# How does a formula unit compare to a crystal lattice for an ionic compound?

The diagram below shows how one sodium atom and one chloride atom form oppositely charged ions, forming a formula unit of sodium chloride, which is part of the crystal lattice. Both the formula unit $\text{NaCl}$ and the crystal lattice have a $1 : 1$ ratio between sodium ions, ${\text{Na}}^{+}$ and chloride ions, $\text{Cl"^(-1)}$.