Formula unit — Science definition
The simplest whole-number ratio of ions in an ionic compound (for example, NaCl).
Where it is taught
- Ionic Bonding — Bonding, Formulas & Molecular Structure, Physical Science: Chemistry, Grade 11
In use
That's why chemists don't call 'NaCl' a molecule; it's a formula unit, the simplest whole-number ratio of ions in the lattice.
Ionic Bonding
Questions using formula unit
Problem 1
An ion with a 3+ charge combines with an ion with a 2− charge. Using the criss-cross rule, how many ions in total (cations plus anions) are in one formula unit of the compound?
Answer and explanation
5
Balancing +3 and −2 needs 2 cations (+6) and 3 anions (−6), so one formula unit has 2 + 3 = 5 ions.
Problem 2
Iron(III) ions (Fe³⁺) combine with oxide ions (O²⁻). How many oxide ions are in one formula unit of the compound?
Answer and explanation
3
Two Fe³⁺ give +6 and three O²⁻ give −6, so the formula is Fe₂O₃ with 3 oxide ions.