Isotopes are atoms of the same element (same atomic number) with different atomic masses. Atomic masses listed on the Periodic Table are average atomic masses that take into account all of the isotopes for that element found on Earth.
Percent abundance tells us the percent of atoms with that specific mass found on Earth. The higher the percent abundance, the more predominant that isotope is and the closer the average atomic mass will be to its value.
Let's now look at the formula for the average atomic mass.
Average atomic mass = Mass of isotope 1 * Percents Abundance of isotope 1 (in a decimal form) + Mass of isotope 2 * Percents Abundance of isotope 2+ ....
To calculate the average atomic mass, we simply change percent to decimal form by dividing by 100 (move the period 2 places to the left). Then we multiply each isotope's mass by its percent abundance and add all of them up.
Example: Calculate the average atomic mass of silicon, given the following data:
Average atomic mass = 27.97693amu*.9223 + 28.97649amu*0.0467 + 29.97376amu*0.0310
Average atomic mass = 28.1 amu