1

AIPMT 2011 Mains

A solid compound XY has NaCl structure. If the radius of the cation is 100 pm, the radius of the anion (Y$-$) will be
A
275.1 pm
B
322.5 pm
C
241.5 pm
D
165.7 pm

Explanation

For NaCl crystal, ${{{r_ + }} \over {{r_ - }}} = 0.414$

Given, r+ = 100 pm

$\therefore$ ${{100} \over {{r_ - }}} = 0.414$

$\Rightarrow$ ${r_ - } = {{100} \over {0.414}}$ = 241.5 pm
2

AIPMT 2010 Prelims

AB crystallizes in a body centered cubic lattice with edge length '$a$' equal to 387 pm. The distance between two oppositely charged ions in the lattice is
A
335 pm
B
250 pm
C
200 pm
D
300 pm

Explanation

For a bcc lattice, 2(r+ + r) = $\sqrt 3 a$

$\therefore$ r+ + r = ${{\sqrt 3 \times 387} \over 2}$ = 335 pm
3

AIPMT 2009

Copper crystallises in a face-centred cubic lattice with a unit cell length of 361 pm. What is the radius of copper atom in pm?
A
157
B
181
C
108
D
128

Explanation

Since Cu crystallises in a face-centred cubic lattice,

Atomic radius, r = ${a \over {2\sqrt 2 }}$

$\therefore$ r = ${{361} \over {2\sqrt 2 }}$ = 128 pm
4

AIPMT 2009

Lithium metal crystallises in a body-centred cubic crystal. If the length of the side of the unit cell of lithium is 351 pm, the atomic radius of lithium will be
A
151.8 pm
B
75.5 pm
C
300.5 pm
D
240.8 pm

Explanation

Since Li crystallises in body-centred cubic

crystal, atomic radius, r = ${{\sqrt 3 a} \over 4}$.

$\therefore$ r = ${{\sqrt 3 } \over 4} \times 351$ = 151.8 pm