What is the standard and easy definition of atomic mass?

The is the weighted-average mass of all known 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")##