Question

What is meant by the term coordination number in solids? What is the coordination number in a face centered cubic close packing structure?

Answer 1

Co-ordination number is the number of spheres (atoms, molecules or ions) directly surrounding a single sphere in a crystal.
The coordination number in a face centered cubic close packing structure is 12. Each sphere in three dimensional arrangement has contact with six spheres in its own layer and three spheres each in the lower layer and in the upper layer. Thus, the nearest neighbours for any sphere becomes 12.