This video explains Close packed structures.
Two dimensional packing can be arranged in two different ways.
(a) Arrange the second layer of spheres in contact with first layer such that each sphere is in contact with four other spheres. (Sphere is aligned horizontally as well as vertically). So the co-ordination number is 4. If the first row is of ‘A’ type and second row is exactly the same as first then this type of arrangement is called ‘AAA’ type of arrangement or square packing.

(b) Arrange the second layer of spheres in contact with first layer such that each sphere is in contact with 6 other spheres (second row spheres fit in the depression of first row spheres). So the co-ordination number is 6. If the first row is of ‘A’ type and second row is of ‘B’ type then this type of packing is called ‘ABAB’ type of packing or Hexagonal packing. The empty space is less when compared to square packing.

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