Alkali halides often crystallize either in the fcc NaCl-type or simple cubic (sc) CsCl-type structure. It is the lattice energy that decides which structure will be stable at a particular temperature and pressure. The energy difference between the fcc Nacl-type and sc CsCl-type structure is small and therefore transition from one to another can be caused by high pressure. For the same nearest neighbor distance the CsCl structure has a slightly (about 1%) higher Coulomb energy than the NaCl structure as the value of Madelung constant \alpha is higher from CsCl. However, each ion has eight nearest neighbor for CsCl structure whereas there are only six in NaCl structure. As the repulsive energy is about 10% of the total energy there is about 2-3% difference in repulsive energy to ouweigh Coulomb energy difference and thus favor the NaCl structure by a small amount. It is indeed the case for most of the ionic crystals that crystallize with NaCl structure. But since the differences in binding energy are small we can only decide the most stable structure only at a given temperature and pressure condition.

number of electrons in the valence give the last layer