3.3.Arrangement of Atoms(Crystallography) 31 FIGURE 3.7.Two polymer chains are mutually at- tracted by van der Waals forces.An applied exter- nal stress can easily slide the chains past each other. Many polymeric chains which consist of covalently bonded atoms contain areas that are permanently polarized.The cova- lent bonding within the chains is quite strong.In contrast to this, the individual chains are mutually attracted by weak van der Waals forces (Figure 3.7).As a consequence,many polymers can be deformed permanently since the chains slide effortlessly past each other when a force is applied.(We will return to this topic in Section 16.4.) One more example may be given.Ice crystals consist of strongly bonded H2O molecules that are electrostatically attracted to each other by weak van der Waals forces.At the melting point of ice, or under pressure,the van der Waals bonds break and water is formed. Mixed As already mentioned above,many materials possess atomic Bonding bonding involving more than one type.This is,for example,true in compound semiconductors (e.g.,GaAs),which are bonded by a mixture of covalent and ionic bonds,or in some transition met- als,such as iron or nickel,which form metallic and covalent bonds. 3.3.Arrangement of Atoms (Crystallography) The strength and ductility of materials depend not only on the binding forces between the atoms,as discussed in Section 3.2, but also on the arrangements of the atoms in relationship to each other.This needs some extensive explanations. The atoms in crystalline materials are positioned in a periodic,Many polymeric chains which consist of covalently bonded atoms contain areas that are permanently polarized. The cova￾lent bonding within the chains is quite strong. In contrast to this, the individual chains are mutually attracted by weak van der Waals forces (Figure 3.7). As a consequence, many polymers can be deformed permanently since the chains slide effortlessly past each other when a force is applied. (We will return to this topic in Section 16.4.) One more example may be given. Ice crystals consist of strongly bonded H2O molecules that are electrostatically attracted to each other by weak van der Waals forces. At the melting point of ice, or under pressure, the van der Waals bonds break and water is formed. As already mentioned above, many materials possess atomic bonding involving more than one type. This is, for example, true in compound semiconductors (e.g., GaAs), which are bonded by a mixture of covalent and ionic bonds, or in some transition met￾als, such as iron or nickel, which form metallic and covalent bonds. 3.3 • Arrangement of Atoms (Crystallography) 31 FIGURE 3.7. Two polymer chains are mutually at￾tracted by van der Waals forces. An applied exter￾nal stress can easily slide the chains past each other. 3.3 • Arrangement of Atoms (Crystallography) Mixed Bonding The strength and ductility of materials depend not only on the binding forces between the atoms, as discussed in Section 3.2, but also on the arrangements of the atoms in relationship to each other. This needs some extensive explanations. The atoms in crystalline materials are positioned in a periodic