Question

How do we know how many ions there are per lattice point in a unit cell?

Answer 1

Lattice points are the positions (or coordinates) where you can place an atom or ion.so the number of ions/atoms in different unit cells as follows.

In FCC unit cells, lattice points are located at the corners and faces of the cube: 8 corners (1/8) + 6 faces (1/2) = 4.

In BCC unit cells, lattice points are located at the corners and center of the cube: 8 corners (1/8) + center (1) = 2.

In Primitive unit cells, lattice points are located at the corners of the cube: 8 corners (1/8) = 1.