1. Size and Charge of Ions:

* Relative sizes: The ratio of the radii of the cation and anion plays a crucial role. If the cation is smaller than the anion, it can fit into the spaces between anions, leading to different crystal structures.

* Charge: The charge of the ions determines the strength of the electrostatic attraction between them, which influences the arrangement and packing of ions in the crystal lattice.

2. Coordination Number:

* The coordination number refers to the number of oppositely charged ions surrounding a given ion in the crystal structure. It is directly related to the relative sizes of the ions and their charges.

3. Packing Efficiency:

* Ions in a crystal structure tend to pack as efficiently as possible to minimize the energy of the system. This leads to different crystal structures with varying packing efficiencies.

4. Temperature and Pressure:

* Temperature and pressure can affect the stability of different crystal structures. At higher temperatures, ions have more kinetic energy and can vibrate more, potentially causing a change in the crystal structure. Pressure can also influence the packing of ions.

5. Lattice Energy:

* The lattice energy is the energy released when ions come together to form a crystal lattice. Different crystal structures have different lattice energies, and the structure with the lowest energy is generally the most stable.

Common Crystal Structures:

* Cubic: NaCl (rock salt), CsCl

* Hexagonal: ZnS (wurtzite), CdS

* Tetrahedral: ZnS (sphalerite), CuCl

* Octahedral: TiO2 (rutile)

The specific shape of an ionic crystal is a result of the interplay of these factors. It's important to note that these factors can often be interconnected, making it complex to predict the exact shape of a crystal.