When calculating the average atomic masses of elements, the isotopes found on Earth are generally taken into consideration. The atomic mass listed on the periodic table is an average value that considers the relative abundance of each isotope of an element found on Earth.
However, it's important to note that the atomic masses listed on the periodic table are average values and can vary slightly depending on the natural abundance of isotopes. Isotopes with higher natural abundances contribute more to the average atomic mass of an element, while isotopes with lower abundances have less impact.
Regarding your specific example, the molar mass of aluminum is approximately 27 grams per mole (g/mol), which is an average value. Aluminum has a single stable isotope, aluminum-27 (^27Al), which is the most abundant and makes up the majority of naturally occurring aluminum on Earth. Although there may be other isotopes of aluminum, such as aluminum-26 (^26Al), which may be more common in stellar environments, they exist in much lower abundances on Earth and therefore have less influence on the average atomic mass.
So, in short, the average atomic masses listed on the periodic table are primarily based on the isotopic composition found on Earth and do not necessarily reflect the isotopic ratios observed in stellar or astronomical contexts.