Question

Which of the following crystals have 6:6 coordination?
(A) \[N{H_4}I\]
(B) MgO
(C) MnO
(D) ZnS

Answer 1

Hint: Try to recall that in NaCl structure, chloride ions have cubic close packing (ccp or fcc) and sodium ions occupy all octahedral sites. Now, by using this you can easily find out the correct option from the given ones..

Complete step by step solution:
- It is known to you that the chloride ions have cubic close packing and all sodium ions occupy all octahedral sites in the cubic unit cell.
- The coordination number of both cation and anion in NaCl structure is 6 and the total number of both cation and anion in NaCl crystal is 4.So, NaCl has 6:6 coordination type.
- Each sodium ion has 6 chloride ions as nearest neighbours, and each chloride ion has 6 sodium ions as nearest neighbours. Also, each sodium ion has 12 sodium ion as second nearest neighbours and each chloride ion has 12 chloride ions as second nearest neighbours.
- This structure is also exhibited by Li, Na, K halides, AgCl, MgO, CaO.
Also, ZnS has 4:4 coordination type and it has zinc ions in alternate tetrahedral voids and all chloride ions form cubic close packing.
-\[N{H_4}I\] shows 8:8 coordination type and MnO shows

Therefore, from above we can conclude that option B is the correct option to the given question.

Note:
- It should be remembered that on applying high pressure coordination numbers increase and NaCl type structure changes to CsCl type.
- Also, you should remember that on heating to about 760 K, the CsCl structure transforms to the NaCl structure.