How is the oxidation of aluminum metal represented?
At first we have
Let's first balance
Now to balance
If the non-stoichiometric coefficients offend thine sensibilities.....then DOUBLE the entire equation......
In either instance, 2 equiv of aluminum metal are oxidized by 3 equiv dioxygen gas to give one equiv alumina.
Note that with regard to dioxygen gas, ALL of the elemental gases (save for the Noble Gases) are diatomic, e.g.