Question

(a) In reference to crystal structure, explain the meaning of 'coordination number'. What is the number of
atoms in a unit cell of (i) a face centred cubic structure ? (ii) a body centred cubic structure ?

(b) The density of lead is 11.35 g cm- and the metal crystallizes with foc unit cell. Estimate the radius of
lead atom. (At. mass of lead 207 g mol-' and N. = 6.02 x 1023 mol-").​

Answer 1

Explanation:

a)It is equal to the number of nearest atoms which will touch a particular atoms.

i) 1/2*6+1=4

ii) 1+1/8*8=2

b) z=4

density = 4*(atomic mass / Avogadro no. ) / a³

[a ³= edge length ]

density = 4*(207/6.02*10²³)/a³

a³=12.11*10^-23

a=2.296(approx.)

=2.3 ( " )

in case of BCC

r=√3a/4 =√3*2.3/4=2.6/4=0.65 cm(approx.)

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