Identify the isotopes of the element
Determine the total atomic mass from the periodic table
Set up an equation using the weighted average of isotopic masses
Let the natural abundance of one isotope be x and the other be 1 – x
Multiply each isotope’s mass by its fractional abundance
Add the products together to equal the average atomic mass
Solve the equation for x
Convert the fractional abundance to a percentage by multiplying by 100
Repeat for each isotope if more than two isotopes are present