The element carbon has 3 naturally occurring isotopes. About 99% of carbon isotopes are C-12, about 1% are C-13 and a tiny amount are C-14. What is the average atomic mass of carbon atoms?
More specifically, each stable isotope will contribute to the average atomic mass of the element proportionally to their abundance.
#color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i))#
In this case, you know that carbon has three naturally occurring isotopes, but that one of them,
That means that you can ignore its contribution to the average atomic mass of carbon. So, you're left with
Here decimal abundance simply means percent abundance divided by
So, the average atomic mass of carbon will be
#"avg. atomic mass" = "12 u" xx 0.99 + "13 u" xx 0.01#
#"avg. atomic mass " = " 12.01 u"#