Ionic Bonds An ionic bond is formed when or more electrons are transferred from a metal to the nonmetal.. +F:: : Li. Li + +F – :: : : Energy must be input to remove an electron from a metal (IE is positive). Generally, the energy released when the non metal accepts an electron (EA) does not compensate for the IE. So why do ionic compounds form? (Section 9.2)

Lattice Energy Remember, IE and EA are for adding/removing an electron to/from an atom in the gaseous state. Ionic compounds are usually solids. The release of energy on forming the solid, called the lattice energy is the driving force for the formation of ionic compounds. Because of high lattice energies, ionic solids tend to be hard and have high melting points. Ionic compounds are insulators in the solid state, because electrons are localized on the ions, but conduct when molten or in solution, due to flow of ions (not electrons). Lattice energies can be calculated using Hess’s law, via a Born-Haber Cycle.

Figure 9.6 The Born-Haber cycle for lithium fluoride

Calculating Lattice Energy Step 1: Convert elements to atoms in the gas state e.g. for Li, Li (s)  Li (g)  H 1 =  H atomization for F, 1/2 F 2 (g)  F (g)  H 2 = 1/2 (Bond Energy) Step 2: Electron transfer to form (isolated) ions Li (g)  Li + (g) + e –  H 3 = IE 1 F (g) + e –  F – (g)  H 4 = EA 1 Step 3: Ions come together to form solid Li + (g) + F – (g)  LiF (s)  H 5 = Lattice Energy Overall: Li (s) + 1/2 F 2 (g)  LiF (s)  H =  H f =  (  H 1–5 ) Lattice Energy =  H f – (  H 1 +  H 2 +  H 3 +  H 4 )

Periodic Trends in Lattice Energy Coulomb’s Law charge A X charge B electrostatic force  distance 2 (since energy = force X distance) charge A X charge B or, electrostatic energy  distance So, lattice energy increases, as ionic radius decreases (distance between charges is smaller). Lattice energy also increases as charge increases.

Figure 9.7 Trends in lattice energy