Question

What is the coordination number of Sodium in Sodium Oxide?

Answer 1

Coordination number:

• The number of particles that surround a single particle in a crystal lattice is known as the coordination number of that particle.

Sodium oxide:

• Sodium oxide $\left({\mathrm{Na}}_2\mathrm{O}\right)$is an example of Face Centred Cubic crystal structure.
• The coordination number of both the ions in a compound may or may not be the same.

The coordination number of sodium in sodium-ion:

• Sodium-ion takes up the tetrahedral void in the lattice and oxide ion takes up all the corners of the FCC.
• In sodium oxide ${\mathrm{Na}}_2\mathrm{O}$, ${\mathrm{Na}}^{+}$ ions are present at tetrahedral voids and ${\mathrm{O}}^{2-}$ ions occupy half of the cubic holes in the face cubic close lattice. ${\mathrm{Na}}^{+}$ions are present at body diagonals.
• One corner sphere and three face spheres combine to produce a tetrahedral void in an FCC structure. Two tetrahedral voids and one cube diagonal are obtained in an FCC structure. One unit cell contains a total of four cube diagonals. As a result, an FCC structure has eight tetrahedral voids overall.
• Each sodium ion is surrounded by $4$ oxide ions and each oxide ion is surrounded by $8$ sodium ions.
• Thus, the number of sodium ions in one unit cell (FCC) = $8\mathrm{x}1=8$
• Number of oxide ions in one unit cell (FCC) = $\left(8\mathrm{x}1/8\right)+(6\mathrm{x}1/2)=4$, as there are $8$ corners and $6$ faces

Thus, as one sodium ion is surrounded by $4$ oxide ions, the coordination number of sodium in Sodium oxide is $4$.