# What structural units make up ionic solids?

Jul 31, 2017

A typical ionic solid consists of a geometric array of ions, alternating between the cation and anion:

It is for this reason why it is generally inappropriate to call a single unit of an ionic compound a "molecule", which usually implies a covalent bond. Instead, an individual unit (one cation, one anion) is called a formula unit (in the image above, it is $\text{NaCl}$, sodium chloride).

The electrostatic attractions between oppositely-charged ions (${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$) are what hold the structure in tight formation.

When an ionic compound is placed in water, the ionic compound generally dissociates into its component ions via the polarity of water molecules; the positive end of several water molecules (the ${\text{H}}^{+}$ end) attract and surround the negatively-charged anion, and vice versa for the cation.

This process is called solvation:

Notice how the negative end of the water molecule (oxygen end, symbol ${\delta}^{-}$) envelops the ${\text{Na}}^{+}$ cation, and the positive end of the water molecule (hydrogen end, symbol ${\delta}^{+}$) envelops the ${\text{Cl}}^{-}$ anion.

• So what structural units comprise an ionic solid?

Possibly the ions themselves!