The unit of atomic mass for aluminum, or any element, is the atomic mass unit (amu).

It's important to note that:

* amu is a relative unit, meaning it's based on a comparison to a standard. One amu is defined as 1/12 the mass of a carbon-12 atom.

* The atomic mass of aluminum is 26.9815386 amu. This value represents the average mass of all naturally occurring isotopes of aluminum.