Structure of Ionic Solids

Ionic compounds form a special type of structure known as a crystal lattice, which is an orderly, repeating pattern extending in three dimensions.
Ionic solids are composed of positively-charged cations and negatively-charged anions. These ions are held together in a tightly-packed structure by strong electrostatic forces, often referred to as ionic bonds.
The arrangement of ions within an ionic solid has a major impact on its overall properties. For instance, ionic solids typically have high melting and boiling points due to the strength of their ionic bonds.
The coordination number of an ion in a crystal lattice refers to the number of oppositely-charged ions it is adjacent to. This number can vary depending on the specific ions that make up the compound.
The key characteristic of a crystal lattice is its symmetry. Whether every ion is surrounded by eight ions of opposite charge (as in a cubic arrangement), or by another number, the arrangement is always symmetrical.
Ionic radii or ionic sizes play a crucial role in determining the structure of ionic compounds. Generally, when ions combine to form an ionic solid, they arrange themselves in such a way that minimises the overall energy of the structure. This usually leads to the smaller cations fitting into the spaces between larger anions.

Properties of Ionic Solids

Ionic solids tend to be hard but brittle. The hardness is due to the strength of the ionic bonds, and the brittleness is due to the orderly arrangement of ions in the crystal lattice.
When external pressure is applied, it can cause the layers of ions to move. When like-charged ions come into proximity, they repel each other, causing the crystal to fracture, explaining the brittleness of ionic solids.
Ionic solids do not conduct electricity in solid form, as their ions are not free to move. But when melted or dissolved in water, they form ions in solution, which are free to move and hence can conduct electricity.
Most ionic solids are water-soluble. The water molecules’ polarity can attract and surround the individual ions, separating them from the overall structure and carrying them into solution, this process is known as dissociation.
Ionic solids have a high thermal stability, which means they can withstand high temperatures without decomposing. This is again due to the strong electrostatic forces between the positive and negative ions.