Here's why:
* Fluorine (F) is always in the -1 oxidation state when it forms ionic compounds.
* Aluminum (Al) is a metal, and metals typically have positive oxidation states.
* The overall charge of a neutral compound is zero.
To balance the charges in AlF₃:
* Three fluorine atoms (3 x -1 = -3)
* One aluminum atom (+3)
Therefore, the oxidation state of aluminum in aluminum fluoride is +3.
