IIT JEE Advanced 2010 Paper 2

If R is the radius of the atom, then for the given unit cell,
a = L = (2√2R)
Then area of the square unit cell is
L × L = (2√2R)2 = 8R2
Area of atoms present in one unit cell is
πR2+4(

πR2

4

) = 2πR2
Therefore, packing efficiency is

2πR2

8R2

× 100 =

π

4

×100 = 78.54 %