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NEET levelSingle correct optionDifficulty 1 of 3ss006

For a face-centred cubic unit cell of atoms of radius $r$ , the edge length $a$ is

A $2 r$
B $4 r$
C $22 r$
D $4 r / 3$

Answer: C

Atoms touch along the face diagonal, which has length

2 a = 4 r

, so

a = 4 r / 2 = 22 r

. The option

4 r / 3

is for bcc and

2 r

for simple cubic.

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