With only 76% of the volume of the close-packed unit cell occupied by spheres, significant space remains unoccupied.Many solid lattices containing two atoms (or a cation and an anion) have a close-packed arrangement of one with the other occupying the spaces between the 'close-packed' atoms. Alloys in which two different metals are present are one example of this.
For example, the image at the right shows that there is a space at the centre of each edge of the unit cell. Those on the edges of the front face are quite obvious.
The four spaces behind the corner spheres of the front face can be seen more clearly if you pass your mouse over the image. These spaces are at the centres of edges around the top and bottom of the unit cell. A space of the same type at the center of the unit cell is also revealed.
In the next few questions, we will explore the characteristics of this type of space.