1

### AIPMT 2015

The correct statement regarding defects in crystalline solids is
A
Frenkel defects decrease the density of crystalline solids
B
Frenkel defect is a dislocation defect
C
Frenkel defect is found in halides of alkaline metals
D
Schottky defects have no effect on the density of crystalline solids.

## Explanation

Frenkel defect is a dislocation defect as smaller ions (usually cations) are dislocated from normal sites to interstitial sites. Frenkel defect is shown by compounds having large difference in the size of cations and anions hence, alkali metal halides do not show Frenkel defect. Also, Schottky defect decreases the density of crystal while Frenkel defect has no effect on the density of crystal.
2

### AIPMT 2014

If $a$ is the length of the side of a cube, the distance between the body centered atom and one corner atom in the cube will be
A
${2 \over {\sqrt 3 }}a$
B
${4 \over {\sqrt 3 }}a$
C
${{\sqrt 3 } \over 4}a$
D
${{\sqrt 3 } \over 2}a$

## Explanation

The distance between the body centered atom and one corner atom is ${{\sqrt 3 } \over 2}a$.
3

### NEET 2013

The number of carbon atoms per unit cell of diamond unit cell is
A
6
B
1
C
4
D
8

## Explanation

Total number of carbon atoms per unit cell

= $8 \times {1 \over 8} + 6 \times {1 \over 2} + 4$ = 8
4

### NEET 2013

A metal has a fcc lattice. The edge length of the unit cell is 404 pm. The density of the metal is 2.72 g cm$-$3. The molar mass of the metal is
(NA Avogadro's constant = 6.02 $\times$ 1023 mol$-$1)
A
27 g mol$-$1
B
20 g mol$-$1
C
40 g mol$-$1
D
30 g mol$-$1

## Explanation

for fcc Z = 4.

d = ${{ZM} \over {{N_A}{a^3}}}$

$\Rightarrow$ M = ${{d{N_A}{a^3}} \over Z}$

= ${{2.72 \times 6.023 \times {{10}^{23}} \times {{\left( {404 \times {{10}^{ - 10}}} \right)}^3}} \over 4}$

= 27 g mol–1