Assign an oxidation number to each atom in the reactants. 4Al(s)+3O2(g)→2Al2O3(s)

(Oxidation number for both Al and O is needed)

Al⁺³ , O⁻²

Explanation:

This is an oxidation reduction reaction (Redox).

Chemical reaction:

4Al + 3O₂   →    2Al₂O₃

The oxidation state on Al and O on left hand side is zero. On right hand side Al is present in +3 oxidation state while O is present in -2 oxidation sate.

Oxygen is reduced and Al is oxidized in this reaction.

The oxidation reduction reactions are called redox reaction. These reactions are take place by gaining or losing the electrons and oxidation state of elements are changed.

Oxidation:

Oxidation involve the removal of electrons and oxidation state of atom of an element is increased.

Reduction:

Reduction involve the gain of electron and oxidation number is decreased.

since electrons are negative the answer is neagtaive