Chemistry
Covalent Network Solid
A covalent network solid is a type of solid in which the atoms are bonded together by a network of covalent bonds, forming a continuous three-dimensional structure. This results in a strong and rigid solid with high melting and boiling points. Examples of covalent network solids include diamond, graphite, and silicon dioxide (quartz).
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4 Key excerpts on "Covalent Network Solid"
eBook - PDFChemistry
Structure and Dynamics
- James N. Spencer, George M. Bodner, Lyman H. Rickard(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
Which of the following solids are held together by an extended network of covalent bonds? (a) sodium chloride (b) CuAl 2 (c) gold (d) calcium carbonate (e) diamond (f) dry ice (solid CO 2 ) 5. Which force must be overcome to sublime dry ice, solid CO 2 ? (a) metallic bonding (b) ionic bonding (c) covalent bonding (d) dispersion forces 6. Which force must be overcome to melt solid pentane, C 5 H 12 ? (a) metallic bonding (b) ionic bonding (c) covalent bonding (d) dispersion forces Physical Properties of Molecular and Network Covalent Solids 7. Compare and contrast the bonding in molecular solids and network covalent solids. 8. Which would you expect to have a higher melting point, molecular solids or network covalent solids? 9. What are the three stable crystalline forms of carbon? How do their structures differ? Metallic Solids 10. What are delocalized electrons? 11. What is a metallic bond? What types of atoms are most likely to form metallic bonds? 12. Describe the bonding in a substance that is held together by metallic bonds. 13. Molecular, ionic, and network covalent solids all have one characteristic in common that makes them different from metallic solids. What is this characteristic? Physical Properties That Result from the Structure of Metals 14. Explain why metals are solids at room temperature. 15. Explain why metals are malleable and ductile. 16. Explain why metals conduct heat and electricity. 17. Which of the following categories is most likely to contain a compound that is a poor conductor of elec- tricity when solid but a very good conductor when molten? (a) molecular solids (b) covalent solids (c) ionic solids (d) metallic solids The Structure of Metals 18. List three common structures for metals. 19. Describe the difference in the way planes of atoms stack to form hexagonal-closest-packed, cubic-closest-packed, body-centered cubic, and simple cubic structures.
eBook - ePub- Anthony R. West(Author)
- 2014(Publication Date)
- Wiley(Publisher)
3 Bonding in Solids 3.1 Overview: Ionic, Covalent, Metallic, van der Waals and Hydrogen Bonding in SolidsCrystalline materials exhibit the complete spectrum of bond types, ranging from ionic to covalent, van der Waals and metallic. Sometimes more than one type of bonding is present, as in salts of complex anions, e.g. Li2 SO4 , which have both ionic bonds between Li+ and SO4 2– ions and covalent bonds linking sulfur and oxygen atoms within SO4 2– ions. Commonly, the bonds are a blend of different types, as in TiO, which is ionic/metallic, and CdI2 , which is ionic/covalent/van der Waals. In discussing structures, it is often convenient to ignore temporarily the complexities of mixed bond types and to treat bonds as though they were purely ionic.Ionic bonding leads to structures with high symmetry in which the coordination numbers are as high as possible. In this way, the net electrostatic attractive force which holds crystals together (and hence the lattice energy) is maximised. Covalent bonding , by contrast, gives highly directional bonds in which atoms prefer a certain coordination environment, irrespective of other atoms that are present. The coordination numbers in covalent structures are usually small and may be less than those in ionic structures which contain similar-sized atoms.The bonding in a particular compound correlates fairly well with the position of the component atoms in the periodic table and, especially, with their electronegativity. Alkali and alkaline earth elements usually form ionic structures (Be is sometimes an exception), especially in combination with small electronegative anions such as O2− and F− . Covalent structures occur especially with (a) small atoms of high valency which, in the cationic state, would be highly polarising, e.g. B3+ , Si4+ , P5+ and S6+ and to a lesser extent with (b) large atoms which in the anionic state are highly polarisable, e.g. I− and S2− . Most non-molecular materials have mixed ionic and covalent bonding and, as discussed later, it is possible to assess the ionicity
eBook - PDF- J.C. Anderson, Keith D. Leaver, Rees D. Rawlings, Patrick S. Leevers(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
The other Group 14 elements, silicon and germanium, also crystallize in the same structure. It is useful to note here how to deduce the number of neighbours with which each atom may bond in a covalent solid of an element. In the covalent bonding mechanism the atom acquires electrons by sharing, until it has stable outer subshells. The number of valence electrons needed thus equals the number of electrons lacking from the outer s and p subshells; if there are N electrons present in these outer subshells of the neutral atom then bonding with (8 — N) neighbours results. It is interesting that compounds of an element of Group 13 with one of Group 15 also form a structure related to that of diamond, and which we will meet in Chapter 6. In these compounds each atom is again tetra-hedrally coordinated with four others, even though the isolated Group 13 and Group 15 atoms have three and five valence electrons respectively. The (8 — N) rule clearly does not apply within compounds. Covalent bonding sometimes results in two, or occasionally three, electron pairs being shared by two atoms. We refer to them as double and triple covalent bonds, and show them as two or three parallel lines in a structural diagram. Thus 0 = 0 and N = N represent two famil-iar molecules, while calcium dicarbide, CaC 2r is an ionic solid comprised of the ions Ca+ and (CEEC) 2 -. 5.6 Metallic solids As the name implies, these are confined to metals and near-metals, many of which are found in Groups 1-3 and 11-13 of the periodic table. If we take copper as an example, we see that shells K, L and M are full, while there is just one 4s electron in the N shell. In solid copper the outer 5.7 COMBINATIONS OF BONDING MECHANISMS 77 electron is readily released from the parent atom and all the valence electrons can move about freely between the copper ions.
eBook - PDF- William W. Porterfield(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
This is obviously hopelessly deficient, and makes it clear that covalent bonding is thermodynamically important to the Agl crystal. Two lattices are often seen in three-dimensional polymeric network crys-tals. The first we have already seen: the R e 0 3 lattice, seen in Fig. 3.2 for the InF 3 crystal. We can distinguish it from a truly “electrostatic” lattice because its approximate close-packing of anions has regular vacancies, which leave large cavities in the crystal that could, under the right circumstances, contain other atoms or ions. We shall return to this possibility shortly. Note that 134 3 DIRECTIONAL BOND NETWORKS AND SOLID STATE CHEMISTRY O C(graphite); B(BN) ® C(graphite); N(BN) O As(arsenic); Sn(SnS) ® As(arsenic); S(SnS) Ο Θ Θ Ο Θ Ο Ο Φ Ο Ο o o oo oo o o o o o oooooo o o o o o o o O O O O O O O o o o o o o o O O O O O O O O O O O Bi2 Se3 edge view; Bi coordination octahedron outlined m Bi O S e o o oo oo o o o o o O © O O © O O O O O 0 Figure 3.3 Perspective views of layer structures. because of the 1 :3 stoichiometry of the compound the coordination number ratio must be 6 : 2 . Another three-dimensional polymeric network crystal lattice adopted by over a hundred compounds is the PbCl 2 lattice, an idealized version of which is seen in Fig. 3.4. In this lattice the Pb atom is nine-coordinate, approximat-ing the tricapped trigonal prism shown, though with distortion. The three- dimensional polymer is formed by first joining a chain of tricapped trigonal prisms together by sharing capping edges. A second such chain can be placed next to the first, sharing each edge, if the capping Cl atoms from the first chain become prism atoms for the second chain so that the two chains are not at the same height and if the direction of the second chain is reversed relative to the first chain. This edge sharing can be extended both in the plane of the figure and perpendicular to it.
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