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SOLID STATE - CLOSE PACKED STRUCTURES (GRADE 12)


Close Packed Structures

In solids, the constituent particles are close-packed, leaving the minimum vacant space.

(a) Close Packing in One Dimension

In close packing one dimension, spheres are arranged in a row such that adjacent atoms are in contact with each other. 


(b) Close packing in Two Dimensions

In two-dimensional close packing, a row of closed packed spheres is stacked to obtain a two-dimensional pattern.

This stacking is done in two ways:

Square close packing and Hexagonal close packing

 Square close packing or AAA... Type arrangement- The spheres of second row are exactly above those of the first row. The second row is exactly same as the first one. If we label the first row as A, the second row is also A. Similarly, we may place more rows to get AAA… type arrangement. In this arrangement, the spheres are found to form squares. This type of packing is known as square close packing in two dimensions.




Hexagonal close-packing or ABABA… Type arrangement- The spheres in the second row are placed in the depressions between the spheres of the first row. The spheres in the third row are placed in the depressions between the spheres of the second row and so on. In this arrangement, the first row is exactly same as the third one and the second row is exactly same as the fourth one. 




If we label the first row as A, the second row is different and may be called B. The spheres of third row is same as that of first row, and therefore, it is also called A. Similarly, fourth row is similar to second row and therefore, it is also called B. Hence the arrangement is called ABAB… type.   

In this arrangement, the spheres are found to form hexagon. This type of packing is known as hexagonal close packing in two dimensions.


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