Hexagonal close packed arrangement of ions is described as

Packing fraction of a layer in $2D$ hexagonal close packing arrangement is:

$A$ compound made up of elements $A$ and $B$ (with a general formula $A_x B_y$),where $B$ forms an $hcp$ lattice and $A$ occupies $2/3$ of the tetrahedral voids. The formula of the compound is

$A$ two-dimensional solid pattern formed by two different atoms $X$ and $Y$ is shown below. The black and white squares represent atoms $X$ and $Y,$ respectively. The simplest formula for the compound based on the unit cell from the pattern is

If a compound (oxide) has an $hcp$ arrangement and the cations $M$ occupy two-thirds of the octahedral voids,the formula of the compound is:

The relationship between the radius $(r)$ of the atom and the edge length $(a)$ of the cubic close packing $(CCP)$ structure is: