28 3·Mechanisms Repulsive energy 0 Net energy FIGURE 3.3.Schematic represen- tation of the potential energy, Attractive Epot,of an Na+and a Cl-ion energy as a function of the internu- clear distance,d.The equilib- rium distance,do,between the Na do two ions is the position of smallest potential energy. Repulsion→ Attraction ativity"between the elements involved,that is,the likelihood of an atom to accept one or more extra electrons.Chlorine,for ex- ample,is strongly electronegative because its outer shell is al- most completely filled with electrons (Figure 3.2).Sodium,on the other hand,is said to be weakly electronegative (actually it is electropositive:Group I of the Periodic Table!)and,therefore, readily gives up its valencel electron. Covalent Covalently bound solids such as diamond or silicon are typically Bond from Group IV of the Periodic Table.Consequently,each atom has four valence electrons.Since all atoms are identical,no elec- trons are transferred to form ions.Instead,in order to achieve the noble gas configuration,double electron bonds are formed by electron sharing [see Figure 3.4(a)].In other words,each Group IV atom,when in the solid state,is "surrounded"by eight elec- trons,which are depicted in Figure 3.4(a)as dots. The two-dimensional representation shown in Figure 3.4(a)is certainly convenient but does not fully describe the characteris- tics of such solids.Indeed,in three dimensions,silicon atoms, for example,are arranged in the form of a tetrahedron2 around a center atom having 10928'angles between the bond axes,as depicted in Figure 3.4(b).Because of this directionality and be- Valentia (Latin)=capacity,strength 2Tetraedros (Greek)=four-faced.ativity” between the elements involved, that is, the likelihood of an atom to accept one or more extra electrons. Chlorine, for ex￾ample, is strongly electronegative because its outer shell is al￾most completely filled with electrons (Figure 3.2). Sodium, on the other hand, is said to be weakly electronegative (actually it is electropositive: Group I of the Periodic Table!) and, therefore, readily gives up its valence1 electron. Covalently bound solids such as diamond or silicon are typically from Group IV of the Periodic Table. Consequently, each atom has four valence electrons. Since all atoms are identical, no elec￾trons are transferred to form ions. Instead, in order to achieve the noble gas configuration, double electron bonds are formed by electron sharing [see Figure 3.4(a)]. In other words, each Group IV atom, when in the solid state, is “surrounded” by eight elec￾trons, which are depicted in Figure 3.4(a) as dots. The two-dimensional representation shown in Figure 3.4(a) is certainly convenient but does not fully describe the characteris￾tics of such solids. Indeed, in three dimensions, silicon atoms, for example, are arranged in the form of a tetrahedron2 around a center atom having 109°28 angles between the bond axes, as depicted in Figure 3.4(b). Because of this directionality and be￾FIGURE 3.3. Schematic represen￾tation of the potential energy, Epot, of an Na and a Cl ion as a function of the internu￾clear distance, d. The equilib￾rium distance, d0, between the two ions is the position of smallest potential energy. 28 3 • Mechanisms Na+ Cl– d0 d Repulsion Attraction 0 Epot Repulsive energy Net energy Attractive energy 1Valentia (Latin)
capacity, strength. 2Tetraedros (Greek)
four-faced. Covalent Bond