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:

#"Isotope"##color(white)(m)##"fractional abundance"##color(white)(m)##"Isotopic Mass "##color(white)(mm)##"Wt Avg"#
#"O-16"color(white)(mmmmm)##0.997620##color(white)(mmmmm)##15.9949##color(white)(mmmm)##color(white)(m)##15.95683#
#"O-17"color(white)(mmmmm)##0.000373##color(white)(mmmmm)##16.9991##color(white)(mmmm)##color(white)(m)##00.00634#
#"O-17"color(white)(mmmmm)##0.002000##color(white)(mmmmm)##17.9992##color(white)(mmmm)##color(white)(m)##00.03510#

#"Sum""of" "Wt" "Avg." "contributions"# = Atomic Mass (amu) =#color(white)(m)#15.9994

Atomic Mass Oxygen published on Periodic Table = 15.9994 amu