Here's why:

* Al represents the element aluminum.

* S represents the element sulfur.

* The subscript "2" after Al indicates there are two aluminum atoms in the compound.

* The subscript "3" after S indicates there are three sulfur atoms in the compound.

Since aluminum is a metal and sulfur is a nonmetal, the compound is an ionic compound. In naming ionic compounds, the metal is named first, followed by the nonmetal with the ending "-ide".