Final answer:
In a simple cubic crystal structure, atoms touch along the edges of the cube, with each unit cell's edge being equal to the diameter of one atom.
Step-by-step explanation:
In the simple cubic crystal structure, atoms touch each other along the edges of the cube. Each unit cell is defined by the centers of eight atoms located at the corners of the cube. As atoms at adjacent corners touch each other, the edge length of this cell is equal to two atomic radii, or one atomic diameter. In this cubic lattice, each atom contacts six others, which gives it a coordination number of six. Despite the simplicity of this structure, it is relatively inefficient as the spheres are not packed closely, only filling about 52% of the volume, and is exemplified by polonium (Po) crystallizing in such an arrangement.