some Living Systems 773
23.2 Cytotoxicity of Some Metal Compounds 784
23.3 Antimalarial Metallodrugs 792
APPENDIX A: Ionization Energies 797
APPENDIX B: Character Tables for Selected Point Groups 801
INDEX 807
x Contents
Trang 13Preface to the Second Edition
The development of inorganic continues at an astonishing pace, and the number of journal pagesdevoted annually to the field continues to increase Since the publication of the first edition of thisbook, exciting results have been obtained in many areas, especially in the application of inorganiccompounds in medicine
The present edition continues the same format as the first with five major areas that constitute atomicand molecular structure; condensed phases; acids, bases, and solvents; chemistry of the elements; andchemistry of coordination compounds Although the structure of the book has not been changed, thereare some significant modifications First, the coverage of superacids in Chapter 10 has been expanded inorder to reflect the increasing utilization of these materials in inorganic chemistry Second, a newchapter on bioinorganic chemistry has been added, with numerous illustrations from the recent liter-ature that describe the use of metal complexes in medicine Third, the coverage of catalysis by complexeshas been expanded with new material on mechanisms Fourth, the entire manuscript has been edited toenhance clarity of presentation The judicious use of color illustrations is also new to this edition.Finally, a significant number of additional end of chapter questions and problems have been added.Although there have been substantial changes in the coverage from the first edition, flexibility in theorder of presentation remains After the introductory chapters on atomic and molecular structure, thenature of inorganic condensed phases is described with a theme of understanding properties of suchmaterials Chapter 8 deals with dynamic processes in solids because of the importance of the topic inmany industrial processes
Explanations do not exist for all observations in the behavior of inorganic compounds quently, throughout the text there are instances mentioned which illustrate unanswered questions inscience It is hoped that some of these might generate an interest that would cause the reader to pursuefurther study in inorganic chemistry
Conse-The author wishes to thank all of those students who have used the first edition and pointed outerrors Their contribution to the preparation of the second edition is gratefully acknowledged Theauthor also wishes to thank his wife, Kathleen, for enduring yet another long project
xi
Trang 14Preface to the First Edition
No single volume, certainly not a textbook, can come close to including all of the important topics ininorganic chemistry The field is simply too broad in scope and it is growing at a rapid pace Inorganicchemistry textbooks reflect a great deal of work and the results of the many choices that authors mustmake as to what to include and what to leave out Writers of textbooks in chemistry bring to the taskbackgrounds that reflect their research interests, the schools they attended, and their personalities Intheir writing, authors are really saying “this is the field as I see it.” In these regards, this book is similar toothers
When teaching a course in inorganic chemistry, certain core topics are almost universally included Inaddition, there are numerous peripheral areas that may be included at certain schools but not at othersdepending on the interests and specialization of the person teaching the course The course content mayeven change from one semester to the next The effort to produce a textbook that presents coverage of
a wide range of optional material in addition to the essential topics can result in a textbook for a onesemester course that contains a thousand pages Even a “concise” inorganic chemistry book can benearly this long This book is not a survey of the literature or a research monograph It is a textbook that
is intended to provide the background necessary for the reader to move on to those more advancedresources
In writing this book, I have attempted to produce a concise textbook that meets several objectives.First, the topics included were selected in order to provide essential information in the major areas ofinorganic chemistry (molecular structure, acidebase chemistry, coordination chemistry, ligand fieldtheory, solid-state chemistry, etc.) These topics form the basis for competency in inorganic chemistry at
a level commensurate with the one semester course taught at most colleges and universities
When painting a wall, better coverage is assured when the roller passes over the same area severaltimes from different directions It is the opinion of the author that this technique works well inteaching chemistry Therefore, a second objective has been to stress fundamental principles in thediscussion of several topics For example, the hardesoft interaction principle is employed indiscussion of acidebase chemistry, stability of complexes, solubility, and predicting reaction prod-ucts Third, the presentation of topics is made with an effort to be clear and concise so that the book
is portable and user friendly This book is meant to present in convenient form a readable account ofthe essentials of inorganic chemistry that can serve as both as a textbook for a one semester course,upper-level course and as a guide for self study It is a textbook not a review of the literature or
a research monograph There are few references to the original literature, but many of the advanced
Trang 15Although the material contained in this book is arranged in a progressive way, there is flexibility inthe order of presentation For students, who have a good grasp of the basic principles of quantummechanics and atomic structure, Chapters 1 and 2 can be given a cursory reading but not included inthe required course material The chapters are included to provide a resource for review and self study.Chapter 4 presents an overview of structural chemistry early so the reader can become familiar withmany types of inorganic structures before taking up the study of symmetry or chemistry of specificelements Structures of inorganic solids are discussed in Chapter 7, but that material could easily bestudied before Chapters 5 or 6 Chapter 6 contains material dealing with intermolecular forces andpolarity of molecules because of the importance of these topics when interpreting properties ofsubstances and their chemical behavior In view of the importance of the topic, especially in industrialchemistry, this book includes material on rate processes involving inorganic compounds in the solidstate (Chapter 8) The chapter begins with an overview of some of the important aspects of reactions insolids before considering phase transitions and reactions of solid coordination compounds.
It should be an acknowledged fact that no single volume can present the descriptive chemistry of allthe elements Some of the volumes that attempt to do so are enormous In this book, the presentation
of descriptive chemistry of the elements is kept brief with the emphasis placed on types of reactions andstructures that summarize the behavior of many compounds The attempt is to present an overview ofdescriptive chemistry that will show the important classes of compounds and their reactions withoutbecoming laborious in its detail Many schools offer a descriptive inorganic chemistry course at anintermediate level that covers a great deal of the chemistry of the elements Part of the rationale foroffering such a course is that the upper-level course typically concentrates more heavily on principles ofinorganic chemistry Recognizing that an increasing fraction of the students in the upper-level inorganicchemistry course will have already had a course that deals primarily with descriptive chemistry, thisbook is devoted to a presentation of the principles of inorganic chemistry while giving a brief overview
of descriptive chemistry in Chapters 12e15, although many topics that are primarily descriptive innature are included in other sections Chapter 16 provides a survey of the chemistry of coordinationcompounds and that is followed by Chapters 17e23 that deal with structures, bonding, spectra, andreactions of coordination compounds The material included in this text should provide the basis forthe successful study of a variety of special topics
Doubtless, the teacher of inorganic chemistry will include some topics and examples of current orpersonal interest that are not included in any textbook That has always been my practice, and itprovides an opportunity to show how the field is developing and new relationships
Most textbooks are an outgrowth of the author’s teaching In the preface, the author should convey
to the reader some of the underlying pedagogical philosophy which resulted in the design of his or herbook It is unavoidable that a different teacher will have somewhat different philosophy and meth-odology As a result, no single book will be completely congruent with the practices and motivations ofall teachers A teacher who writes the textbook for his or her course should find all of the needed topics
in the book However, it is unlikely that a book written by someone else will ever contain exactly theright topics presented in exactly the right way
The author has taught several hundred students in inorganic chemistry courses at Illinois StateUniversity, Illinois Wesleyan University, University of Illinois, and Western Kentucky University usingthe materials and approaches set forth in this book Among that number are many who have gone ontograduate school, and virtually all of that group have performed well (in many cases very well!) on
xiiiPreface to the First Edition
Trang 16registration and entrance examinations in inorganic chemistry at some of the most prestigious tutions Although it is not possible to name all of those students, they have provided the inspiration tosee this project to completion with the hope that students at other universities may find this book useful
insti-in their study of insti-inorganic chemistry It is a pleasure to acknowledge and give thanks to Derek Colemanfor his encouragement and consideration as this project progressed Finally, I would like to thank mywife, Kathleen, for reading most of the manuscript and making many helpful suggestions Her constantencouragement and support have been needed at many times as this project was underway
xiv Preface to the First Edition
Trang 171 PART
Structure of Atoms and Molecules
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Trang 19CHAPTER 1
Light, Electrons, and Nuclei
The study of inorganic chemistry involves interpreting, correlating, and predicting the properties andstructures of an enormous range of materials Sulfuric acid is the chemical produced in the largesttonnage of any compound A greater number of tons of concrete is produced, but it is a mixture ratherthan a single compound Accordingly, sulfuric acid is an inorganic compound of enormous impor-tance On the other hand, inorganic chemists study compounds such as hexaaminecobalt(III) chloride,[Co(NH3)6]Cl3, and Zeise’s salt, K[Pt(C2H4)Cl3] Such compounds are known as coordinationcompounds or coordination complexes Inorganic chemistry also includes areas of study such asnonaqueous solvents and acidebase chemistry Organometallic compounds, structures and properties
of solids, and the chemistry of elements other than carbon comprise areas of inorganic chemistry.However, many compounds of carbon (e.g CO2 and Na2CO3) are also inorganic compounds Therange of materials studied in inorganic chemistry is enormous, and a great many of the compounds andprocesses are of industrial importance Moreover, inorganic chemistry is a body of knowledge that isexpanding at a very rapid rate, and knowledge of the behavior of inorganic materials is fundamental tothe study of the other areas of chemistry
Because inorganic chemistry is concerned with structures and properties as well as the synthesis ofmaterials, the study of inorganic chemistry requires familiarity with a certain amount of informationthat is normally considered to be physical chemistry As a result, physical chemistry is normally
a prerequisite for taking a comprehensive course in inorganic chemistry There is, of course, a great deal
of overlap of some areas of inorganic chemistry with the related areas in other branches of chemistry.Knowledge of atomic structure and properties of atoms is essential for describing both ionic andcovalent bonding Because of the importance of atomic structure to several areas of inorganic chemistry,
it is appropriate to begin our study of inorganic chemistry with a brief review of atomic structure andhow our ideas about atoms were developed
1.1 SOME EARLY EXPERIMENTS IN ATOMIC PHYSICS
It is appropriate at the beginning of a review of atomic structure to ask the question, “How do we knowwhat we know?” In other words, “What crucial experiments have been performed and what do theresults tell us about the structure of atoms?” Although it is not necessary to consider all of the earlyexperiments in atomic physics, we should describe some of them and explain the results The first ofthese experiments was that of J.J Thompson from 1898 to 1903, which dealt with cathode rays In theexperiment, an evacuated tube that contains two electrodes has a large potential difference generatedbetween the electrodes as shown inFigure 1.1
Inorganic Chemistry DOI: http://dx.doi.org/10.1016/B978-0-12-385110-9.00001-7
3
Trang 20Under the influence of a high electric field, the gas in the tube emits light The glow is the result ofelectrons colliding with the molecules of gas that are still present in the tube even though the pressurehas been reduced to a few torr The light that is emitted is found to consist of the spectral lines char-acteristic of the gas inside the tube Neutral molecules of the gas are ionized by the electrons streamingfrom the cathode, which is followed by recombination of electrons with charged species Energy (in theform of light) is emitted as this process occurs As a result of the high electric field, negative ions areaccelerated toward the anode and positive ions are accelerated toward the cathode When the pressureinside the tube is very low (perhaps 0.001 torr), the mean free path is long enough that some of thepositive ions strike the cathode, which emits rays Rays emanating from the cathode stream toward theanode Because they are emitted from the cathode, they are known as cathode rays.
Cathode rays have some very interesting properties First, their path can be bent by placing a magnetnear the cathode ray tube Second, placing an electric charge near the stream of rays also causes the paththey follow to exhibit curvature From these observations, we conclude that the rays are electricallycharged The cathode rays were shown to carry a negative charge because they were attracted to
a positively charged plate and repelled by the one that carried a negative charge
The behavior of cathode rays in a magnetic field is explained by recalling that a moving beam ofcharged particles (they were not known to be electrons at the time) generates a magnetic field The sameprinciple is illustrated by passing an electric current through a wire that is wound around a compass Inthis case, the magnetic field generated by the flowing current interacts with the magnetized needle of thecompass causing it to point in a different direction Because the cathode rays are negatively chargedparticles, their motion generates a magnetic field that interacts with the external magnetic field In fact,some important information about the nature of the charged particles in cathode rays can be obtained
by studying the curvature of their path in a magnetic field of known strength
Consider the following situation.Suppose a cross wind of 10 mph is blowing across a tennis court
If a tennis ball is moving perpendicular to the direction the wind is blowing, the ball will follow
a curved path It is easy to rationalize that if a second ball had a cross sectional area that was twice that
of the tennis ball but the same mass, it would follow a more curved path because the wind pressure on
it would be greater On the other hand, if a third ball having twice the cross sectional area and twice themass of the tennis ball were moving perpendicular to the wind direction, it would follow a path withthe same curvature as the tennis ball The third ball would experience twice as much wind pressure asthe tennis ball, but it would have twice the mass, which tends to cause the ball to move in a straight line(inertia) Therefore, if the path of a ball is being studied when it is subjected to wind pressure applied
+ Cathode rays –
FIGURE 1.1
Design of a cathode ray tube
4 CHAPTER 1: Light, Electrons, and Nuclei
Trang 21perpendicular to its motion, an analysis of the curvature of the path could be used to determine theratio of the cross sectional area to the mass of the ball, but neither property alone.
A similar situation exists for a charged particle moving under the influence of a magnetic field Thegreater the mass is, the greater will be the tendency of the particle to travel in a straight line On the otherhand, the higher its charge is, the greater will be its tendency to travel in a curved path in the magneticfield If a particle has two units of charge and two units of mass, it will follow the same path as one thathas one unit of charge and one unit of mass From the study of the behavior of cathode rays in
a magnetic field, Thompson was able to determine the charge to mass ratio for cathode rays, but not thecharge or the mass alone The negative particles in cathode rays are electrons, and Thompson is creditedwith the discovery of the electron From his experiments with cathode rays, Thompson determined thecharge to mass ratio of the electron to be 1.76 108coulomb/g
It was apparent to Thompson that if atoms in the metal electrode contained negative particles(electrons), then they must also contain positive charges because atoms are electrically neutral.Thompson proposed a model for the atom in which positive and negative particles were embedded insome sort of matrix The model became known as the plum pudding model because it resembled plumsembedded in a pudding Somehow, an equal number of positive and negative particles were held inthis material Of course we now know that this is an incorrect view of the atom, but the model didaccount for several features of atomic structure
The second experiment in atomic physics that increased our understanding of atomic structurewas conducted by Robert A Millikan in 1908 This experiment has become known as the MillikanOil Drop experiment because of the way in which oil droplets were used In the experiment, oildroplets (made up of organic molecules) were sprayed into a chamber where a beam of X-rays wasdirected on them The X-rays ionized molecules by removing one or more electrons producingcations As a result, some of the oil droplets carried an overall positive charge The entire apparatuswas arranged in such a way that a negative metal plate, the charge of which could be varied, was atthe top of the chamber By varying the (known) charge on the plate, the attraction between the plateand a specific droplet could be varied until it exactly equaled the gravitational force on the droplet.Under this condition, the droplet could be suspended with an electrostatic force pulling the dropupward that equaled the gravitational force pulling downward on the droplet Knowing the density
of the oil and having measured the diameter of the droplet, the mass of the droplet was calculated Itwas a simple matter to calculate the charge on the droplet because the charge on the negative platewith which the droplet interacted was known Although some droplets may have had two or threeelectrons removed, the calculated charges on the oil droplets were always a multiple of the smallestcharge measured Assuming that the smallest measured charge corresponded to that of a singleelectron, the charge on the electron was determined That charge is 1.602 1019 coulomb or
4.80 1010
esu (electrostatic units: 1 esu ¼ 1 g½cm3/2s1) Because the charge to mass ratio wasalready known, it was now possible to calculate the mass of the electron, which is 9.11 1031kg or9.11 1028g
The third experiment that is crucial to understanding atomic structure was carried out by ErnestRutherford in 1911 and is known as Rutherford’s experiment It consists of bombarding a thin metalfoil with alpha (a) particles Thin foils of metals, especially gold, can be made so thin that the thickness
of the foil represents only a few atomic diameters The experiment is shown diagrammatically in
Figure 1.2
51.1 Some Early Experiments in Atomic Physics
Trang 22It is reasonable to ask why such an experiment would be informative in this case The answer lies inunderstanding what the Thompson plum pudding model implies If atoms consist of equal numbers ofpositive and negative particles embedded in a neutral material, a charged particle such as an a particle(which is a helium nucleus) would be expected to travel near an equal number of positive and negativecharges when it passes through an atom As a result, there should be no net effect on the a particle, and itshould pass directly through the atom or a foil that is only a few atoms in thickness.
A narrow beam of a particles impinging on a gold foil should pass directly through the foil becausethe particles have relatively high energies What happened was that most of the a particles did just that,but some were deflected at large angles and some came essentially back toward the source! Rutherforddescribed this result in terms of firing a 16-inch shell at a piece of tissue paper and having it bounce back
at you How could an a particle experience a force of repulsion great enough to cause it to changedirections? The answer is that such a repulsion could result only when all of the positive charge in a goldatom is concentrated in a very small region of space Without going into the details, calculationsshowed that the small positive region was approximately 1013cm in size This could be calculatedbecause it is rather easy on the basis of electrostatics to determine what force would be required tochange the direction of an a particle with a þ2 charge traveling with a known energy Because theoverall positive charge on an atom of gold was known (the atomic number), it was possible todetermine the approximate size of the positive region
Rutherford’s experiment demonstrated that the total positive charge in an atom is localized in a verysmall region of space (the nucleus) Because the majority of a particles passed through the gold foil, itwas indicated that they did not come near a nucleus In other words, most of the atom is empty space.The diffuse cloud of electrons (which has a size on the order of 108cm) did not exert enough force onthe a particles to deflect them The plum pudding model simply did not explain the observations fromthe experiment with a particles
Although the work of Thompson and Rutherford had provided a view of atoms that was tially correct, there was still the problem of what made up the remainder of the mass of atoms It hadbeen postulated that there must be an additional ingredient in the atomic nucleus, and it wasdemonstrated in 1932 by James Chadwick In his experiments, a thin beryllium target was bom-barded with a particles Radiation having high penetrating power was emitted, and it was initially
essen-Gold foil particles α
FIGURE 1.2
A representation of Rutherford’s experiment
6 CHAPTER 1: Light, Electrons, and Nuclei
Trang 23assumed that they were high-energygrays From studies of the penetration of these rays in lead, itwas concluded that the particles had an energy of approximately 7 MeV Also, these rays were shown
to eject protons having energies of approximately 5 MeV from paraffin However, in order to explainsome of the observations, it was shown that if the radiation were grays, they must have an energythat is approximately 55 MeV If an a particle interacts with a beryllium nucleus so that it becomescaptured, it is possible to show that the energy (based on mass difference between the products andreactants) is only about 15 MeV Chadwick studied the recoil of nuclei that were bombarded by theradiation emitted from beryllium when it was a target for a particles and showed that if the radiationconsists ofgrays, the energy must be a function of the mass of the recoiling nucleus, which leads to
a violation of the conservation of momentum and energy However, if the radiation emitted fromthe beryllium target is presumed to carry no charge and consist of particles having a mass approx-imately that of a proton, the observations could be explained satisfactorily Such particles were calledneutrons, and they result from the reaction
9
4Be þ42He/
h13
6C
i/12
Atoms consist of electrons and protons in equal numbers and in all cases except the hydrogen atom,some number of neutrons Electrons and protons have equal but opposite charges, but greatly differentmasses The mass of a proton is 1.67 1024g In atoms that have many electrons, the electrons are notall held with the same energy so we will discuss later the shell structure of electrons in atoms At thispoint, we see that the early experiments in atomic physics have provided a general view of the structures
of atoms
1.2 THE NATURE OF LIGHT
From the early days of physics, a controversy had existed regarding the nature of light Some prominentphysicists, such as Isaac Newton, had believed that light consisted of particles or “corpuscles.” Otherscientists of that time believed that light was wave-like in its character In 1807, a crucial experiment wasconducted by T Young in which light showed a diffraction pattern when a beam of light was passedthrough two slits Such behavior showed the wave character of light Other work by A Fresnel and F.Arago had dealt with interference, which also depended on light having a wave character
The nature of light and the nature of matter are intimately related It was from the study of lightemitted when matter (atoms and molecules) was excited by some energy source or the absorption oflight by matter that much information was obtained In fact, most of what we know about the structure
of atoms and molecules has been obtained by studying the interaction of electromagnetic radiationwith matter or electromagnetic radiation emitted from matter These types of interactions form the basis
of several types of spectroscopy, techniques that are very important in studying atoms and molecules
In 1864, Maxwell showed that electromagnetic radiation consists of transverse electric and magneticfields that travel through space at the speed of light (3.00 108m sec1) The electromagnetic spectrumconsists of the several types of waves (visible light, radio waves, infrared radiation, etc.) that form
a continuum as shown inFigure 1.3 In 1887, Hertz produced electromagnetic waves by means of anapparatus that generated an oscillating electric charge (an antenna) This discovery led to the devel-opment of radio
71.2 The Nature of Light
Trang 24Although all of the developments that have been discussed are important to our understanding ofthe nature of matter, there are other phenomena that provide additional insight One of them concernsthe emission of light from a sample of hydrogen gas through which a high voltage is placed The basicexperiment is shown inFigure 1.4 In 1885, Balmer studied the visible light emitted from the gas bypassing it through a prism that separates the light into its components.
The four lines observed are as follows
Ha ¼ 656:28 nm ¼ 6562:8 A
Hb ¼ 486:13 nm ¼ 4861:3 A
Hg ¼ 434:05 nm ¼ 4340:5 A
Hd ¼ 410:17 nm ¼ 4101:7 AThis series of spectral lines for hydrogen became known as the Balmer Series, and the wavelengths ofthese four spectral lines were found to obey the relationship
1
l ¼ RH
1
n equals infinity, there is a limit reached That limit is known as the series limit for the Balmer Series.Keep in mind that these spectral lines, the first to be discovered for hydrogen, were in the visibleregion of the electromagnetic spectrum Detectors for visible light (human eyes and photographicplates) were available at an earlier time than were detectors for other types of electromagneticradiation
Eventually, other series of lines were found in other regions of the electromagnetic spectrum TheLyman Series was observed in the ultraviolet region, whereas the Paschen, Brackett, and Pfund Series
10 -12 eV 10 -9 eV 10 -3 eV 1 eV 1 keV 1 MeV 1 GeV
Energy
-rays x-rays
Long wave radio
Short wave radio Infrared Uv
Visible light
R O Y G B I V
R a d i o
γ
10 -6 eV
FIGURE 1.3
The electromagnetic spectrum
8 CHAPTER 1: Light, Electrons, and Nuclei
Trang 25were observed in the infrared region of the spectrum All of these lines were observed as they wereemitted from excited atoms, so together they constitute the emission spectrum or line spectrum of hydrogenatoms.
Another of the great developments in atomic physics involved the light emitted from a device known
as a black body Because black is the best absorber of all wavelengths of visible light, it should also bethe best emitter Consequently, a metal sphere, the interior of which is coated with lampblack, emitsradiation (black body radiation) having a range of wavelengths from an opening in the sphere when it
is heated to incandescence One of the thorny problems in atomic physics was trying to predict theintensity of the radiation as a function of wavelength In 1900, Max Planck arrived at a satisfactoryrelationship by making an assumption that was radical at that time Planck assumed that absorptionand emission of radiation arises from oscillators that change frequency However, Planck assumed thatthe frequencies were not continuous but rather that only certain frequencies were allowed In otherwords, the frequency is quantized The permissible frequencies were multiples of some fundamentalfrequency,n0 A change in an oscillator from a lower frequency to a higher one involves the absorption
of energy whereas energy is emitted as the frequency of an oscillator decreases Planck expressed theenergy in terms of the frequency by means of the relationship
where E is the energy, v is the frequency, and h is a constant (known as Planck’s constant,6.63 1027erg s ¼ 6.63 1034J s) Because light is a transverse wave (the direction the wave ismoving is perpendicular to the displacement), it obeys the relationship
wherelis the wavelength,nis the frequency, and c is the velocity of light (3.00 1010cm sec1) Bymaking these assumptions, Plank arrived at an equation that satisfactorily related the intensity andfrequency of the emitted black body radiation
The importance of the idea that energy is quantized is impossible to overstate It applies to all types
of energies related to atoms and molecules It forms the basis of the various experimental techniques forstudying the structure of atoms and molecules The energy levels may be electronic, vibrational, orrotational depending on the type of experiment conducted
In the 1800s, it was observed that when light was shone on a metal plate contained in anevacuated tube an interesting phenomenon occurs The arrangement of the apparatus is shown in
Figure 1.5
Prism Source
FIGURE 1.4
Separation of spectral lines due to refraction in a prism spectroscope
91.2 The Nature of Light
Trang 26When light was shone on the metal plate, an electric current flows Because light and electricity areinvolved, the phenomenon became known as the photoelectric effect Somehow, light is responsible forthe generation of the electric current Around 1900, there was ample evidence that light behaved as
a wave, but it was impossible to account for some of the observations on the photoelectric effect byconsidering light in that way Observations on the photoelectric effect include the following:
1 The incident light must have some minimum frequency (the threshold frequency) in order forelectrons to be ejected
2 The current flow is instantaneous when the light strikes the metal plate
3 The current is proportional to the intensity of the incident light
In 1905, Albert Einstein provided an explanation of the photoelectric effect by assuming that theincident light acts as particles This allowed for instantaneous collisions of light particles (photons) withelectrons (called photoelectrons), which resulted in the electrons being ejected from the surface of themetal Some minimum energy of the photons was required because the electrons are bound to themetal surface with some specific binding energy that depends on the type of metal The energy required
to remove an electron from the surface of a metal is known as the work function (w0) of the metal.Ionization potential (which corresponds to removal of an electron from a gaseous atom) is not thesame as the work function If an incident photon has an energy that is greater than the work function ofthe metal, the ejected electron will carry away part of the energy as kinetic energy In other words, thekinetic energy of the ejected electron will be the difference between the energy of the incident photonand the energy required to remove the electron from the metal This can be expressed by the equation
By increasing the negative charge on the plate to which the ejected electrons move, it is possible tostop the electrons and thereby stop the current flow The voltage necessary to stop the electrons isknown as the stopping potential Under these conditions, what is actually being determined is the kineticenergy of the ejected electrons If the experiment is repeated using incident radiation with a differentfrequency, the kinetic energy of the ejected electrons can again be determined By using light havingseveral known incident frequencies, it is possible to determine the kinetic energy of the electronscorresponding to each frequency and make a graph of the kinetic energy of the electrons vsn As can be
Light
Ejected electrons
FIGURE 1.5
Apparatus for demonstrating the photoelectric effect
10 CHAPTER 1: Light, Electrons, and Nuclei
Trang 27seen from Eqn (1.5), the relationship should be linear with the slope of the line being h, Planck’sconstant, and the intercept is w0 There are some similarities between the photoelectric effectdescribed here and photoelectron spectroscopy of molecules that is described in Section 3.4.
Although Einstein made use of the assumption that light behaves as a particle, there is no denyingthe validity of the experiments that show that light behaves as a wave Actually, light has characteristics
of both waves and particles, the so-called particleewave duality Whether it behaves as a wave or
a particle depends on the type of experiment to which it is being subjected In the study of atomic andmolecular structure, it is necessary to use both concepts to explain the results of experiments
1.3 THE BOHR MODEL
Although the experiments dealing with light and atomic spectroscopy had revealed a great deal aboutthe structure of atoms, even the line spectrum of hydrogen presented a formidable problem to thephysics of that time One of the major obstacles was that energy was not emitted continuously as theelectron moves about the nucleus After all, velocity is a vector quantity that has both a magnitude and
a direction A change in direction constitutes a change in velocity (acceleration) and an acceleratedelectric charge should emit electromagnetic radiation according to Maxwell’s theory If the movingelectron lost energy continuously, it would slowly spiral in toward the nucleus and the atom would
“run down.” Somehow, the laws of classical physics were not capable of dealing with this situation,which is illustrated inFigure 1.6
Following Rutherford’s experiments in 1911, Neils Bohr proposed in 1913 a dynamic model of thehydrogen atom that was based on certain assumptions The first of these assumptions was that therewere certain “allowed” orbits in which the electron could move without radiating electromagneticenergy Further, these were orbits in which the angular momentum of the electron (which for a rotatingobject is expressed as mvr) is a multiple of h/2p(which is also written as Z),
mvr ¼ nh
where m is the mass of the electron, v is its velocity, r is the radius of the orbit, and n is an integer that cantake on the values 1, 2, 3, , and Z is h/2p The integer n is known as a quantum number, or morespecifically, the principal quantum number
Bohr also assumed that electromagnetic energy was emitted as the electron moved from a higherorbital (larger n value) to a lower one and absorbed in the reverse process This accounts for the fact that
+ e-
FIGURE 1.6
As the electron moves around the nucleus, it is constantly changing direction
111.3 The Bohr Model
Trang 28the line spectrum of hydrogen shows only lines having certain wavelengths In order for the electron tomove in a stable orbit, the electrostatic attraction between the electron and the proton must be balanced
by the centrifugal force that results from its circular motion As shown inFigure 1.7, the forces areactually in opposite directions, so we equate only the magnitudes of the forces
The electrostatic force is given by the coulombic force as e2/r2, and the centrifugal force on theelectron is mv2/r Therefore, we can write
½ðg cm2=sec2Þsec2=½gðg1=2cm3=2=secÞ2 ¼ cm: (1.12)
Forces acting on an electron moving in a hydrogen atom
12 CHAPTER 1: Light, Electrons, and Nuclei
Trang 29From Eqn(1.7), we see that
in the cm-g-s system of units, the energy calculated will be in ergs Of course 1 J ¼ 107erg and
1 cal ¼ 4.184 J
By assigning various values to n, we can evaluate the corresponding energy of the electron inthe orbits of the hydrogen atom When this is done, we find the energies of several orbits asfollows
These energies can be used to prepare an energy level diagram such as that shown inFigure 1.8.Note that the binding energy of the electron is lowest when n ¼ 1, and the binding energy is 0 when
n ¼ N
Although the Bohr model successfully accounted for the line spectrum of the hydrogen atom, itcould not explain the line spectrum of any other atom It could be used to predict the wavelengths ofspectral lines of other species that had only one electron such as Heþ, Li2þ, Be3þ, etc Also, the modelwas based on assumptions regarding the nature of the allowed orbits that had no basis in classicalphysics An additional problem is also encountered when the Heisenberg Uncertainty Principle is
Trang 30considered According to this principle, it is impossible to know exactly the position and momentum of
a particle simultaneously Being able to describe an orbit of an electron in a hydrogen atom is alent to knowing its momentum and position The Heisenberg Uncertainty Principle places a limit onthe accuracy to which these variables can be known simultaneously That relationship is
where D is read as the uncertainty in the variable that follows Planck’s constant is known as thefundamental unit of action (it has units of energy multiplied by time), but the product of momentummultiplied by distance has the same dimensions The essentially classical Bohr model explained the linespectrum of hydrogen, but it did not provide a theoretical framework for understanding atomicstructure
1.4 PARTICLEeWAVE DUALITY
The debate concerning the particle and wave nature of light had been lively for many years when in
1924 a young French doctoral student, Louis V de Broglie, developed a hypothesis regarding the nature
of particles In this case, the particles were “real” particles such as electrons De Broglie realized that forelectromagnetic radiation, the energy could be described by the Planck equation
n = 1
n = 2
Lyman Series
Balmer Series
Paschen Series
Brackett Series
An energy level diagram for the hydrogen atom
14 CHAPTER 1: Light, Electrons, and Nuclei
Trang 31However, one of the consequences of Einstein’s special theory of relativity (in 1905) is that a photonhas an energy that can be expressed as
This famous equation expresses the relationship between mass and energy, and its validity has beenamply demonstrated This equation does not indicate that a photon has a mass It does signify thatbecause a photon has energy, its energy is equivalent to some mass However, for a given photon, there isonly one energy So
of the moving electrons, and the value corresponds exactly to the wavelength predicted by the deBroglie equation Since this pioneering work, electron diffraction has become one of the standardexperimental techniques for studying molecular structure
De Broglie’s work clearly shows that a moving electron can be considered as a wave If it behaves inthat way, a stable orbit in a hydrogen atom must contain a whole number of wavelengths or otherwisethere would be interference that would lead to cancellation (destructive interference) This conditioncan be expressed as
151.4 ParticleeWave Duality
Trang 321.5 ELECTRONIC PROPERTIES OF ATOMS
Although we have not yet described the modern methods of dealing with theoretical chemistry(quantum mechanics), it is possible to describe many of the properties of atoms For example, theenergy necessary to remove an electron (the ionization energy or ionization potential) from a hydrogenatom is the energy that is equivalent to the series limit of the Lyman Series Therefore, atomic spec-troscopy is one way to determine ionization potentials for atoms
If we examine the relationship between the first ionization potentials for atoms and their atomicnumbers, the result can be shown graphically as in Figure 1.9 Numerical values for ionizationpotentials are shown in Appendix A
Several facts are apparent from this graph Although we have not yet dealt with the topic of electronconfiguration of atoms, you should be somewhat familiar with this topic from earlier chemistrycourses We will make use of some of the ideas that deal with electron shells here but delay presentingthe details until later
1 The helium atom has the highest ionization potential of any atom It has a nuclear charge of þ2,and the electrons reside in the lowest energy level close to the nucleus
2 The noble gases have the highest ionization potentials of any atoms in their respective periods.Electrons in these atoms are held in shells that are completely filled
3 The Group IA elements have the lowest ionization potentials of any atoms in their respectiveperiods As you probably already know, these atoms have a single electron that resides in a shelloutside of other shells that are filled
4 The ionization potentials within a period generally increase as you go to the right in that period.For example, B < C < O < F, etc However, in the case of nitrogen and oxygen, the situation isreversed Nitrogen, which has a half-filled shell, has a higher ionization potential than oxygen,which has one electron more than a half-filled shell There is some repulsion between the twoelectrons that reside in the same orbital in an oxygen atom, which makes it easier to removeone of them
0 400 800 1200 1600 2000 2400
Na
Mg Al Si P Cl S Ar
K
Zn
Ga Ge
As Se Br Kr
FIGURE 1.9
The relationship betweenfirst ionization potential and atomic number
16 CHAPTER 1: Light, Electrons, and Nuclei
Trang 335 In general, the ionization potential decreases for the atoms in a given group going down in thegroup For example, Li > Na > K > Rb > Cs and F > Cl > Br > I The outer electrons are farther fromthe nucleus for the larger atoms, and there are more filled shells of electrons between the nucleusand the outermost electron.
6 Even for the atom having the lowest ionization potential, Cs, the ionization potential is
approximately 374 kJ mol1
These are some of the general trends that relate the ionization potentials of atoms with regard to theirpositions in the periodic table We will have opportunities to discuss additional properties of atomslater
A second property of atoms that is vital to understanding their chemistry is the energy released when
an electron is added to a gaseous atom,
XðgÞ þ eðgÞ/XðgÞ DE ¼ electron addition energy (1.24)For most atoms, the addition of an electron occurs with the release of energy so the value ofDE isnegative There are some exceptions, most notably the noble gases and Group IIA metals These atomshave completely filled shells so any additional electrons would have to be added in a new empty shell.Nitrogen also has virtually no tendency to accept an additional electron because of the stability of thehalf-filled outer shell
After an electron is added to an atom, the “affinity” that it has for the electron is known as the electronaffinity Because energy is released when an electron is added to most atoms, it follows that energywould be required to remove the electron so the electron affinity is positive for most atoms Theelectron affinities for most of the main group elements are shown inTable 1.1 It is useful to rememberthat 1 eV per atom is equal to 96.48 kJ mol1
Several facts are apparent when the data shown inTable 1.1are considered In order to see some ofthe specific results more clearly,Figure 1.10has been prepared to show how the electron affinity varies
Table 1.1 Electron Affinities of Atoms in kJ mol1
a 845 kJ mol 1 for addition of two electrons.
b 531 kJ mol 1 for addition of two electrons.
171.5 Electronic Properties of Atoms
Trang 34with position in the periodic table (and therefore orbital population) From studyingFigure 1.10andthe data shown inTable 1.1, the following relationships emerge.
1 The electron affinities for the halogens are the highest of any group of elements
2 The electron affinity generally increases in going from left to right in a given period In general, theelectrons have been added to the atoms in the same outer shell Because the nuclear chargeincreases in going to the right in a period, the attraction for the outer electron increases accordingly
3 In general, the electron affinity decreases going downward for atoms in a given group
4 The electron affinity of nitrogen is out of line with those of other atoms in the same period because
it has a stable half-filled shell
5 Whereas nitrogen has an electron affinity that is approximately zero, phosphorus has a valuegreater than zero even though it also has a half-filled outer shell The effect of a half-filled shelldecreases for larger atoms because that shell has more filled shells separating it from thenucleus
6 In the case of halogens (Group VIIA), the electron affinity of fluorine is lower than that of chlorine.This is because the fluorine atom is small and the outer electrons are close together and repellingeach other Adding another electron to an F atom, although very favorable energetically, is not asfavorable as it is for chlorine, which has the highest electron affinity of any atom For Cl, Br, and I,the trend is in accord with the general relationship
7 Hydrogen has a substantial electron affinity, which shows that we might expect compoundscontaining Hto be formed
8 The elements in Group IIA have negative electron affinities showing that the addition of an electron
to those atoms is not energetically favorable These atoms have two electrons in the outer shell,which can hold only two electrons
9 The elements in Group IA can add an electron with the release of energy (a small amount) becausetheir singly occupied outer shells can hold two electrons
H
He Li
Be
B C
N O F
Ne Na
Mg
Al
Si P S Cl
Ar K
Ca
Ga
Ge As Se Br
Kr
-3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
E A., eV
Atomic number 0
FIGURE 1.10
Electron affinity as a function of atomic number
18 CHAPTER 1: Light, Electrons, and Nuclei
Trang 35As is the case with ionization potential, electron affinity is a useful property when considering thechemical behavior of atoms, especially when describing ionic bonding, which involves electrontransfer.
In the study of inorganic chemistry, it is important to understand how atoms vary in size The relativesizes of atoms determine to some extent the molecular structures that are possible.Table 1.2shows thesizes of atoms in relationship to the periodic table
Some of the important trends in the sizes of atoms can be summarized as follows
1 The sizes of atoms in a given group increase as one progresses down the group For example, thecovalent radii for Li, Na, K, Rb, and Cs are 134, 154, 227, 248, and 265 pm, respectively For F, Cl,
Br, and I the covalent radii are 71, 99, 114, and 133 pm, respectively
2 The sizes of atoms decrease in progressing across a given period Nuclear charge increases in such
a progression as long as electrons in the outer shell are contained in the same type of shell
Therefore, the higher the nuclear charge is (farther to the right in the period), the greater will be theattraction for the electrons and the closer to the nucleus they will reside For example, the radii forthe first long row of atoms are as follows
Other rows in the periodic table follow a similar trend However, for the third row, there is a
general decrease in radius except for the last two or three elements in the transition series Thecovalent radii of Fe, Co, Ni, Cu, and Zn are 126, 125, 124, 128, and 133 pm, respectively This effect
is a manifestation of the fact that the 3d orbitals shrink in size as the nuclear charge increases (going
to the right), and the additional electrons populating these orbitals experience greater repulsion
As a result, the size decreases to a point (at Co and Ni), but after that the increase in repulsionproduces an increase in size (Cu and Zn are larger than Co and Ni)
Table 1.2 Atomic Radii in pm
Trang 363 The largest atoms in the various periods are the Group IA metals The outermost electron resides in
a shell that is outside other completed shells (the noble gas configurations), so it is loosely held(low ionization potential) and relatively far from the nucleus
An interesting effect of nuclear charge can be seen by examining the radius of a series of species thathave the same nuclear charge but different numbers of electrons One such series are the ions that have
10 electrons (the neon configuration) The ions include Al3þ, Mg2þ, Naþ, F, O2, and N3, for whichthe nuclear charge varies from 13 to 7.Figure 1.11 shows the variation in size of these species withnuclear charge
Note that the N3ion (radius 171 pm) is much larger than the nitrogen atom, for which the covalentradius is only 71 pm The oxygen atom (radius 72 pm) is approximately half the size of the oxide ion(radius 140 pm) Anions are always larger than the atoms from which they are formed On the otherhand, the radius of Naþ(95 pm) is much smaller than the covalent radius of the Na atom (radius
154 pm) Cations are always smaller than the atoms from which they are formed
Of particular interest in the series of ions is the Al3þion, which has a radius of only 50 pm, whereasthe atom has a radius of 126 pm As will be described in more detail later (see Chapter 7), the small sizeand high charge of the Al3þion causes it (and similar ions with high charge to size ratio or chargedensity) to have some very interesting properties It has a great affinity for the negative ends of polarwater molecules so that when an aluminum compound is dissolved in water, evaporating the waterdoes not remove the water molecules that are bonded directly to the cation The original aluminumcompound is not recovered
Because inorganic chemistry is concerned with the properties and reactions of compounds that maycontain any element, understanding the relationships between properties of atoms is important Thistopic will be revisited numerous times in later chapters, but the remainder of this chapter will bedevoted to a brief discussion of the nuclear portion of the atom and nuclear transformations We nowknow that it is not possible to express the weights of atoms as whole numbers that represent multiples
of the mass of a hydrogen atom as had been surmised about two centuries ago Although Dalton’satomic theory was based on the notion that all atoms of a given element were identical, we now knowthat this is not correct As students in even elementary courses now know, the atomic masses represent
40 60 80 100 120 140 160 180
Radii of ions having the neon configuration
20 CHAPTER 1: Light, Electrons, and Nuclei
Trang 37averages resulting from most elements existing in several isotopes The application of mass spectroscopytechniques has been of considerable importance in this type of study.
1.6 NUCLEAR BINDING ENERGY
There are at present 116 known chemical elements However, there are well over 2000 known nuclearspecies as a result of several isotopes being known for each element About three-fourths of the nuclearspecies are unstable and undergo radioactive decay Protons and neutrons are the particles that arefound in the nucleus For many purposes, it is desirable to describe the total number of nuclear particleswithout regard to whether they are protons or neutrons The term nucleon is used to denote both ofthese types of nuclear particles In general, the radii of nuclides increase as the mass number increases,with the usual relationship being expressed as
where A is the mass number and r0is a constant that is approximately 1.2 1013
Any nuclear species is referred to as a nuclide Thus,1H,23Na, 126C, and23892U are different nizable species or nuclides A nuclide is denoted by the symbol for the atom with the mass numberwritten to the upper left, the atomic number written to the lower left, and any charge on the species, q
recog-to the upper right For example,
A
ZXq
As was described earlier in this chapter, the model of the atom consists shells of electronssurrounding the nucleus, which contains protons and, except for the isotope1H, a certain number ofneutrons Each type of atom is designated by the atomic number, Z, and a symbol derived from thename of the element The mass number, A, is the whole number nearest to the mass of that species Forexample, the mass number of11H is 1 although the actual mass of this isotope is 1.00794 atomic massunits (amu) Because protons and neutrons have masses that are essentially the same (both areapproximately 1 atomic mass unit, amu), the mass number of the species minus the atomic numbergives the number of neutrons, which is denoted as N Thus, for157N, the nucleus contains seven protonsand eight neutrons
When atoms are considered to be composed of their constituent particles, it is found that the atomshave lower masses than the sum of the masses of the particles For example,42He contains two electrons,two protons, and two neutrons These particles have masses of 0.0005486, 1.00728, and 1.00866 amu,respectively, which gives a total mass of 4.032977 amu for the particles However, the actual mass of4
2He is 4.00260 amu so there is a mass defect of 0.030377 amu That “disappearance” of mass occursbecause the particles are held together with an energy that can be expressed in terms of the Einsteinequation,
Trang 38When the mass being converted to energy is 1 amu (1.66054 1024g), the amount of energyreleased is 1.49 103erg This energy can be converted to electron volts by making use of theconversion that 1 eV ¼ 1.60 1012erg Therefore, 1.49 103erg/1.60 1012erg/eV is9.31 108eV When dealing with energies associated with nuclear transformations, energies areordinarily expressed in MeV with 1 MeV being 106eV Consequently, the energy equivalent to 1 amu is
931 MeV When the mass defect of 0.030377 amu found for4He is converted to energy, the result is28.3 MeV In order to make a comparison between the stability of various nuclides, the total bindingenergy is usually divided by the number of nucleons, which in this case is 4 Therefore, the binding energyper nucleon is 7.07 MeV
As an aside issue, it may have been noted that we neglected the attraction energy between theelectrons and the nucleus The first ionization energy for He is 24.6 eV and the second is 54.4 eV Thus,the total binding energy of the electrons to the nucleus in He is only 79.9 eV, which is only0.000079 MeV and is totally insignificant compared to the 28.3 MeV represented by the total bindingenergy Attractions between nucleons are enormous compared to binding energies of electrons inatoms Neutral atoms have the same number of electrons and protons, the combined mass of which isalmost exactly the same as that of a hydrogen atom Therefore, no great error is introduced whencalculating mass defects by adding the mass of an appropriate number of hydrogen atoms to that of thenumber of neutrons For example, the mass of168O can be approximated as the mass of eight hydrogenatoms and eight neutrons The binding energy of the electrons in the eight hydrogen atoms is ignored.When similar calculations are performed for many other nuclides, it is found that the binding energyper nucleon differs considerably The value for168O is 7.98 MeV, and the highest value is approximately8.79 MeV for56Fe This suggests that for a very large number of nucleons, the most stable arrangement
is for them to make5626Fe, which is actually abundant in nature.Figure 1.12shows the binding energyper nucleon as a function of mass number of the nuclides
0 1 2 3 4 5 6 7 8 9 10
Mass number BE/nucleon, MeV
FIGURE 1.12
The average binding energy per nucleon as a function of mass number
22 CHAPTER 1: Light, Electrons, and Nuclei
Trang 39With the highest binding energy per nucleon being for species such as5626Fe, we can see that the fusion
of lighter species to produce nuclides that are more stable should release energy Because very heavyelements have lower binding energy per nucleon than do nuclides having mass number from about 50
to 80, fission of heavy nuclides is energetically favorable One such nuclide is23592U, which undergoesfission when bombarded with low energy neutrons
235
92U þ10n/9236Kr þ14156Ba þ 310n (1.27)When23592U undergoes fission, many different products are obtained because there is not a great deal ofdifference in the binding energy per nucleon for nuclides having a rather wide range of mass numbers Ifthe abundances of the products are plotted against the mass numbers, a double humped curve isobtained, and the so-called symmetric split of the23592U is not the most probable event Fission productshaving atomic numbers in the ranges of 30e40 and 50e60 are much more common than two46Pdisotopes
50, and 82 with a separate series for protons and neutrons It was known early in the development ofnuclear science that these numbers of nucleons represented stable arrangements although it was notknown why these numbers of nucleons were stable Consequently, they were referred to as magicnumbers
Another difference between nucleons and electrons is that nucleons pair whenever possible Thus,even if a particular energy level can hold more than two particles, two particles will pair when they arepresent Thus, for two particles in degenerate levels, we show two particles as [Y rather than [ [
As a result of this preference for pairing, nuclei with even numbers of protons and neutrons haveall paired particles This results in nuclei that are more stable than those which have unpaired particles.The least stable nuclei are those in which both the number of neutrons and the number of protons isodd This difference in stability manifests itself in the number of stable nuclei of each type.Table 1.3
shows the numbers of stable nuclei that occur
Figure 1.13shows graphically the relationship between the number of neutrons and the number ofprotons for the stable nuclei The data show that there does not seem to be any appreciable difference
231.7 Nuclear Stability
Trang 40in stability when the number of protons or neutrons is even whereas the other is odd (the eveneoddand oddeeven cases) The small number of nuclides that have odd Z and odd N (so-called oddeoddnuclides) is very small, which indicates that there is an inherent instability in such an arrangement Themost common stable nucleus which is of the oddeodd type is147N.
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
The relationship between the number of neutrons and protons for stable nuclei
Table 1.3 Numbers of Stable Nuclides Having Different Arrangements of Nucleons