What is average atomic mass?

Have you ever wondered where those atomic mass decimal numbers are from?

The decimal numbers are from the average of ALL natural occurring isotopes. Recall, isotopes are elements that have different number of neutrons, and also recall that atomic masses are calculated by protons adding to the number of neutrons.  So if the number of neutrons change, so does the atomic mass! But since the protons are still the same, they are still the same element, just with a different atomic mass.

So where does this lead us?

Average atomic masses (or really the atomic mass) is taking all the isotopes of that element, and finding the average between them.

A sample question:

Carbon has an isotope of Carbon-12 that has a mass of 12.000000 that is 98.9% of all natural occurring carbons. The remaining amount of carbon is Carbon-13 with a mass of 13.003355. What is the average atomic mass?

To calculate: You must take each portion individually, and sum them up

(98.9% of 12.000000) + (1.1% of 13.003355) = 12.011 atomic mass unit (amu).


Calculating average atomic mass is similar to how you calculate your final grade of your chemistry class.

Let’s say:

In your chemistry course, this is your grading rubric for your course grade:

30% Final exam

50% In class work

20% Tests

If you scored 30% on your final exam, but 10% of your In class work, and 20% on your tests, you can pretty much see that your average is not going to be 100% even though you scored 100% on your Final exam and tests (great test taker! but not such a great student).

In order to calculate your final grade, you need to take the component of each and then sum them all up!

100% of 30% = 30 points out of 30

10% out of 50% = 5 points out of 50

100% out of 20% = 20 points out of 20

Grand Total: 30 + 5 + 20 = 55% is your final score.