How does the atomic mass change depending on the isotope?
Because isotopes reflect changes in the number of neutrons, they have higher or lower atomic masses depending on the isotopes you are comparing.
Atomic masses on the periodic table are the number of grams per mole of a substance. Isotopes are atoms that have the same number of protons, but different numbers of neutrons. Moles are just a specific number of atoms, so changing the number of neutrons will increase or decrease the atomic mass depending on isotopes you are comparing.