[6.022e23]

Alright, so, for instance, determine lattice energy using Borne-Landi for NaCl2 (theoretical ionic lattice)...

What I did was assume the lattice was fcc, so I calculated the radius ratio (r+/r-) using CN=6 for both ions. Table values (Shannon-Prewitt Ionic Radii in Å) were Na+, CN 6 =  1.16 and Cl-, CN 6 = 1.67. So: r+/r- = 0.695. Alright, so assuming fcc and with NaCl2's stoichiometry, Na+ must occupy all tetrahedral holes in fcc. Therefore, CN=4 for Na+ and CN=6 for Cl-... Am I on the right track or am I f&#^$*@ it up? If I am doing the latter, please set me forth on the right track...