The atomic mass on your Periodic Table is an average of
the known istopes making up that element. This average
is known as a weighted average, since it includes the
abundance of each isotope. The most abundant isotope
contributes the most 'weight' to the average:
ex C-12 is the most abundant isotope of carbon;
O-16 is the most abundant isotope of oxygen.

How to determine the average atomic mass?

When finding the average atomic mass
I must bring along my tools and come to class
Each decimal times the isotope mass
Then taking my total will help me to pass
I must take care with what I decide
I multiply and add but never divide

Weighted Average Atomic Mass
I can cal-cu-late in Chemistry class