1

### AIPMT 2015 Cancelled Paper

A given metal crystallises out with a cubic structure having edge length of 361 pm. If there are four metal atoms in one unit cell, what is the radius of one atom?
A
80 pm
B
108 pm
C
40 pm
D
127 pm

## Explanation

As Z = 4, so, structure is fcc.

Hence, r = ${a \over {2\sqrt 2 }} = {{361} \over {2\sqrt 2 }}$ = 127.65 pm = 127 pm
2

### 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.
3

### AIPMT 2015

The vacant space in bcc lattice unit cell is
A
48%
B
23%
C
32%
D
26%

## Explanation

Packing efficiency of bcc lattice = 68% Hence, empty space = 32%.
4

### 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 ${{\sqrt 3 } \over 2}a$.