Aluminium (Atomic mass = 27) crystallises in a cubic system with edge length of 4 A∘. Its density is 2.7 g cm−3. The number of aluminium atoms present per unit cell is
A
6
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B
4
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C
2
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D
3
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Solution
The correct option is B 4 Density of the unit cell, ρ=Z×Ma3×NA
Given : ρ=2.7g/cm3,M=27,a=4A∘=4×10−8cm NA=6.023×1023 Z=ρ×a3×NAM Z=2.7×(4×10−8)3×(6.023×1023)27Z=3.85≈4
Hence option (b) is correct.