Here's how to break down the formula and determine the number of atoms:

* Aluminum (Al): The subscript "2" indicates there are 2 aluminum atoms.

* Chromium (Cr): The subscript "2" indicates there are 2 chromium atoms.

* Oxygen (O): The subscript "7" indicates there are 7 oxygen atoms.

Total: 2 + 2 + 7 = 11 atoms

Therefore, one formula unit of aluminum dichromate (Al₂Cr₂O₇) represents 11 atoms.