Average Atomic Mass

Every element has at least 2 naturally occurring isotopes.

This begs the question, "If elements can have different masses, why is there only one mass on the Periodic Table?"

The mass on the Periodic Table is actually the Average Atomic Mass of all of that element's isotopes.

Quick notes on Average Atomic Mass:

• Units: amu (Atomic Mass Unit)
• The mass on the PT
• Comprised of the average of all isotopic masses

This is the formula for Average Atomic Mass:



This is what the formula means:

1. Isotopes don't exist in equal amounts
2. Take each isotope's mass
3. Multiply it by its % abundance (in decimal form)
4. Do this for all isotopes of the element
5. Add them together

Ex.

Carbon has 2 main isotopes: Carbon-12 and Carbon-13.

C-12 makes up about 98% of all of the Carbon in the world.
C-13 makes up about 1.1% of all of the Carbon in the world.

To find the Average Atomic Mass of Carbon:

1) Change % to decimal

98% = .98
1.1% = .011

2) Multiply decimal by corresponding mass

.98 x 12 = 11.76 amu
.011 x 13 = .143 amu

3) Add the products together

11.76 amu + .143 amu = 11.903 amu

4) The Average Atomic Mass of Carbon is 11.903 amu

Websites
Here is a video explaining how to calculate Average Atomic Mass.
This website gives a brief explanation and a few examples.
Worksheets
This is a great worksheet on Average Atomic Mass.