In hexogonal systems of crystals, a frequently encountered arrangement ofatoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexongonas and threeatoms are sandwiched inbetween them. A space filling model of this structure called haxagonal closed packed (HCP) is constituted of a sphere on a flat surface surrounded in the same plane by six identical spheres as closely as possible . Three spheres are then placed over the first layer so that they touch each other and represent the second layer. Finally, the second layer is covered with a third layer that is identical to the bottom layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be 'r'. The number of atoms in this HCP unit cell is

In hexogonal systems of crystals, a frequently encountered arrangement

1.	In hexogonal systems of crystals, a frequently encountered arrangement ofatoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexongonas and threeatoms are sandwiched inbetween them. A space filling model of this structure called haxagonal closed packed (HCP) is constituted of a sphere on a flat surface surrounded in the same plane by six identical spheres as closely as possible . Three spheres are then placed over the first layer so that they touch each other and represent the second layer. Finally, the second layer is covered with a third layer that is identical to the bottom layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be 'r'. The number of atoms in this HCP unit cell is
Answer» 4 6 12 17 Solution :According to the given DESCRIPTION, the ARRANGEMENT of atoms in HCP will be as shownFig. 1.61. Effective number of atoms present in the unit CELL of HCP `12 xx1/6 + 3 + 2 xx 1/2 = 6`