Question

What is the chemical formula of alumina?
A. ${ Al }_{ 2 }{ O }_{ 3 }$
B. ${ Al }_{ 2 }{ O }_{ 3 }.2{ H }_{ 2 }O$
C. ${ Al }_{ 2 }{ O }_{ 3 }.{ H }_{ 2 }O$
D. ${ NaAlO }_{ 2 }$

Answer 1

Hint: There is some charge in the aluminium and oxygen ions. Aluminium has a +x charge whereas oxygen has -y charge. On applying the criss-cross method we get the formula of alumina.

Complete step by step answer:

Alumina is the common name for the compound aluminium oxide.
First of all, we know that aluminium is a metal and Oxygen is a non-metal making it an ionic compound.
There is some charge in the aluminium and oxygen ions. Aluminium has a +3 charge whereas oxygen has -2 charge.
On applying the criss-cross method we get the formula of aluminium oxide. Usually, aluminium loses three electrons while oxygen gains two electrons in the chemical process and this is mainly done to achieve stability.
Since the overall charge of a compound must always be equal to zero (neutral), we need 2 aluminum atoms and 3 oxygen atoms in order to balance out the charge and make the compound neutral. This means that the chemical formula for aluminum oxide is simply ${ Al }_{ 2 }{ O }_{ 3 }$. That's 2 aluminum atoms for every 3 oxygen atoms.
Therefore, we can conclude that the correct answer is ${ Al }_{ 2 }{ O }_{ 3 }$.

So, the correct answer is option A.

Additional information:
Bauxite - Bauxite is a group of hydrated aluminum oxides. It can be written as ${ Al }_{ 2 }{ O }_{ 3 }.n{ H }_{ 2 }O$. Where n represents that it can have different numbers of water molecules per alumina based on concentration of mineral in the ore on an average. The option B and C are Bauxite.

Sodium aluminate - Option D is sodium aluminate. It is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications.
Pure sodium aluminate is a white crystalline solid having a formula variously given as ${ NaAlO }_{ 2 }$.

Note: Aluminium oxide is amphoteric. It has reactions as both a base and an acid. Aluminum metal rapidly develops a thin layer of aluminum oxide of a few millimeters that prevents the metal from reacting with water.