Question

What is the formula of aluminum carbonate?

Answer 1

Aluminium carbonate is an ionic compound consisting of elements namely aluminium, carbon and oxygen. The elements like carbon and oxygen combine covalently to give an anionic molecule with two negative charges called carbonate.

Complete answer:
The chemical formula of any ionic compound is written by writing the symbol of the cation followed by the anionic species. The naming of the compounds is done in a similar manner where the electropositive species (usually the metal) is named first and then the anionic species is named.
Aluminium is an element belonging to the thirteenth group of the periodic table and is a metal due to its electropositive character. It has a tendency to form tri-positive cations and therefore its valency is three. The carbonate molecules are actually formed from the bonding of carbon and oxygen atoms that leads to the formation of the carbonate anion that has a negative charge of magnitude two.
The formula can be written using a criss-cross method. Where the valency of the cation is used as the stoichiometric number of the anion and that of anion is used as the stoichiometric number of of the cation.
$\Rightarrow$ Therefore two aluminum cations react with three carbonate ions to give aluminum carbonate that can be represented by the formula $A{l_2}{(CO)_3}$ .

Note:
The criss-cross method of involving valencies is only useful for the formation of ionic compounds and not molecular or covalent compounds. It is actually the charge on each atom that needs to be balanced so as to have an overall neutral molecule.