Here's why:

* Atomic Radius: Atomic radius refers to the size of an atom. Aluminum has a larger atomic radius than chlorine.

* Electron Configuration: Aluminum has three electrons in its outermost shell, while chlorine has seven. This means aluminum's outermost electrons are further from the nucleus, making the atom larger.

* Shielding Effect: The inner electrons in aluminum shield the outermost electrons from the nucleus's pull, which further increases the atomic radius.

In summary: Aluminum has a larger atomic radius because it has fewer electrons in its outermost shell and a greater shielding effect.