The ion formula of aluminum fluoride is related to chemical principles. For aluminum, the common valence is positive trivalent; for fluoride, the common valence is negative monovalent.

In compounds, the algebra sum of the valences of each element is zero. To obtain the ion formula of aluminum fluoride, it is necessary to balance the valences of aluminum and fluoride. The combination of aluminum ion $Al ^ {3 +} $and fluoride ion $F ^{ - }$, in order to achieve the valence algebra sum of zero, three fluoride ions need to be combined with one aluminum ion.

Therefore, the ionic formula of aluminum fluoride is $AlF_ {3} $. This formula accurately expresses the proportional relationship between aluminum and fluorine in compounds, which is a crucial foundation for chemical research and application.