Chemistry
Giant Covalent Structures
Giant covalent structures are a type of chemical bonding where atoms are held together by strong covalent bonds, forming a continuous network. Examples include diamond, graphite, and silicon dioxide. These structures have high melting and boiling points due to the strength of the covalent bonds, and they are typically insoluble in solvents.
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5 Key excerpts on "Giant Covalent Structures"
eBook - PDFChemistry for Technologists
The Commonwealth and International Library: Electrical Engineering Division
- G. R. Palin, N. Hiller(Authors)
- 2014(Publication Date)
- Pergamon(Publisher)
16. The coval-ent bonding in crystals of this type is much stronger than the inter-molecular forces in molecular crystals. The solids are extremely hard, and have very high melting points. Apart from diamond, silicon carbide, aluminium nitride and silica are among the more common solids which are covalent crystals. There are not many compounds of this type, but some of them are very useful because of their extreme hardness. Graphite is another form of carbon, the atoms being held together by covalent bonds in a different arrangement to that of diamond. The car-bon is in the sp 2 hybrid state, and the covalent bonding is similar to that described in Chapter 3 for benzene. Each carbon atom forms three coval-ent bonds with adjoining carbon atoms. These bonds involve the sp 2 hy- 50 C H E M I S T R Y F O R T E C H N O L O G I S T S brid orbitals, and are in one plane with an angle of 120° between each bond. The result is a plate of fused hexagonal rings as shown in Fig. 17. The unhybridized 2p electrons enter delocalized orbitals which are com-mon to all the atoms in the plate. The bonding within the plates is co-TFT • carbon covalent bond FIG. 16. Structure of diamond. valent, but the plates stack together in the solid with only the weak van der Waals forces between them. Mechanically the plates are very strong, but the weak non-directional forces between them allow large distortions under small loads. This accounts for graphite being a solid lubricant. Molybdenum disulphide, which is another solid lubricant, has a similar plate structure. Amorphous solids with covalent bonding throughout also occur, the • carbon covalent van der Waals forces bonds FIG. 17. Structure of graphite. F O R C E S IN S O L I D S 51 most important being the thermosetting plastics. These consist of vast networks of covalently bonded atoms with no overall regular pattern, although similar patterns of grouped atoms are repeated.
eBook - PDFCrystal Structures
Lattices and Solids in Stereoview
- M Ladd(Author)
- 1999(Publication Date)
- Woodhead Publishing(Publisher)
3 Looking at covalent structures 3.1 INTRODUCTION The nature of the classification of structures adopted in this book results in the covalent class being relatively small, although not unimportant. Solids in which the structural units are held together in the solid state by covalent forces need a three-dimensional disposition of covalent interatomic bonds, and we shall see how this can come about. The many compounds of organic chemistry are not included here: although covalent forces exist between the atoms in organic molecules, the molecules themselves are linked in the solid state by weaker intermolecular forces, so that organic solids will be considered under the heading of molecular solids. It is not difficult to understand how charged species, such as Na* and Cl can attract one another, as we have discussed in the previous chapter. But how do two neutral hydrogen atoms unite to form a hydrogen molecule, and how do neutral carbon atoms bond to form the structure of diamond? We expect that the forces involved will be electrical, because such is the nature of matter. The qualitative concept of covalency is based on shared pairs of electrons but, further, we need to be able to explain features such as the near equivalence of bond lengths and angles involving given species across a range of compounds. We find the answer in the wave mechanics of the covalent bond, and we shall discuss this topic next. 3.2 WAVE MECHANICS OF THE COVALENT BOND An electron that is free to move in a single dimension can be represented as a wave by the Schrodinger wave equation, which may be stated as (3.1) KA 2 /2m e ) (d 2 /d* 1 ) + V(x)]y(x) = Ey(x) , where h ('cross-/*') = hl2%, h being the Planck constant, m e is the rest mass of an electron, V(x) is the potential energy of the electron at a position x, y^x) is the one-dimensional wavefunction, and E is the total energy, the sum of kinetic and potential energies.
eBook - PDFEngineering Materials Science
Properties, Uses, Degradation, Remediation
- H McArthur, D Spalding(Authors)
- 2004(Publication Date)
- Woodhead Publishing(Publisher)
Sec. 1.13] Mixed Bonding 47 a) Graphite (orientation) b) Diamond (covalent) c) Diamond (orientation) Figure 1.20 Structure of covalently bonded carbon (graphite and diamond) The graphite structure is elongated in one direction (the C-C interatomic distance is 0.340 nm, compared to 0.142 nm within the plane and 0.154 nm in the diamond structure). The softness of graphite is ascribed to the atoms in one direction being further apart due to weak (van der Waals) forces or bonds (1.14), which means that the planes can accommodate occluded gases, water, oxygen etc. (These gases are removed in a vacuum, leading to rapid wear of the carbon brushes of high-flying airliners). Conversely, the diamond structure is modelled as a three-dimensional structure in which the carbon atoms are all equal distances apart (interatomic distance 0.154 nm), bonded by very strong covalent bonds (1.11). This structure gives diamonds have very high melting (>3800°C) and boiling points (5100°C). Graphite may be changed into diamond at very high pressure (5 x 10 7 N/m 2 ) and temperatures (1500°C); this process is used industrially, but only a small amount of diamond is produced. Covalent structures are also formed between silicon (Si) (group 4) and oxygen (0) (group 6) in the silicates (1.13). Applying the (8-N) rule, each atom of silicon must have (8-4=) 4 nearest neighbours and each oxygen (8-6=) 2 nearest neighbours (Figure 1.21). ~ ~ Figure 1.21 Structure of the silicates The scientist's model is shown as a three-dimensional repeating structure, whose unit cell is shown in Figure 1.21a (a plan view of the unit cell is shown in Figure 1.21b; Table 1.11 gives the ordinate system). The silicate tetrahedra, i.e., silica dioxide ('silica') (SiOJ, is covalently bond together in a close packed arrangement to give a large molecule (sand) with very little free space. The schematic three dimensional view of the structure of sand illustrates the bond direction of 109° within the tetrahedra.
eBook - PDF- J.C. Anderson, Keith D. Leaver, Rees D. Rawlings, Patrick S. Leevers(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
26 A substance whose melting point is — 154°C must consist of small covalent molecules because only van der Waals forces could give rise to such a low melting point. 27 C0 2 and Si0 2 , whose melting points are respectively -56°C and 1700°C, both form small covalent molecules because carbon and silicon are both in Group 14 of the periodic table. Answers i 5 9 13 17 21 25 (a) (b) (b) (b) (b) (c) (a) 3 (b) 7 (b), 11 (b), 15 (a) 19 (b), 23 (a) 27 (d) (d) (c) (c) 2 (c) 6 (b) 10 (b) 14 (a), (c) 18 (a) 22 (b) 26 (d) 4 (a) 8 (b) 12 (c) 16 (c) 20 (i) (a) (ii) (e) 24 (c) The internal structure of crystals 6.1 Introduction It was mentioned in the last chapter that the properties of a solid depend as much upon the arrangement of atoms as on the strength of the bonds between them. We met there some examples of such arrangements, diamond and rock salt (NaCl), and also an irregular solid, glass. We know that regular internal structure often leads to regular crystalline external shapes, as in many natural precious stones, and that amorphous solids do not naturally form regular external shapes. Between these extremes lie other categories such as the metals, which show regularity in the way their atoms pack together, but which do not normally take on crystalline shapes. Materials in which perfectly aligned rows of atoms extend throughout their volume are commonly seen in use only as precious stones, but artificially grown crystals containing such aligned rows of silicon atoms are used inside every desktop computer, television and electronic controller of equipment. Mobile phones use crystals of both silicon and gallium arsenide, while the internal crystalline nature of metals, ceramics and plastics is crucial to their strength and ductility (or lack of it). The role of structure in controlling material properties will become much clearer in later chapters. In this chapter, we introduce the concept of atomic order within crystals.
eBook - PDF- Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
INTRODUCTION CHAPTER 4 Chemical Bonding and Molecular Geometry 4.1 Ionic Bonding 4.2 Covalent Bonding 4.3 Chemical Nomenclature 4.4 Lewis Symbols and Structures 4.5 Formal Charges and Resonance 4.6 Molecular Structure and Polarity It has long been known that pure carbon occurs in different forms (allotropes) including graphite and diamonds. But it was not until 1985 that a new form of carbon was recognized: buckminsterfullerene. This molecule was named after the architect and inventor R. Buckminster Fuller (1895–1983), whose signature architectural design was the geodesic dome, characterized by a lattice shell structure supporting a spherical surface. Experimental evidence revealed the formula, C 60 , and then scientists determined how 60 carbon atoms could form one symmetric, stable molecule. They were guided by bonding theory—the topic of this chapter—which explains how individual atoms connect to form more complex structures. 4.1 Ionic Bonding LEARNING OBJECTIVES By the end of this section, you will be able to: • Explain the formation of cations, anions, and ionic compounds • Predict the charge of common metallic and nonmetallic elements, and write their electron configurations Figure 4.1 Nicknamed “buckyballs,” buckminsterfullerene molecules (C 60 ) contain only carbon atoms (left) arranged to form a geometric framework of hexagons and pentagons, similar to the pattern on a soccer ball (center). This molecular structure is named after architect R. Buckminster Fuller, whose innovative designs combined simple geometric shapes to create large, strong structures such as this weather radar dome near Tucson, Arizona (right). (credit middle: modification of work by “Petey21”/Wikimedia Commons; credit right: modification of work by Bill Morrow) CHAPTER OUTLINE As you have learned, ions are atoms or molecules bearing an electrical charge.
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