Answer to Question #55650 in Physical Chemistry for Misha Jain

Question

In Na2O crystal, the coordination number of Na+ and O2- respectively are? And how?

Answer 1

The coordination number of Na+ is of 4. Since the charge of anion is in two times larger than for cation the coordination number of O2- must be 8. This occurs because the substance (Na2O) has to be neutral. The total charge should be zero.
In general, sodium cation can reveal the coordination number of 4 and 6 in its compounds. However, for the type A2B, it is found in the tetrahedral coordination environment (Coordination number is 4). This is determined by the ratio of radii of sodium cation and oxygen anion as well as their charges.
For instance, if sodium had the coordination number of 6, the oxygen would show the coordination number of 12. It is impossible because oxygen can only show the maximal number which equals 8.