Calculating Average Atomic Mass from Isotopic Data
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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.
Forgetting to convert percentages into decimal fractions before multiplying inflates the result by a factor of 100; the fractions used in the sum must always add to 1.00, not 100.