The three-dimensional aspects of molecular shape can be crucial to both properties and reactions. The Third Dimension explores the arrangements of atoms in molecules and in different types of solids. Initial chapters describe the common crystal structures and how they are related to close-packed arrangements of ions. Metallic, ionic, molecular and extended covalent crystals are covered; major types of crystal defects are also discussed. The book then introduces isomerism, and explores the stereochemical consequences of the tetrahedral carbon atom. Chirality is also investigated. The book concludes with a Case Study on Liquid Crystals, which describes structures, properties and applications. As visualisation in 3D is an important part of this book, the accompanying CD-ROMs provide video material, interactive questions and exercises using models to aid understanding of crystals, organic molecules and stereochemistry. All necessary programs are provided. The Molecular World series provides an integrated introduction to all branches of chemistry for both students wishing to specialise and those wishing to gain a broad understanding of chemistry and its relevance to the everyday world and to other areas of science. The books, with their Case Studies and accompanying multi-media interactive CD-ROMs, will also provide valuable resource material for teachers and lecturers. (The CD-ROMs are designed for use on a PC running Windows 95, 98, ME or 2000.)
The Third Dimension
By Lesley Smart, Michael GaganThe Royal Society of Chemistry
Copyright © 2002 The Open University
All rights reserved.
ISBN: 978-0-85404-660-7Contents
PART 1 CRYSTALS Lesley Smart,
1 INTRODUCTION, 11,
2 STRUCTURES OF METALS, 15,
3 THE INTERNAL STRUCTURE OF A CRYSTAL, 29,
4 IONIC SOLIDS, 46,
5 IONIC RADII, 69,
6 EXTENDED COVALENT STRUCTURES, 82,
7 MOLECULAR CRYSTALS, 89,
8 DEFECTS IN CRYSTALS, 100,
9 SUMMARY OF THE THIRD DIMENSION: CRYSTALS, 105,
LEARNING OUTCOMES, 106,
QUESTIONS: ANSWERS AND COMMENTS, 109,
ANSWERS TO COMPUTER ACTIVITIES, 117,
FURTHER READING, 118,
ACKNOWLEDGEMENTS, 118,
PART 2 MOLECULAR SHAPE Michael Gagan,
1 THE TETRAHEDRAL CARBON ATOM, 121,
2 MOLECULAR CONFORMATION, 126,
3 THE REPRESENTATION OF MOLECULES, 128,
4 CONSTITUTIONAL (STRUCTURAL) ISOMERISM, 136,
5 STEREOISOMERS OF MOLECULES CONTAINING DOUBLE BONDS, 139,
6 CHIRALITY, 148,
7 MOLECULES WITH MORE THAN ONE CHIRAL ATOM, 160,
8 STEREOCHEMISRTY OF SATURATED RING COMPOUNDS, 165,
9 CONCLUSION, 167,
APPENDIX A BRIEF SURVEY OF ORGANIC FUNCTIONAL GROUPS, 169,
LEARNING OUTCOMES, 175,
QUESTIONS: ANSWERS AND COMMENTS, 177,
ANSWERS TO EXERCISES, 186,
ANSWERS TO MODEL EXERCISES, 188,
FURTHER READING, 194,
ACKNOWLEDGEMENTS, 194,
CASE STUDY: LIQUID CRYSTALS — The fourth state of matter Corrie Imrie,
1 THE DISCOVERY, 197,
2 THE FOURTH STATE OF MATTER, 202,
3 TYPES OF LIQUID CRYSTALS, 203,
4 LIQUID CRYSTAL PHASES, 206,
5 IDENTIFYING LIQUID CRYSTAL PHASES, 208,
6 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY, 210,
7 LIQUID CRYSTAL DISPLAY DEVICES, 212,
8 CHIRALITY AND LIQUID CRYSTALS, 215,
9 DISCOTIC LIQUID CRYSTALS, 217,
10 LIQUID CRYSTAL POLYMERS, 219,
11 SIDE-GROUP LIQUID CRYSTAL POLYMERS, 223,
12 LYOTROPIC LIQUID CRYSTALS, 226,
13 LIQUID CRYSTALS AND LIFE, 228,
14 WHAT NEXT?, 230,
FURTHER READING, 231,
ACKNOWLEDGEMENTS, 231,
INDEX, 232,
CD-ROM INFORMATION, 240,
CHAPTER 1
Part 1
Crystals
Lesley Smart
1 INTRODUCTION
You will be aware how atoms sometimes bond together to make individual molecules, as in carbon dioxide, chlorine and iodine for instance, but how in other cases, like diamond, silica, and sodium chloride, it is not possible to distinguish individual molecules (Figures 1.1a-d). You should also remember that bonding, and thus the arrangement of electrons, influences the shape of a molecule such as ammonia (Figure 1.1e). Many chemicals are solids at normal temperatures, and in the first part of this Book we are going to investigate the variety of structures adopted by elements and compounds in the crystalline solid state. The second part of the Book will concentrate only on the structure and shape of individual molecules.
To begin with, we are going to look at the structure of metals and of ionic solids. Such materials are very different from those containing individual molecules, in that they comprise extended arrays of atoms or ions in which discrete molecules are not identifiable. For instance, as well as diamond (Figure 1.1c) we can consider another familiar example, sodium chloride (common salt, Figure 1.2a), which has the empirical formula NaCl. Although the formula gives us the highly important information that there is one sodium atom present for every chlorine atom, it disguises the fact that crystalline sodium chloride does not comprise discrete NaCl molecules.
* What is the structure of sodium chloride?
* It is an aggregate of Na+ ions and Cl- ions arranged together in a regular, repeating three-dimensional pattern, with six ions of one type octahedrally surrounding an ion of the other type (Figure 1.2b).
The structure adopted by NaCl is also adopted by many other compounds, but it is not the only type of crystal structure. Ionic solids adopt a variety of patterns in the arrangements of their constituent ions. We shall show you a few of the ones that are more commonly found, and discuss the reasons for the variations.
We shall then move from ionic solids to look at a variety of structures adopted by solids where covalent rather than ionic bonding is predominant. For instance, silica (quartz), with empirical formula SiO2 (Figure 1.3), is covalently bonded: in its crystals we find an extended structure of covalently linked silicon and oxygen atoms, but no distinguishable individual molecules of SiO2.
In the final kind of solid state structure that we consider — molecular structures — small individual, covalently bound molecules such as iodine form crystals in which the molecules are held together by weak bonding (Figure 1.4).
Not everyone finds it an easy matter to visualize things in three dimensions, and so we have devised several ways of demonstrating the structures; you will have to find out which method helps you most. First and foremost is the use of WebLab Viewer Lite™ to look at the structures. Most of the diagrams of crystal structures in this Book can be viewed using WebLab ViewerLite. You will find the files on one of the CD-ROMs associated with this Book; you can access them via the CD-ROM Guide (p. 240) under 'Figures'. WebLab ViewerLite allows you to view a structure in any orientation: the instructions for using it can be found on one of the CD-ROMs, and if you haven't used it before, you should practise now with the four structures in this introduction. In addition to WebLab ViewerLite on one of the CD-ROMs, there is a program Virtual Crystals to work through, model-building exercises to view, and a program entitled Crystals, which contains some computer animations. If at all possible, try to study this Book near your computer, so that you can more easily use some or all of these aids.
Many ionic crystal structures can be regarded as based on a few very simple ways of packing spheres together in three dimensions. The simple sphere-packing schemes are best illustrated by the structures of metals, and so it is with these that we begin in the next Section.
2
STRUCTURES OF METALS
2.1 Close-packing in two dimensions
Before moving on to three-dimensional structures, it's convenient first to take a look at the efficiency of close-packing in two dimensions, by looking at two different ways in which circles may be arranged. Figure 2.la shows a close-packed arrangement, whereas Figure 2.1 b contains a square arrangement. The first apparent difference is that each circle touches six others in Figure 2.la, but only four others in Figure 2.1b. How do the two arrangements compare for efficient use of space? The bounded areas highlighted on the two diagrams both enclose a total of four complete circles. If we overlay these two areas (Figure 2.lc), we can see clearly that the four circles (comprising two whole circles, two half circles, and four quarter circles) in Figure 2.la occupy less total area than the four circles in Figure 2.lb (comprising one whole circle, four half circles, and four quarter circles) — about 15% less in fact. Now let's move on to the three-dimensional case.
COMPUTER ACTIVITY 2.1 Using computer animation and models to study close-packing
The close-packing of spheres is demonstrated by building models in Model Building on one of the CD-ROMs associated with this Book.
There is a computer animation of close-packing in the first part of...
