GATE 2020 MT – Question 35

Materials Science and Non-Metallic Materials · Crystal structures and bonding · 1 mark · Numerical answer

Find the number of atoms per nm² in the (100) plane of FCC Pb, rounded to an integer. Atomic radius is 0.175 nm.

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Correct answer: 7 to 9

Explanation

FCC geometry gives $a=2\sqrt2r$. A (100) square contains four corner quarters plus one face-centred atom, for two atoms per area a². Planar density is $2/a^2=1/(4r^2)=1/[4(0.175)^2]=8.163$ atoms/nm², rounding to 8.

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