# An imaginary element Y occurs 30% of the time with a mass of 57.9 a.m.u. The remaining sample of Y has a mass of 60.9 a.m.u. What is the average atomic mass of Y?

##### 1 Answer

#### Explanation:

The *average atomic mass* of an element is calculated by taking the **weighted average** of the atomic masses of its naturally occurring isotopes.

In essence, this means that each stable isotope will contribute to the average atomic mass of the element **In proportion** to its *abundance*.

#color(blue)(|bar(ul(color(white)(a/a)"avg. atomic mass" = sum_i "isotope"_i xx "abundance"_icolor(white)(a/a)|)))#

This equation uses **decimal abundance**, which is simply *percent abundance* divided by

So, you know that your imaginary element

*How do you know that this second isotope has a* *abundance?*

That is the case because the percent abundances of the stable isotopes **must** add up to give

The average atomic mass of

#"avg. atomic mass" = "57.9 u" xx 30/100 + "60.9 u" xx 70/100#

#"avg. atomic mass " = color(green)(|bar(ul(color(white)(a/a)" 60.0 u"color(white)(a/a)|)))#