Question

1. Calculate the % ionic bonding character in the three compounds below and then when appropriate use ionic charge and ionic radii, predict crystal structures for the following materials:

(a) CoO

(b) KI

(c) NiS

2. (Callister 3.30) Magnesium oxide has the rock salt crystal structure and a density of 3.58 g/cm³.

(a) Determine the unit cell edge length or lattice parameter.

(b) How does this result compare with the edge length as determined from the radii in Table 3.4, assuming that the Mg²⁺ and O^2- ions just touch each other along the edges?

Answer 1

1.(a) Percentage ionic character can be calcualted from the following formulae

%IC = [1 - (exp)(0.25x(X_A - X_B)²]x100

For CoO: X_Co = 1.8 and X_O = 3.5

=> %IC = [1 - (exp)(0.25x(3.5 - 1.8)²]x100 = 51.4 % (answer)

r(O2-) = 140 pm

r(Co2+) = 88 pm

r(Co2+) / r(O2-) = 0.629

Hence CoO has rock salt(NaCl) structure.

(b)

Percentage ionic character can be calcualted from the following formulae

%IC = [1 - (exp)(0.25x(X_A - X_B)²]x100

For KI: X_K = 0.8 and X_I = 2.5

=> %IC = [1 - (exp)(0.25x(2.5 - 0.8)²]x100 = 51.4 % (answer)

r(K+) = 138 pm

r(I-) = 220 pm

r(K+) / r(I-) = 0.627

Hence KI has rock salt(NaCl) structure and coordination number is 6.

(c)

Percentage ionic character can be calcualted from the following formulae

%IC = [1 - (exp)(0.25x(X_A - X_B)²]x100

For NiS: X_Ni = 1.8 and X_S = 2.5

=> %IC = [1 - (exp)(0.25x(2.5 - 1.8)²]x100 = 11.5 % (answer)

r(Ni2+) = 69 pm

r(S2-) = 184 pm

r(K+) / r(I-) = 0.375

Hence NiS has zinc blend(ZnS) structure and coordination number is 4.