In hexagonal system of crystals, a frequently 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 sandwiched in between them. A space-filling model of this structure,called hexagonal closed packed, is constituted of a sphere on a flat surface surrounded in the same plane by 6 identical spheres as closely as possible.three spheres are placed over the first layer so that they touch each other and touch the second layer. Each one of the three spheres touch the three spheres of the bottom layer. Finally the second layer is covered with a third layer identical to bottom layer in relative position. Assume the radius of every sphere to be 'r'. find the number of atoms in hcp unit cell?