Important Board Question- Chemistry

 Important Solved Question- Chemistry

Question: The pattern of successive layers of ccp arrangement can be designated as:

(        (A) AB AB AB

   (B) AB ABC AB ABC

   (C) ABC ABC ABC 

   (D) AB BA AB BA

Solution: Hint:-‘ccp’ arrangement stands for cubic closed packing arrangement. The arrangement in a cubic closed packing is such that its packing efficiency is 74%. For such type of problems you need to high imagination skills in three dimensions. But the best approach can be try to think of all layers individually and then try to figure out the pattern. For example – simple cubic lattice formed by AAA…. Arrangement.

Step-by-step answer:

•               The ccp arrangement is obtained when third layer of a structure is placed over the second layer. 
•                Now when third layer is placed over the second, there are only two possibilities.
•     First possibility by covering tetrahedral voids and the second by covering octahedral voids.         First we will understand about tetrahedral voids and octahedral voids. So tetrahedral voids are formed when sphere of second layer is above the void of first layer. Octahedral voids are present at edge Centre and body Centre.
•                When third layer is placed over the second by covering tetrahedral voids then in this possibility sphere of third layer will be symmetric with first layer and if we follow the pattern the spheres of second layer will be aligned with fourth layer and so on. So, now we can conclude that pattern of spheres is repeated in alternate layers. The pattern can be written as ABAB……..  . This pattern is called hexagonal packed structure (hcp).
•               Now when third layer is placed over second layer such that it covers the octahedral voids. In this manner third layer are not aligned with either of the first or second. So if we consider first layer as A, second layer as B and the third layer formed as C and so on. The pattern designated will be ABCABCABC……
•               Final Result: The pattern of successive layers of ccp arrangement can be designated as ABC ABC ABC.
•                       Correct option is (C).

Note:

•       There is very important point to remember in ABC ABC ABC ….....pattern and that is if spheres
•     of third layer are placed into tetrahedral voids, the packing is hcp (hexagonal closed packing).
•        ccp is cubic closed packing, it is also called fcc(face centered cubic).
•        In fcc(face centered cubic), spheres of third layer are placed into octahedral voids.
•        The number of these two types of voids depend upon the number of close packed spheres.
•         Let the number of closed packed spheres be N, then:
•     The number of octahedral voids generated will be N.
•     The number of tetrahedral voids generated will be 2N.