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**Hint :**Molecular formula is a chemical formula that helps us to find the total number of atoms in every element of each molecule of a substance. It will give you an exact number of each atom in a molecule. It has chemical symbols of the constituent elements and numerical subscripts which describe the number of atoms of each element present in a particular molecule.

**Complete Step By Step Answer:**

To write the molecular formula of aluminium oxide we must know the valency of both aluminium and oxygen.

Valency- the combining capacity of an atom or the maximum number of electrons that an atom can gain or lose to stabilize itself i.e. to achieve an octet state.

The valency of aluminium$(Al)$ is $3$and the valency of oxide is nothing but valency of oxygen$(O)$ which is$2$.

So now for finding the molecular formula after finding the valency we will write the symbol of element and their respective valency in fraction form, as shown below for aluminium and oxygen:

$\dfrac{{Al}}{3}$ and $\dfrac{O}{2}$

After writing them in fraction form we will just cross multiply, as shown below:

$\dfrac{{Al}}{3} = \dfrac{O}{2}$

Simply cross multiply the valencies i.e. multiply $Al$with $2$and $O$with $3$ and write them as the subscript of element/ion, alongside the symbol of the element.

Hence in case of aluminium oxide, we cross multiply the valencies and finally, we get:

$A{l_2}{O_3}$

(Where the subscripts denote the number of that particular element/ion)

Therefore the molecular formula of aluminum oxide is$A{l_2}{O_3}$.

**Note :**

Molecular formulas of any molecule can be found out by this method. The things that one must know are the symbols as well as the valencies of those elements/ions and. And then just interchange their valency. Sometimes in question, the weight of an element might be given, for finding the molecular formula of those compounds different formulas are used and for that empirical formula and molar mass need to be calculated.

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