# How do isotopes affect the atomic mass of an element?

By way of example we could look to the hydrogen atom. The VAST majority of hydrogen atoms (in this universe) are the protium isotope. i.e. ""^1H, whose nuclei contain JUST the defining proton. There is a smaller percentage (>1%) of hydrogen atoms WITH one NEUTRON in their nuclei to give the deuterium isotope. i.e. ""^2H, and because this is relatively cheap, and easily incorporated into a molecule, deuterium labelling is routinely used in analysis.
And there is even a smaller percentage of hydrogen atoms with TWO NEUTRONS in their nuclei, to give the tritium isotope. i.e. ""^3H. The weighted average of the isotopic percentages gives $1.00794 \cdot g \cdot m o {l}^{-} 1$...the mass printed on the Periodic Table.