Question

What is the coordination number of sodium in $N{a_2}O$?

Answer 1

Coordination number is defined as the number of the nearest neighboring touching atom to the central metal atom and it is also known as a ligancy. We have a difference in the method of determination of the coordination number of molecules and crystals.

Complete step by step answer:
The structure $N{a_2}O$ is ${A_2}B$ type and it is similar to fluorite structure except that the position of the cations and anions are reversed. Hence the compounds having ${A_2}B$ type structure are said to possess an antifluorite structure. In the case of $N{a_2}O$, $O_2^ -$ ions have a cubic close-packed arrangement and $N{a^ + }$ ions occupy all the tetrahedral voids also in this structure, each $O_2^ -$ ion is surrounded by eight $N{a^ + }$ ions and each $N{a^ + }$ ion is surrounded by four $O_2^ -$ ions. Hence the coordination number of $N{a_2}O$ is in ratio 4:8.
Hence the coordination number of sodium in $N{a_2}O$ is 4.
There exist different oxides and sulphides of alkali metals such as ${\text{L}}{{\text{i}}_{\text{2}}}{\text{O, }}{{\text{K}}_{\text{2}}}{\text{O, R}}{{\text{b}}_{\text{2}}}{\text{O and N}}{{\text{a}}_{\text{2}}}{\text{S}}$ that have antifluorite structure.

Note:
The coordination number of the central atom determines the geometric shapes of the compound and the possible shapes are:
If the coordination number is two then the shape of the molecule would be Linear.
For the case of coordination number three, the shape would be Trigonal planar, Trigonal pyramidal.
The modules with coordination number four on the central atom have a square planar shape.
If the coordination number is five then we get the structures with trigonal bipyramidal shape.
The compounds with coordination number six have an octahedral shape
The compounds having coordination number eight on the central atom have hexagonal bipyramidal shape.