In hexagonal systems of crystals , a frequenctly encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are regular hexagons and three atoms are sandwiched inbetween them. A space filling model of this structure called hexagonal 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 in relative position . Assume radius of every sphere to be 'r'. The number of atoms in this HCPunit cell is

In hexagonal systems of crystals , a frequenctly encountered arrangeme

1.	In hexagonal systems of crystals , a frequenctly encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are regular hexagons and three atoms are sandwiched inbetween them. A space filling model of this structure called hexagonal 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 in relative position . Assume radius of every sphere to be 'r'. The number of atoms in this HCPunit cell is
Answer» 4 6 12 17 Solution :According to the given descripition, the ARRANGEMENT of ATOMS in HCP will. EFFECTIVE number of atoms PRESENT in the UNIT cell of HCP `=12xx1/6+3+2xx1/2`=6