The atomic mass of an element is the average relative mass of its atoms as compared with an atom of carbon-12 whose mass is taken as 12. In terms of percentages of different isotopes, the average atomic mass can be determined as follows: [tex]A_{average}[/tex] = ∑([tex]p_{i}[/tex]X [tex]A_{i}[/tex])/100. Where, [tex]p_{i}[/tex] is the percentage abundance of isotope with atomic number [tex]A_{i}[/tex].
So, average atomic mass of neon = [tex]\frac{(90.51 X 19.99) + (0.27 X 20.99) + (9.22 X 21.99)}{100}[/tex] = 20.177 amu.
