# 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

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

#### Explanation:

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

$27.9769 \times .9218 + 28.9765 \times .0471 + 29.9738 \times .0312$ (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:

$25.7891 + 1.3648 + 0.9352 = 28.0891$

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.