Question

A crystal of sodium hydride has an fcc unit cell of ${\text{H}}^{-}$ ions with ${\text{Na}}^{+}$ ions at the body centres of a unit cell and in the centre of edges. The number of ${\text{H}}^{-}$ that touch each ${\text{Na}}^{+}$ is:

• A. $6$
• B. $4$
• C. $10$
• D. $2$

Answer 1

The correct option is A $6$

Hydride ions form the fcc structure.
They are present at the corners of the cube and the face centres.
Sodium ions are present at the edge centres and the body centre.
Each sodium ion (present at the edge centre) is surrounded by six hydride ions out of which $2$ are present at the corners and $4$ are present at the face centres.
The sodium ion present at the body centre is surrounded by six hydride ions present at six face centres.

Hence, the correct option is $\text{A}$