Coordination number

One characteristic of a metal lattice is the coordination number for an individual metal atom.

The coordination number of an atom equals the number of other atoms that the metal atom would touch if the lattice were extended infinitely in all directions.

In determining the coordination number by counting the number of atoms touched in a picture of a lattice, remember that pictures only show a very small portion of the lattice, and it may be necessary imagine the other spheres that would be present if the lattice were extended.

The atoms in the diagram shown are all the same, but some have been shaded for illustration purposes.

Note the darker atom at the right touches four atoms in the front face of the lattice shown, BUT it does NOT have a coordination number of 4 because it also touches atoms behind it in the picture and other atoms that would be added at the front to extend the lattice.
 

The coordination number depends on the density of atoms (number of atoms per unit volume) in the lattice.  Atoms in more dense metallic lattices have higher coordination numbers.