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How will you distinguish between the following pairs of terms: (i) Hexagonal close-packing and cubic close-packing?

1.7   How will you distinguish between the following pairs of terms:

              (i) Hexagonal close-packing and cubic close-packing?

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(i) Hexagonal close-packing:- In this, tetrahedral voids of the second layer are covered by the spheres of the third layer. In this case, the atoms (spheres) of the third layer are aligned with those of the first layer. This results in the pattern of spheres to be repeated in alternate layers. We get a pattern as ABAB ....... . This structure is known as the hexagonal close-packed (hcp) structure.

(ii) Cubic close-packing:-  In this case, the third layer is placed above the second layer such that its spheres cover the octahedral voids. In this manner, the atoms (spheres) of the third layer are not aligned with those of either the first or the second layer. This pattern of layers is written as ABCABC ........... . This structure is called cubic close-packed (ccp)

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