How would you find atomic mass?
Isotopes of elements are a set of elements all having the same atomic number (number of protons) but different atomic mass (number of neutrons). Typically a mixture of isotopes is separated via mass spectroscopy and characterized as to %Abundance and Isotopic mass. The atomic mass is then determined by multiplying fractional abundance times isotopic mass (atomic mass units-amu) giving the weighted average of the isotopes of the mix.
For example, for this example isotopes of oxygen unclude Oxygen -16, Oxygen - 17 & Oxygen -18. Arranging these in a tabular format one gets the following:
Atomic Mass Oxygen published on Periodic Table = 15.9994 amu