3.2.Binding Forces Between Atoms 27 M M FIGURE 3.2.Schematic representation of the formation of ionically bound Nacl by transferring one electron from Na to Cl Na CI to yield Nat and Cl-ions. whereas a negatively charged chlorine ion is formed by gaining one electron.The net effect is that the two oppositely charged ions attract each other electrostatically and thus produce the ionic bond.Ionic bonds are extremely strong(see Table 3.1).Any mechanical force that tries to disturb this bond would upset the electrical balance.It is partly for this reason that ionically bound materials are,in general,strong and mostly brittle.(Still,in par- ticular cases for which the atomic structure is quite simple,plas- tic deformation may be observed,such as in NaCl single crystals, which can be bent by hand under running water. The question arises why the oppositely charged ions do not ap- proach each other to the extent that fusion of the two nuclei would occur.To answer this,one needs to realize that the elec- trons which surround the nuclei exert strong repulsive forces upon each other that become exceedingly stronger the closer the two ions approach.Specifically,the orbits of the electrons of both ions start to mutually overlap.It is this interplay between the electrostatic attraction of the ions and the electrostatic repulsion (caused by the overlapping electron charge distributions)that brings about an equilibrium distance,do,between the ions,as shown in Figure 3.3. Characteristic examples of materials which are held together in part or entirely by ionic bonds are the alkali halides,many ox- ides,and the constituents of concrete.The strength and the ten- dency for ionic bonding depend on the difference in "electroneg- TABLE 3.1 Bonding energies for various atomic bonding mechanisms Bonding mechanism Bonding energy [kJ.mol-1] Ionic 340-800 Covalent 270-610 Metallic 20-240 Van der Waals <40whereas a negatively charged chlorine ion is formed by gaining one electron. The net effect is that the two oppositely charged ions attract each other electrostatically and thus produce the ionic bond. Ionic bonds are extremely strong (see Table 3.1). Any mechanical force that tries to disturb this bond would upset the electrical balance. It is partly for this reason that ionically bound materials are, in general, strong and mostly brittle. (Still, in par￾ticular cases for which the atomic structure is quite simple, plas￾tic deformation may be observed, such as in NaCl single crystals, which can be bent by hand under running water.) The question arises why the oppositely charged ions do not ap￾proach each other to the extent that fusion of the two nuclei would occur. To answer this, one needs to realize that the elec￾trons which surround the nuclei exert strong repulsive forces upon each other that become exceedingly stronger the closer the two ions approach. Specifically, the orbits of the electrons of both ions start to mutually overlap. It is this interplay between the electrostatic attraction of the ions and the electrostatic repulsion (caused by the overlapping electron charge distributions) that brings about an equilibrium distance, d0, between the ions, as shown in Figure 3.3. Characteristic examples of materials which are held together in part or entirely by ionic bonds are the alkali halides, many ox￾ides, and the constituents of concrete. The strength and the ten￾dency for ionic bonding depend on the difference in “electroneg- 3.2 • Binding Forces Between Atoms 27 + 11 + 17 K L M K L M Na Cl FIGURE 3.2. Schematic representation of the formation of ionically bound NaCl by transferring one electron from Na to Cl to yield Na and Cl ions. TABLE 3.1 Bonding energies for various atomic bonding mechanisms Bonding mechanism Bonding energy [kJmol1] Ionic 340–800 Covalent 270–610 Metallic 20–240 Van der Waals 40