In a body-centered cubic unit cell, how many atoms are in contact with the central atom?

• A. One atom in contact with eight identical atoms located at the corners.
• B. No atoms in contact.
• C. All atoms in contact with twelve.
• D. Two atoms at the corners.

Answer: (A)

In a body-centered cubic structure, the central atom sits at the center of the cell and touches each of the eight corner atoms along the body diagonals. The distance from the center to a corner equals sqrt(3)/2 times the cell edge, which for atoms of the same size equals twice the atomic radius. Therefore the central atom is in contact with all eight corner atoms. This yields eight neighboring atoms in contact with the central one. The other options conflict with this geometry: there are contacts, not none; it’s not twelve contacts (that would be the coordination in a face-centered cubic lattice); and two at the corners is not how contact works in this structure.