Question

What is the correct formula for aluminium oxide?
(A) $Al{O_2}$
(B) $A{l_2}{O_2}$
(C) $Al{O_{}}$
(D) $A{l_2}{O_3}$

Answer 1

Aluminium oxide is a chemical compound containing aluminium and oxygen. It is commonly called alumina. The most natural and commonly existing form of Aluminium oxide is corundum.The molecular formula of the compound can be calculated by knowing the valencies of elements.

Complete step by step answer:
Bauxite is the ore of aluminium. Aluminium hydroxide is a main component of bauxite. Bauxite is purified by Bayer’s process as :
$A{l_2}{O_3} + {H_2}O + NaOH \to NaAl{(OH)_4} \\\ Al{(OH)_3} + NaOH \to NaAl{(OH)_4} \\\$
On cooling,
$NaAl{(OH)_4} \to Al{(OH)_3} + NaOH$
Solid aluminium hydroxide is then calculated to give aluminium oxide.
$2Al{(OH)_3} \to A{l_2}{O_3} + 3{H_2}O$
Hence, molecular formula of aluminium oxide is $A{l_2}{O_3}$
The molecular formula of Aluminium oxide can also be determined by Criss-crossing their valencies as shown below:
$A{l_ \nwarrow }_{ - 2}{}_{{}_{ + 3} \nearrow }O = A{l_2}{O_3}$
Aluminium has a positive valency of $3$ , whereas oxygen has a negative valency of $2$. On crisscrossing the $2$ of oxygen atom is written as a subscript with aluminium. In the same way, the $3$ of Aluminium is written with oxygen as a subscript. Hence, giving the molecular formula of the compound.

Hence, the correct option is D $A{l_2}{O_3}$ .

Note:
Aluminium oxide occurs naturally in crystalline polymorphic phase $\alpha - A{l_2}{O_3}$ as corundum. Varieties of corundum give rise to the precious gemstones ruby and sapphire. Aluminium oxide is used upto $90\%$ to produce aluminium metal, as an abrasive as it is hard and as a refractory material because of its high melting point.The positive and negative valency signs does not have any effect on the molecular formula of the compound, only the number of valency is considered while assigning a molecular formula to any compound.