# Exercise 7 | Liquids and Solids

Crystalline potassium fluoride has a face-centered cubic array of anions with an interpenetrating face-centered cubic cation lattice.

How many potassium ions and fluoride ions are there per unit cell?

Face-centered cubic lattice:

- 1 atom at the corner ($\frac{1}{8}$ x 8 corners)

- 3 atoms at the center of each face ($\frac{1}{2}$ x 6 faces)

⇒  4 anions

⇒ 4 fluoride ions

Same for cations:

⇒ 4 cations

⇒ 4 potassium ions

(We know also that potassium fluoride is neutral ⇒ same number of fluoride ions and potassium ions)