Why do we use the term "formula unit" rather than "molecule" when we refer to the simplest repeating unit of an ionic solid?

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Chemistry

Kamila Woodward

14 April, 15:51

Why do we use the term "formula unit" rather than "molecule" when we refer to the simplest repeating unit of an ionic solid?

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Abagail Hale · Answer 1

In order to understand the difference between Molecule and Formula Unit lets take one one example for each.

NH₃:

Ammonia is a covalent compound which contains three covalent bonds between N and three H's. One molecule of Ammonia can exist independently, so it is possible to find a single molecule of NH₃ comprising of one nitrogen and three hydrogen atoms bonded covalently.

NaCl:

Sodium chloride is made up of Na⁺ and Cl⁻ ions. Now, the formula NaCl is not molecular formula, instead it is a formula unit. Formula unit means that this compound consists of giant structure of Na⁺ ions and Cl⁻. There numbers can not be counted but the ratio of these two ions to each other is countable and for each Na⁻ ion there is one Cl⁻ ion. Hence, Formula unit is an Empirical formula of any Ionic compound.