# What is the average atomic mass of silicon?

## The isotopic distribution of the silicon element is 92.2% ""^28Si $\left(\text{mass of the isotope is 27.968 amu}\right)$, 3.1% ""^30Si $\left(\text{mass of the isotope is 29.9737 amu}\right)$, and 3.1% ""^29Si $\left(\text{mass of the isotope is 28.9765 amu}\right)$.

Sep 7, 2016

""^29Si and ""^30Si have been multiplied by the wrong scaling factor.

#### Explanation:

27.9768xx92.2%+28.9765xx4.68%+29.9737xx3.10% $=$ $27.9768 \times 0.922 + 28.9765 \times 0.0468 + 29.9737 \times 0.0310$ $=$ $28.0800$ $\text{amu}$.

i.e. 1% $=$ $\left(0.01\right)$ or $\left(\frac{1}{100}\right) \times \text{the original quantity}$

It's all too easy to make a mistake with 'rithmetic! We have all done it.

The atomic mass of $S i$ that appears on the Periodic Table is $28.0855$ "amu" ± 0.0003, so we are within error. Good luck in your exam.