Here's why:

* Atomic Number: Aluminum has an atomic number of 13, meaning it has 13 protons and 13 electrons.

* Electron Configuration: The electron configuration of aluminum is 1s² 2s² 2p⁶ 3s² 3p¹.

* Outermost Shell: The outermost shell is the third shell (n=3). This shell contains 3 electrons (2 in the 3s subshell and 1 in the 3p subshell).