Here's why:

* Valence electrons: Aluminum has 3 valence electrons, which are the electrons in its outermost shell.

* Tendency to lose electrons: Metals tend to lose electrons to achieve a stable electron configuration. Aluminum will lose its 3 valence electrons to become a positively charged ion (Al³⁺).

* Oxidation number: The oxidation number represents the charge an atom would have if all its bonds were ionic. Since aluminum loses 3 electrons, its oxidation number is +3.