Iron (II) oxide has a cubic structure and each unit cell has a size of `5 Å`. If density of this oxide is 4 g `cm^(-3)`, calculate the number of `Fe^(2+) and O^(2-)` ions present in each unit cell.
(Atomic mass of Fe = 56, O = 16, `N_(A) = 6.023 xx 10^(23) and 1 Å = 10^(-8)` cm)

Explanation

(a) When amorphous solid (glass) are heated and cooled slowly they acquire crystallinity at same temperature. Glass objects of ancient monuments, over a period of years, are exposed to sunlight and cooled during night times, which has resulted in the crystalization of the glass object which imparts milky colour to the ancient monuments.
(b) Solve by using
`d = (Z xx M)/(a^(3) xx N_(A) xx 10^(-30))`
`Z = (4.0 g cm^(-3) xx (5 xx 10^(-8)cm)^(3)(6.023 xx 10^(23) mol^(-1)))/((56 + 16) g mol^(-1))`
= 4 i.e., `4Fe^(2+) and 4O^(2-)` ions present in each unit cell.