Ionic compounds, molecular orbitals and

FIGURE 5.14 Approximate LiF Molecular Orbitals. (Approximate LiF Molecular Orbitals by Kaitlin Hellie. Reprinted by permission.) 

Addition of these elementary steps, beginning with solid Li and gaseous F2, results in formation of the corresponding gas-phase ions, and provides the net enthalpy change for this chemical change  

The free energy change (AG = tsU - TtAS) must be negative for a reaction to  

The lattice enthalpy for crystal formation is sufficiently large and negative to render AG for Li(i) + F2(g) ------- LiF(i) negative. Consequently, the reaction is spontaneous. 

The methods described previously for diatomic molecules can be extended to molecules consisting of three or more atoms, but this approach becomes more challenging as molecules become more complex. We will first consider several examples of linear molecules to illustrate the concept of group orbitals and then proceed to molecules that benefit from the application of formal group theory methods. 

In a more accurate picture of ionic crystals, the ions are held together in a three-dimensional lattice by a combination of electrostatic attraction and covalent bonding. Although there is a small amount of covalent character in even the most ionic compounds, there are no directional bonds, and each Li ion is surrounded by six F ions, each of which in turn is surrounded by six Li ions. The crystal molecular orbitals form energy bands, described in Chapter 7. 

---