Question

In an FCC unit cell, what fraction of the edge is not covered by atoms?

• A. 0.134
• B. 0.293
• C. 0.26
• D. 0.32

Answer 1

The correct option is B 0.293

Fraction of the edge that is not covered by atoms $=\frac{a-2r}{a}$ (a = length of edge of the unit cell, r = radius of the atom)
In an FCC unit cell, $r=\frac{a}{2\sqrt{2}}$
Therefore, fraction of edge that is not covered$=\frac{a-2\times \frac{a}{2\sqrt{2}}}{a}=0.293$
Fraction of the edge that is not covered = 0.293