The atomic mass is the weighted-average mass of all known isotopes for a particular atom. (The greatest contribution is due to the most abundant isotopes.)
For example, carbon has an atomic mass of about #12.011 "g/mol"#. This is determined by noting that there are isotopes for #""^8C# through #""^22C# (see the wikipedia entry).
With #M_i# indicating isotopic mass and #M_a# indicating atomic mass in #"g/mol"#, and the most abundant isotopes being #""^12C# and #""^13C#:
#M_(i,""^12C) = 12#
#M_(i,""^13C) = 13.0033548378_(10)# (subscript digits are most uncertain)
The isotopic abundances of these are #~~ 98.93%# and #~~1.07%#, respectively, so the atomic mass is calculated as:
(With #A_i =# isotopic, or natural abundance)
#M_(a,C) = sum_"isotopes" (A_(i,""^nC)*M_(i,""^nC))#
#= (A_(i,""^12C)*M_(i,""^12C) )+ (A_(i,""^13C)*M_(i,""^13C))#
#= (0.9893*12 "g/mol") + (0.0107*13.0033548378_(10) "g/mol")#
#~~ 12.010735897 "g/mol" => color(blue)(12.011 "g/mol")#
