In BCC, the distance between the two nearest atoms will be:

1. $\sqrt{3}$ $a$

2. $\frac{\sqrt{3}}{2}a$

3. $\frac{\sqrt{3}}{4}a$

4. $\frac{a}{2}$

The pyknometric density of sodium chloride crystal is 2.165 × 10³ kg m^–3 while its X-ray density is 2.178 × 10³ kg m^–3. The fraction of unoccupied sites in sodium chloride crystal is:

1. 5.96

2. 5.96 × 10^–2

3. 5.96 × 10^–1

4. 5.96 × 10^–3

A compound formed by elements X and Y crystallizes in a cubic structure in which the X atoms are at the corners of a cube and the Y atoms are at the face centers. The formula of the compound is:

1. X₃Y

2. XY

3. XY₂

4. XY₃