To confirm this definition, the tutor used both a textbook and an internet search. It’s not discussed by everybody, but the idea is useful.

I navigated high school, plus three second-year chem courses (I was a math major), without ever knowing the definition of formula unit. I likely would never have known, except that one of my high school students asked me about it.

The concept of a formula unit is the ionic compound’s answer to the idea of a molecule. Covalent compounds, such as CO₂ or H₂O, exist in self contained units which we commonly call molecules. Molecules can be separated from each other. It’s very easy to think of CO₂ molecules buzzing around independently of one another like kids in a gym.

Ionic compounds – aka, salts – (you may want to read my post on them here) don’t generally exist in separate, self contained units. They more often exist as matrices of alternating metal and nonmetal participants that spread out in all directions. Here’s a quick example of NaCl:

In such a structure, it’s hard to tell which Na atom belongs to which Cl atom, if they were to be separated into partnerships. The lines between the atoms are not strictly “bonds”. (A Cl atom, for instance, can only form one bond; it’s the same for Na.) Rather, they can be thought of as associations or else rotating bonds that move to each adjacent atom in turn.

By the diagram, we can see that for every Na atom there is one Cl atom. For this case, then, a formula unit is the ratio between the Na and the Cl atoms: NaCl. In the case of CaCl₂, a formula unit is CaCl₂. The math works the same as if it were a molecule. It’s just best not to call CaCl₂ a molecule, because it likely isn’t usually found as separate units. Hence, for salts, such as CaCl₂ or NaCl, we refer to formula units, rather than molecules.

HTH:)

Sources:

dictionary.reference.com

Turner, A. Mason and Curtis T. Sears, Jr: Inquiries in Chemistry. Toronto: Allyn and    Bacon Canada Ltd., 1977.

HTH:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.