Question

In sapphire, oxide ions are arranged in hexagonal close packing and aluminium ions occupy two-thirds of the octahedral voids. What is the formula of sapphire?

Answer 1

Correct Answer - `Al_(2)O_(3)`
Let number of anion `O^(2-) = 1//"atom"`
Number of `TV = 2// "atom"`
Number of `OV = 1//"atom"`
Number of cation `(Al^(3+)) = (2)/(3) xx 1 = (2)/(3)`
Formula: `Al^(3+)_(2//3)O^(2-)` or `Al_(2)O_(3)`.