Question

In an oxide of aluminium, oxide ions are arranged in ccp arranged and the aluminium ions occupy two third of the octahedral voids. What is the formula of the compound?

Answer 1

To determine the formula of an oxide of aluminium we are given that:

Oxide ions (O²⁻) are arranged in ccp (cubic close-packed) structure. 

Aluminium ions (Al³⁺) occupy two-thirds (2/3) of the octahedral voids.

In a ccp lattice, the number of octahedral voids is equal to the number of atoms in the lattice.

Let the number of oxide ions = 1

Then, Number of octahedral voids = 1

Aluminium ions of octahedral voids = `2/3`

Now we have,

Aluminium (Al) = `2/3`

Oxygen (O) = 1

To eliminate the fraction, multiply both by 3.

Al : O = `2/3 xx 3 : 1 xx 3`

= 2 : 3

∴ The formula of the given compound is Al₂O₃.