26 3·Mechanisms the type of interatomic forces that hold the atoms in their posi- tion.These interatomic bonds,which result from electrostatic in- teractions,may be extremely strong (such as for ionic or cova- lent bonds;see below).Materials whose atoms are held together in this way are mostly hard and brittle.In contrast,metallic and particularly van der Waals bonds are comparatively weak and are thus responsible,among others,for the ductility of materials.In brief,there is not one single physical mechanism that determines the mechanical properties of materials,but instead,a rather com- plex interplay between interatomic forces,atomic arrangements, and defects that causes the multiplicity of observations described in Chapter 2.In other words,the interactions between atoms play a major role in the explanation of mechanical properties.We shall endeavor to describe the mechanisms leading to plasticity or brittleness in the sections to come. In contrast to this,the electrical,optical,magnetic,and some thermal properties can be explained essentially by employing the electron theory of condensed matter.This will be the major theme of Part II of the present book,in which the electronic properties of materials will be discussed. 3.2.Binding Forces Between Atoms Ionic Bond Atoms consist of a positively charged nucleus and of negatively charged electrons which,expressed in simplified terms,orbit around this nucleus.Each orbit (or "shell")can accommodate only a maximum number of electrons,which is determined by quantum mechanics;see Appendix I.In brief,the most inner "K- shell"can accommodate only two electrons,called s-electrons. The next higher "L-shell"can accommodate a total of eight electrons,that is,two s-electrons and six p-electrons.The fol- lowing "M-shell"can host two s-electrons,six p-electrons,and ten d-electrons;and so on. Filled outermost s+p shells constitute a particularly stable (nonreactive)configuration,as demonstrated by the noble (in- ert)gases in Group VIII of the Periodic Table (see Appendix). Chemical compounds strive to reach this noble gas configuration for maximal stability.If,for example,an element of Group I of the Periodic Table,such as sodium,is reacted with an element of Group VII,such as chlorine,to form sodium chloride (NaCl), the sodium gives up its only electron in the M-shell,which is transferred to the chlorine atom to fill the p-orbit in its M-shell; see Figure 3.2.The sodium atom that gave up one electron is now positively charged (and is therefore called a sodium ion),the type of interatomic forces that hold the atoms in their posi￾tion. These interatomic bonds, which result from electrostatic in￾teractions, may be extremely strong (such as for ionic or cova￾lent bonds; see below). Materials whose atoms are held together in this way are mostly hard and brittle. In contrast, metallic and particularly van der Waals bonds are comparatively weak and are thus responsible, among others, for the ductility of materials. In brief, there is not one single physical mechanism that determines the mechanical properties of materials, but instead, a rather com￾plex interplay between interatomic forces, atomic arrangements, and defects that causes the multiplicity of observations described in Chapter 2. In other words, the interactions between atoms play a major role in the explanation of mechanical properties. We shall endeavor to describe the mechanisms leading to plasticity or brittleness in the sections to come. In contrast to this, the electrical, optical, magnetic, and some thermal properties can be explained essentially by employing the electron theory of condensed matter. This will be the major theme of Part II of the present book, in which the electronic properties of materials will be discussed. Atoms consist of a positively charged nucleus and of negatively charged electrons which, expressed in simplified terms, orbit around this nucleus. Each orbit (or “shell”) can accommodate only a maximum number of electrons, which is determined by quantum mechanics; see Appendix I. In brief, the most inner “K￾shell” can accommodate only two electrons, called s-electrons. The next higher “L-shell” can accommodate a total of eight electrons, that is, two s-electrons and six p-electrons. The fol￾lowing “M-shell” can host two s-electrons, six p-electrons, and ten d-electrons; and so on. Filled outermost s p shells constitute a particularly stable (nonreactive) configuration, as demonstrated by the noble (in￾ert) gases in Group VIII of the Periodic Table (see Appendix). Chemical compounds strive to reach this noble gas configuration for maximal stability. If, for example, an element of Group I of the Periodic Table, such as sodium, is reacted with an element of Group VII, such as chlorine, to form sodium chloride (NaCl), the sodium gives up its only electron in the M-shell, which is transferred to the chlorine atom to fill the p-orbit in its M-shell; see Figure 3.2. The sodium atom that gave up one electron is now positively charged (and is therefore called a sodium ion), Ionic Bond 26 3 • Mechanisms 3.2 • Binding Forces Between Atoms