Chemistry / Unit 2 Chem 0238 worked example, 60 to 90 seconds
Calculating Average Atomic Mass from Isotopic Data
The idea
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.
In practice
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.
Where people slip
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.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Something wrong in this lesson? Flag it · Sources for Chemistry