The unit cube length for LiCl ( NaCl structure) is 5 . 14 A ∘ . Assuming anion-anion contact, calculate the ionic radius for chloride ion.

In NaCl structure, anions have face centred cubic arrangement. In such a unit cell face diagonal is four times the radius of anion. In LiCl crystal structure Li + ions precisely fit into the octahedral voids of closed packed structure of chloride ions. Interionic distance of LiCl = 5 . 14 2 = 2 . 57 A ° From the diagram, ⇒ BC = AB 2 + AC 2 = ( 2 . 57 ) 2 + ( 2 . 57 ) 2 = 3 . 63 A ∘ The ionic radius of chloride ion = 1 2 × 3 . 63 A ° = 1 . 81 A ° .

An element A crystallises in fcc structure. 200 g of this element has 4.12 × 10 24 atoms. The density of A is 7 .2 g cm - 3 . Calculate the edge length of the unit cell.

Given, density, ρ = 7 .2 g cm - 3 Since, it is given that 200 g of this element has 4 . 12 × 10 24 atoms, Molar mass, M = w × N A N = 200 × 6 . 023 × 10 23 4 . 12 × 10 24 = 29 . 23 g / mol The face-centred cubic structure has atoms located at each of the corners and the centres of all the cubic faces. Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Therefore, effective number of atoms z in a face -centred cubic unit cell is four. By using the equation of density of cubic crystal system, ρ = z × M a 3 × N A a 3 = z × M ρ × N A a 3 = 4 × 29 . 23 7 .2 × 6 . 023 × 10 23 = 26 . 96 × 10 - 24 a = 2 .997 × 10 - 8 cm a = 299 .7 pm Hence, the edge length is 299 .7 pm .

ZnO turns yellow on heating. Why?

In ZnO , Zn + 2 ions occupy the interstitial sites and electrons are trapped in the interstitial sites for neutralization. Crystals that show such type of defects are generally coloured. Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.