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QUESTION

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")