Aluminium is protected from rust by a layer of oxidised Al2O3. Can you write a balanced equation for the formation of this layer?

To form Al2O3 you need both Al and O2 (oxygen gas is a dimer):

Al + O2 -> Al2O3

But this equation is not balanced. To balance it, first equate the number of oxygen atoms:

Al + 3O2 -> 2Al2O3

Now you have 6 oxygens on each side, but the Al is not balanced so you must add Al on the left hand side.

4Al + 3O2 -> 2Al2O3

And its balanced!