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

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Hint:We know that this question is taken from the chapter of solid state. And coordination numbers we can find by counting the number of atoms attached with a single atom.

Complete step by step solution:
> The number of atoms nearest to an atom in the crystalline structure is called the coordination number. Coordination number, also called Ligancy, the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal.
> The term "closest packed structures" refers to the most tightly packed or space-efficient composition of crystal structures (lattices).
- A unit cell is the smallest representation of an entire crystal. All crystal lattices are built of repeating unit cells.
- The face-centered cubic (fcc) has a coordination number of 12 and contains 4 atoms per unit cell. Let’s take an atom at the corner, this corner atom is attached with 6 other atoms of the corner and 6 atoms of face.

Note: The packing of spheres can describe the solid structures of crystals. In a crystal structure, the centers of atoms, ions, or molecules lie on the lattice points.