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12 Solids and Modern Materials Review Questions 12.1 Graphene is one-atom-thick graphite. It is the thinnest and strongest known material. It conducts heat and electricity, it is transparent, and it is completely impermeable to all substances, including helium. 12.3 A crystalline lattice is the regular arrangements of atoms within a crystalline solid. The crystalline lattice can be represented by a small collection of atoms, ions, or molecules-a fundamental building block called the unit cell. When the unit cell is repeated over and over-like tiles in a floor or the pattern in a wallpaper design, but in three dimensions-the entire lattice can be reproduced. 12.5 Atoms in a simple cubic cell structure have a coordination number of 6, an edge length of 2r, and one atom in the unit cell. Atoms in a body-centered cubic cell structure have a coordination number of 8, an edge length of and two atoms in the unit cell. Atoms in a face-centered cubic cell structure have a coordination number of 12, an edge length of and four atoms in the unit cell. 12.7 The three types of solids are molecular solids, ionic solids, and atomic solids. Molecular solids are those solids whose composite units are molecules. The lattice sites in a crystalline molecular solid are therefore occupied by molecules. Ice (solid H₂O) and dry ice (solid CO₂) are examples of molecular solids. Molecular solids are held together by the kinds of intermolecular dispersion forces, dipole-dipole forces, and hydrogen bonding. Ionic solids are those solids whose composite units are ions. Table salt (NaCl) and calcium fluoride (CaF₂) are good examples of ionic solids. Ionic solids are held together by the coulombic interactions that occur between the cations and anions occupying the lattice sites in the crystal, which is an ionic bond. Atomic solids are those solids whose composite units are individual atoms. Atomic solids can themselves be divided into three categories-nonbonding atomic solids, metallic atomic solids, and network covalent atomic solids-each held together by a different kind of force. Nonbonding atomic solids, which include only the noble gases in their solid form, are held together by relatively weak dispersion forces. Metallic atomic solids, such as iron and gold, are held together by metallic bonds, which in the simplest model are represented by the interaction of metal cations with a sea of electrons that sur- round them. Network covalent atomic solids, such as diamond, graphite, and silicon dioxide, are held together by covalent bonds. 12.9 Nonbonding atomic solids, which include only the noble gases in their solid form, are held together by relatively weak dispersion forces. Metallic atomic solids, such as iron and gold, are held together by metallic bonds, which in the simplest model are represented by the interaction of metal cations with a sea of electrons around them. Network covalent atomic solids, such as diamond, graphite, and silicon dioxide, are held together by covalent bonds. 12.11 The coordination number of the unit cell for an ionic compound represents the number of close cat- ion-anion interactions. Because these interactions lower potential energy, the crystal structure of a particular ionic compound will be the one that maximizes the coordination number while accommo- dating both charge neutrality (each unit cell must be neutral) and the different sizes of the cations and anions that compose the particular compound. In general, the more similar the radii of the cation and the anion, the higher the possible coordination number. 238 Copyright © 2017 Pearson Education, Inc.