Silver (atomic weight= 108 g mol-1 and its  density of 10.5 g cm-3 ).The number of silver atoms on a surface of area 10-12 m2 can be expressed in scientific notation as1.25 × 10x. The value of x is (nearest integer)

Ag crystallizes in fcc unit cell with 4 atoms per nit cell p=4×1086.023×1023×a3=10.5 g cm−3⇒a3 (volume of unit cell) = 6.83 × 10-23 cm3⇒a= 4 × 10-8 cm = 4 × 10-10 Cm⇒Surface area of unit cell = a2 = 1.6 × 10-19 m2⇒Number of unit cell on 10-12 m2 surface =10-121.6×10-19=0.625×107 m2∵ There are two atom (effectively) on one face of unit cell. Number of atoms on 10-12 m2 surface= 2 × number of unit cell= 1.25 × 107⇒ x = 7