Question

In the \(NaCl\) (Rock salt) structure, what would be the distance for the nearest cation \((Na^+)\) from an anion \((Cl^-)\) ?
Here \(a\) is the edge cell length.

Answer 1

In the \(NaCl\) (Rock salt) structure,
\(Cl^-\) ions form the FCC lattice,while \(Na^+\) ions occupy all the octahedral voids.

In the above figure, \(Na^+\) ion is surrounded octahedrally by six \(Cl^-\) ions and \(Cl^-\) ion is surrounded octahedrally by six \(Na^+\) ions
For an anion \(Cl^-\)
Distance of the nearest cation \(Na^+\) will be \(\dfrac{a}{2}\).
This is the distance from the corner atom \(Cl^-\) to the edge atom \(Na^+\)
Option (b) is correct.
Number of nearest cations are 6
Nearest anion-anion distance is \(\dfrac{a}{\sqrt{2}}\)
This is the distance between face centre and corner atoms.
In a rock salt \(NaCl\) structure:
Effective number of \(Cl^-\) ions in each unit cell are 4
Effective number of \(Na^+\) ions in each unit cell are 4
Effective total number of \(NaCl\) units in a unit cell are 4