* Identify the polyatomic ion: The polyatomic ion in this compound is nitrate (NO₃⁻).
* Count the oxygen atoms in the polyatomic ion: There are three oxygen atoms in each nitrate ion.
* Consider the subscript outside the parentheses: The subscript "3" outside the parentheses means there are three nitrate ions in the formula unit.
* Calculate the total number of oxygen atoms: 3 oxygen atoms per nitrate ion * 3 nitrate ions = 9 oxygen atoms
Therefore, there are 9 oxygen atoms in one formula unit of Al(NO₃)₃.
