What is the unit cell volume when edge length a equals 3 Å?

• A. 9 Å^3
• B. 18 Å^3
• C. 27 Å^3
• D. 81 Å^3

Answer: (C)

In a cubic unit cell, the volume comes from multiplying the three edge lengths. Since all edges are equal to a, the volume is V = a^3. With a = 3 Å, you get V = 3^3 = 27 Å^3.

The other numbers come from misreading the geometry: 9 Å^3 is a^2, which would be the face area, not a volume; 81 Å^3 is a^4, not a unit-cell volume; 18 Å^3 doesn’t correspond to the cube of the given edge length.