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Question:

Calculate the efficiency of packing in case of a metal crystal for

(i) simple cubic

(ii) body-centred cubic

(iii) face-centred cubic (with the assumptions that atoms are touching each other).

Answer:

(i) Simple cubic:

Suppose the edge length of the unit cell = a

&

Radius of the sphere = r

Then,since the sphere are touching each other along the edge,therefore a = 2r

Now there are 8 spheres at the corners of the cube & each sphere at the corner is shared by 8 unit cells & the contribution per unit cell is 1/8 so that

Number of spheres per unit cell is 8 x 1/8 = 1

Volume of sphere =4/3πr3 & volume of cube = a3 = (2r)3 = 8r3

Now packing efficiency = (volume of one sphere / total volume of cubic unit cell) x 100

Or

(4/3 πr3 / 8r3) x 100 = 52.4%

Therefore the volume occupied in simple cubic arrangement = 52.4%

 

(ii) Body centered cubic:

Let us suppose the edge leght = a & radius of each sphere = r then there are 8 spheres at the corners & 1 in the body of unit cell

Therefore number of spheres per unit cell = (8 x1/8) + 1 = 2

Now volume of unit cell = a3 = (4r / √3)3

and volume of a sphere = 4 / 3πr3

Total volume of two spheres = 2 x 4/3πr3

Packing efficiency = (volume of two spheres in unit cell/total volume of unit cell ) x 100

= (2 x 4/3πr3 / (4r/√3)3 ) x 100 = 68%

Therefore volume occupied in bcc arrangement = 68%

 

(iii) Face centered:

let us suppose the edge length of the unit cell = a

Radius of each sphere = r

Now there are 8 spheres at the corner & 6 at the faces

Therefore number of spheres in unit cell = (8 x 1/8 + 6 x1/2) = 4

From the arrangement of fcc, we get a = 2√2r

Now volume of a unit cell = a3 = (2√2r)3 = 16√2r3

Total volume of 4 spheres = 4 x 4/3 πr3 = 16/3 πr3

Packing efficiency = (volume of 4 spheres in the unit cell/total volume of unit cell) x 100

= (16/3 πr3 /16√2r3) x 100 = 74%

Therefore volume occupied in fcc = 74%


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