IONIC CRYSTAL STRUCTURES

General Bonding considerations.

For compound (A)_n(B)_m we can expect ionic bonding to predominate when atom A has low electronegativity and atom B has a high electronegativity.  In this case electron transfer from one atom to another leads to the formation of A⁺B^-.  For the main group elements the electron transfer continues until the ions have closed shell configurations.

For ionic compounds the bonding forces are electrostatic and therefore omni-directional.  The bonding forces should be maximized by packing as many cations around each anion, and as many cations around each anion as is possible.  The number of nearest neighbor ions of opposite charge is called the coordination number.  We must realize however that the coordination numbers are constrained by the stoichiometry of the compound and by the sizes of the atoms.

e.g. For sodium chloride, Na⁺Cl^-, there are 6 anions around each cation (coordination number Na = 6); because of the 1:1 stoichiometry there must also be 6 Na cations around each Cl anion.  For Zr⁴⁺O^2-₂there are 8 anions around each cation, therefore there must be only 4 cations around each anion.


Réference: https://www.seas.upenn.edu/~chem101/sschem/ionicsolids.html