In a simple cubic lattice, the lattice parameter a relates to the atomic radius R by which expression?

• A. a = 4R
• B. a = 2R
• C. a = √3 R
• D. a = R

Answer: (B)

In a simple cubic lattice, the lattice parameter a is the edge length of the cube. Along that edge, atoms at adjacent corners touch each other, so the center-to-center distance between those touching atoms is 2R. Since that distance equals the edge length, you get a = 2R.

The other expressions would correspond to different contact directions or geometries (for example, touching along a body diagonal or implying an impossible edge size), so they don’t describe the simple cubic packing.