Question

.Nickel oxide crystallizes in the NaCl type of crystal structure. The length of an edge of the unit cell of NiO is 4.18 Å. What is the NiO bond length? Assuming the anions touch along the face diagonal, what is the radius of an O2 anion? What is the approximate radius of the Ni2 ion? eaup foxo bi bo 40 Jobholdo eieoO 68 8 athanic Al ADOrep

Answer 1

The rock-salt (NaCl) structure consists of two mutually interpenetrating face-centred cubic arrays of ions.The rock-salt structure can be viewed  based on a ccp array of bulky anions with cations in all the octahedral holes. NiO also have similar type of FCC structure.

Edge length of Unit cell of NiO = 4.18 A

Since in FCC of this type, corner are occupied by anions, an edge consists of a cation and 2 halves of anion :

edge (a) = 2 r_Ni²⁺ + 2 r_O^2- = 2 (r_Ni²⁺ + r_O² ) = 4.18 A

or, Bond length = (r_Ni²⁺ + r_O² ) = 2.09 A

We assume, that on the face of NiO unit cell, O^2- ions contact each other across the diagonal (d) of the face:

So, a²+a²  = d² or 2a²  = d²

d = 4 r_O^2- (along the diagonal of Face)

2*(4.18 A)2 = (4 rO2- )2 = 16 rO2- 2

r_O^2-  = 1.48 A

Approximate r_Ni²⁺  = 0.61 A