In hexagonal systems of crystals, a frequently encounteredarrangement of atoms is described as a hexagonal prism. Here, thetop
and bottom of the cell are regular hexagons and three atoms aresandwiched in between them. A space-filling model of this
structure, called hexagonal close-packed (HCP), is constitutedof a sphere on a flat surface surrounded in the same plane bysix
identical spheres as closely as possible. Three spheres are thenplaced over the first layer so that they touch each other and
represent the second layer. Each one of these three spherestouches three spheres of the bottom layer. Finally, the secondlayer is
covered with a third layer that is identical to the bottom layerin relative position. Assumer radius of every sphere to be