Final answer:
If two atoms of aluminum have different numbers of neutrons, they are isotopes and will have different atomic masses, while their chemical properties remain unchanged.
Step-by-step explanation:
If two atoms of aluminum have different numbers of neutrons, the primary property that will be different between the two is their atomic mass. Atoms that have the same number of protons but different numbers of neutrons are known as isotopes. Although isotopes have different mass numbers, they exhibit the same chemical properties because they have the same number of protons and electrons. For example, Aluminum-27 and Aluminum-26 are isotopes of aluminum, both having 13 protons but differing in neutron count, which gives them atomic masses of 27 and 26, respectively.