•Lattice energy determines the stability of the ionic compound (↑ Lattice Energy = ↑ Stability).
•Lattice Energy can be calculated using Coulomb’s law OR the Born-Haber Cycle
•Coulomb’s law can be used to calculate the potential energy (E) between two ions in an ionic compound.
Since one ion is (+) and the other one is (-), E < 0 and the formation of the bond between 2 ions is an
exothermic process. Hence, separating an ionic compound into ions (lattice energy) is endothermic (-E).
•The Born-Haber cycle assumes that the formation of an ionic compound takes place in a series of steps,
and each step is involves ionization energies/electron affinities/other properties. An example of the Born-
Haber cycle for the formation of LiF is shown in the PPT slide.
Sublimation is the conversion of a solid to a
gas (endothermic process). Dissociation is the
splitting of F2 (g) into F (g) (endothermic
process). It is 150.6 kJ/mol. Since only ½ mol
of F2 are converted to F, the value is 0.5(150.6
kJ) = 75.3 kJ. The ionization of Li to Li+
requires energy and is endothermic. The
electron affinity of F is 328 kJ. However,
since enthalpy is the negative of electron
affinity, the energy change is – (electron
affinity) or -328 kJ. The energy released when
bonding the gaseous Li+ and F- into solid LiF
(∆H5º) can be calculate from the other values.
Since this value is the energy to for LiF(s), the energy to break LiF(s), or the lattice energy is -∆H5º is 1017
•For a compound to be stable, the lattice energy must be greater than the sum of the ionization energies and
electron affinities involved in the formation of ions.
oThe lattice energy of LiF(s) is greater than the net energy required to form Li+ (ionization energy)
and the F- (electron affinity).
9.4 – The Covalent Bond
•The attraction of the nucleus to the electrons of both atoms involved in the covalent bond helps to maintain
•The Lewis structures use dots for lone pairs. Bonded pairs can be shown using dots for the electrons, or
lines representing the bonds.
•Octet Rule states that all atoms other than H form covalent bonds so that they have 8 valence electrons.