Cubic unit cells

There a three main varieties of the cubic crystal system.

Simple cubic (or primitive cubic):

The atoms in the corners are shared between 8 other unit cells, so they are regarded as 1/8th of an atom. This unit cell therefore has 8 x (1/8) atoms, or 1 in atom in total.

Body-centered cubic:

This has a a single atom inside the cube, so the unit cell contains a total of two atoms.

Face-centered cubic:

This has an atom on each face shared between two unit cells. So in total it contains (8 x (1/8)) + (6 x (1/2)), or 4 atoms in total.