Question

What is the coordination number of sodium in sodium oxide?

Answer 1

The coordination number of a particular atom cannot be determined by simply looking at the number of counter ions it is attached to in its molecular formula. An ionic compound like sodium oxide has a well-defined lattice that can be used for finding out the coordination number.

Complete answer:
Coordination number is the measure of the number of atoms that are directly attached to or in direct contact of a particular atom in its three-dimensional crystal lattice.
The formula of sodium oxide $(N{a_2}O)$ is only based upon the respective valency or charges of sodium and oxide ions. It does not give us any relevant information about the actual spatial arrangement of sodium and oxide ions in the lattice.
Like all other ionic compounds sodium oxide also exists in a crystalline lattice that follows a uniform order.
Sodium oxide is known to exist in an antifluorite type of crystal structure which gives a face centered cubic lattice.
A close look into its lattice suggests that the sodium ions is surrounded by four other atoms directly bonded to it.
Hence the coordination number of sodium ions is four in sodium oxide.

Note:
The coordination number of both the ions in the ionic lattice need not be the same. In sodium oxide, the sodium ions have a coordination number of four and the oxide ions have a coordination number of eight. Hence its coordination ratio is $4:8$ which is the opposite of what is found in compounds having a fluorite-like structure.