Question

The element carbon has 3 naturally occurring isotopes. About 99% of carbon isotopes are C-12, about 1% are C-13 and a tiny amount are C-14. What is the average atomic mass of carbon atoms?

Answer 1

`"12.01 u"`

Explanation:

As you know, the average atomic mass of an element is calculated by taking the weighted average of the atomic mass of its stable isotopes.

More specifically, each stable isotope will contribute to the average atomic mass of the element proportionally to their abundance.

`color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i))`

In this case, you know that carbon has three naturally occurring isotopes, but that one of them, `""^14"C"`, is only present in trace amounts.

That means that you can ignore its contribution to the average atomic mass of carbon. So, you're left with `""^12"C"`, which has a decimal abundance of `0.99`, and `""^13"C"`, which ahs a decimal abundance of `0.01`.

Here decimal abundance simply means percent abundance divided by `100`.

So, the average atomic mass of carbon will be

`"avg. atomic mass" = "12 u" xx 0.99 + "13 u" xx 0.01`

`"avg. atomic mass " = " 12.01 u"`