In corrundum, oxid ions are arranged in `h.c.p.` array and the aluminum ions occupy two`-`thirds of octahedral voids. What is the formula of currundum ?

Answer

`Al_2O_3`

Explanation

In hcp arrangement , there is one octahedral void corresponding to each atom in the close packing.
Since `2/3` of the octahedral voids are occupied by aluminimum ions, therefore, for each oxide ion there will be `2/3` aluminium ions.
Therefore, ratio between aluminium ion nad oxide ion is :
`2/3 : 1 " or " 2 : 3`
Formula of corrundum = `Al_2O_3`.