What is the average atomic mass of silicon?

Given the following data, calculate the average atomic mass of silicon.

isotope = Si-28
amu =27.9769
abundance(%) =92.18

isotope = Si-29
amu =28.9765
abundance(%) =4.71

isotope =Si-30
amu =29.9738
abundance(%) =3.12

1 Answer


Multiply the amu by the percentage of occurrence to arrive at an average atomic mass of #28.0891#


We take the amu of each isotope, multiply it by the percentage of occurrence, and end up with a weighted average:

#27.9769xx.9218+28.9765xx.0471+29.9738xx.0312# (quick note - there is a bit of rounding in here - the abundance percentages add up to 1.01, so the number we calculate is going to be slightly off).

This simplifies to:


Checking "the internet" to verify our answer, I found 28.0855 u ± 0.0003 u, so the answer calculated for our question is pretty close.