Calculating Average Atomic Mass from Isotopic Data

Calculating an element's average atomic mass means multiplying each isotope's mass by its fractional abundance, then summing those products across every naturally occurring isotope.

Neon occurs as three isotopes: 91.84% neon-20 at 19.9924 u, 0.47% neon-21 at 20.9940 u, and 7.69% neon-22 at 21.9914 u. Converting each percentage to a decimal fraction and multiplying by its isotope's mass gives three products: 0.9184 × 19.9924 ≈ 18.361, 0.0047 × 20.9940 ≈ 0.099, and 0.0769 × 21.9914 ≈ 1.691. Adding the three products gives an average atomic mass of about 20.15 u for this particular sample.