Final answer:
A single unit of an ionic compound is not referred to as a crystal; instead, it is called a formula unit, which represents the smallest ratio of ions in a crystalline structure.
Step-by-step explanation:
The statement that a single unit of an ionic compound is referred to as a crystal is false. The smallest repeating unit that shows the characteristic proportions of ions in an ionic compound is known as a formula unit. For instance, in the case of sodium chloride (NaCl), the formula unit is a ratio of one sodium ion to one chloride ion, which reflects the lowest whole number ratio in the compound's crystalline structure. Ionic compounds form a crystalline lattice, where each ion is surrounded by ions of opposite charge in a repeating three-dimensional pattern. This structure is referred to as the crystal lattice, not a single crystal, which might suggest a macroscopic structure.