Question

How many sodium ions and chloride ions are present in a unit cell of sodium chloride crystal?

For a crystal of sodium chloride, state the number of sodium ions and chloride ions present in a unit cell of sodium chloride.

Answer 1

NaCl has a face-centred cubic (fcc) structure:

Cl^– ions occupy the corners and face the centre of the cube.

`8  "corner" xx 1/8 = 1`

`6  "face" xx 1/2 = 3`

∴ Total Cl^– ions = 1 + 3 = 4

Na⁺ ions occupy the edge centre and body centre.

`12  "edge" xx 1/4 = 3`

1 face centre body centre = 1

∴ Total Na+ ions = 3 + 1 = 4

In a unit cell of NaCl crystal:

Number of Na⁺ ions = 4

Number of Cl^– ions = 4