```SIZES OF TETRAHEDRAL AND OCTAHEDRAL VOIDS
(i) Derivation of the relationship between the radius (r) of the octahedral void and the radius (R) of the atoms in close packing.

A sphere into the octahedral void is shown in the diagram. A sphere above and a sphere below this small sphere have not been shown in the figure. ABC is a right angled triangle. The centre of void is A.

Applying Pythagoras theorem.        BC2 = AB2 + AC2         (2R)2 + (R + r)2 + (R + r)2 = 2(R + r)2 => 4R2/2 = (R + r)2

=> (√2) = (R + r)2
=> √2R = R +  r
=> r = √2R – R = (1.414 –1)R
r = 0.414 R
(ii) Derivation of the relationship between radius (r) of the tetrahedral void and the radius (R) of the atoms in close packing: To simplify calculations, a tetrahedral void may be represented in a cube as shown in the figure. In which there spheres form the triangular base, the fourth lies at the top and the sphere occupies the tetrahedral void.

Let the length of the side of the cube = a        From right angled triangle ABC, face diagonal       AB = √AC2 + BC2 = √a2 + a2 = √2a       As spheres A and B are actually touching each other, face diagonal AB = 2R       ∴ 2R = √2a           or           R = 1/√2 a        ....(i)          Again from the right angled triangle ABD        AD √AB2 + BD2 = √(√2a)2 + a2 = √3a

But as small sphere (void) touches other spheres, evidently body diagonal AD = 2(R + r).
∴ 2(R + r) = √3a
=> R + r = √3/2 a                ...(ii)
Dividing equation (ii) by equation (i)
R + r/R = √3 / 2 × a / a / √2 = √3 / √2
=> 1 + r/R = √3 / √2 = 1.225 => r/R = 1.225 – 1 = 0.225
=> r = 0.225 R
RADIUS RATIO IN 1:1 OR AB TYPE STRUCTURE

Structural   Arrangement

Coordination number

Example

0.225 – 0.414

Tetrahedral

4

CuCl, CuBr, CuI, BaS, HgS

0.414 – 0.732

Octahedron

6

MgO, NaBr, CaS, MnO, KBr, CaO

0.732 – 1

Cubic

8

CsI, CsBr, TlBr, NH4Br

Illustration 20. The two ions A+ and B- have radii 88 and 200 pm respectively. In the close packed crystal of compound AB, predict the coordination number of A+.
Solution:          r+ / r– = 88/200 = 0.44
It lies in the range of 0.414 – 0.732
Hence, the coordination number of A+ = 6
Illustration 21. Br- ion forms a close packed structure. If the radius of Br- ions is 195 pm. Calculate the radius of the cation that just fits into the tetrahedral hole. Can a cation A+ having a radius of 82 pm be slipped into the octahedral hole of the crystal A+Br-?
Solution:          (i)   Radius of the cations just filling into the tetrahedral hole
= Radius of the tetrahedral hole = 0.225´195
= 43.875 pm
(ii)   For cation A+ with radius = 82 pm
Radius ratio r+ / r– = 82/195 = 0.4205
As it lies in the range 0.414 – 0.732, hence the cation A+ can be slipped into the octahedral hole of the crystal A+Br-.
Illustration 22. Why is co-ordination number of 12 not found in ionic crystals?
Solution: Maximum radius ratio in ionic crystals lies in the range 0.732 – 1 which corresponds to a coordination number of 8. Hence coordination number greater than 8 is not possible in ionic crystals.
Illustration 23. Iron changes its crystal structure from body-centred to cubic close-packed structure when heated to 916oC. Calculate the ratio of the density of the bcc crystal to that of ccp crystal, assuming that the metallic radius of the atom does not change.
Solution: n the bcc packing, the space occupies is 68% of the total volume available while in ccp, the space occupied is 74%. This means that for the same volume masses of bcc and ccp are in the ratio of 68 : 74. As the volume is same, ratio of density is also same viz 68 : 74
i.e. d(bcc) / d(ccp) = 68/74 = 0.919
Illustration 24. In BeO (Zinc Blende structure), is introduced in available tetrahedral voids. Then ions are removed from a single body diagonal of the unit cell. What will be the molecular formula of the unit cell?
Solution: In BeO (Zinc Blende structure), alternative ‘Td’ voids of FCC of  ions are occupied by So  can be introduced in four available alternative ‘Td’ voids and unit cell formula will be. If now ions are removed from a single body diagonal then unit cell formula will be.
Illustration 25. In a crystal oxide ions are arranged in fcc and A+2 ions are at 1/8th of the tetrahedral voids, and ions B+3 occupied ½ of the octahedral voids. Calculate the packing fraction of the crystal if O-2 of the removed from alternate corner and A+2 is being place at 2 of the corners.
Solution: Since oxide ions are fcc so 4O-2 ∴ unit cell
A+2 are at 1/8th of the tetrahedral so 1A+2|unitcell
B+3 occupies ½ of the octahedral voids ∴ 2B+2/unit cell
After removing O-2 ions
Oxide ion / unit cell = 4/8 + 3 = 3.5
A+2 ions/ unit cell = 2/8 + 1 = 10/8 = 1.25
B+3 ions/unit cell = 2
∴ P.F = 3.5 × 4/3 πr3 + 1.25 × 43πr3A+2 + 2 × 4/3 πr3B+3
We know that a = 4r / √2
rA+2 = 0.225,         ∴ rA, = 0.225r–
rB+3 / r– =0.414       ∴ rB+3 = 0.414r-
Putting all the values
P.F = 0.676
Illustration 26. A binary solid  has a rock salt structure. If the edge length is 400 pm, and radius of cation is 75 pm the radius of anion is
(A) 100 mm                                          (B) 125 pm
(C) 250 pm                                           (D) 325 pm
Solution:          (B) Edge = 2(r+ + r–)
=> 400 – 2(75 + r–)
∴r– = 125 pm
Illustration 27. In closest packing of atoms
(A) The size of tetrahedral void is greater than that of the octahedral void.
The size of the tetrahedral void is smaller than that of the octahedral void.
The size of tetrahedral void is equal to that of the octahedral void.
(D) The size of tetrahedral void may be larger or smaller or equal to that of the octahedral void depending upon the size of atoms.
Solution:          (B)For tetrahedral voids
r+ / r– = 0.225, r+ = 0.225 r–                         …(i)
Similarly for octahedral voids
r+ = 0.414 r–                                         …(ii)
From equation (i) and (ii) it is clear that size of octahedral void is larger than that of tetrahedral voids.
Exercise 7.
(i) If the radius of Mg+2 ion, Cs+ ion, O2- ion S-2 ion and Cl- ion are 0.65A°, 1.69A°, 1.40A°, 1.84A°, and 1.81A° respectively. Calculate the coordination number of the cation in the crystals of MgS, MgO and CsCl.
(ii) Predict the structure of MgO crystal and coordination number of its cation in which cation and anion radii respectively are to 65 pm and 140 pm.
Exercise 8. The radius of calcium ion is 94 pm and of an oxide ion is 146 pm. The coordination number of calcium is ……………```
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