In corundum, oxide ions are arranged in hexagonal close packing and aluminium ionsa occpy tow-third of the octaheral voids. What is the formula of corrundum ? .

Explanation

Let the number of oxide ions in the packing be N.
Then Octahedral viods = N.
`because " " Al^(3+)" ions" = (2)/(3) xx N = (2N)/(3)`
`therefore ` Ratio of `Al^(3+) : O^(2-) =(2N)/(3) : N = 2: 3`
Hence, formula of corrudum is `Al_(2)O_(3)`