Average Atomic Mass: Definition & Formula

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• 2:05 Calculations: Example 1
• 3:46 Calculations: Example 2
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Did you know there are three types of oxygen? Atoms of elements have different versions of each other called isotopes. In this lesson, we will learn how to find the average atomic mass of the elements from its isotopes.

Average Atomic Mass

Have you ever gone through a whole bag of multi-colored M&Ms? If you look inside the bag of M&M's, the shape and size of each M&M is relatively the same. However, there are different colors, so we can say that they have different versions. If we count how many M&Ms there are for each color, sometimes we find that the most popular color in one bag is brown, and the other times, it is green or yellow.

Have you ever wondered how pennies have changed over the years? The penny has undergone several design changes and composition changes - some pennies are made purely of copper, some of nickel, tin, and zinc. If we weigh pennies from different years, their masses are different because their composition is different. In 1793, the mass of the penny is 13.48 g and now, it is only 2.5 g.

Just like M&Ms and pennies have different versions, atoms of the elements in the periodic table have different versions of each other called isotopes. Isotopes are different versions of the same elements with different numbers of neutrons and different atomic masses.

The whole numbers on the isotopes (superscript on the upper left side of the elements) are rounded to the nearest whole number of the mass that is indicated beside the element in the tables below. For example, for silicon-28 (28Si), the 28 is rounded from the mass, which is 27.976927 amu. When we see the atomic masses of elements in the periodic table, these are actually the averages of the atomic masses of their different isotopes. Isotopes of an element have different natural abundances, as shown in the tables below. The natural abundance indicates the percentage of the isotope that is naturally found on the planet.

The average atomic mass of an element refers to the atomic masses of the isotopes of the element, taking into account the different abundances of the element's isotopes. The average atomic mass can be calculated using the following formula:

Calculating Average Atomic Mass - Example 1

Let us go over a couple of examples on how to calculate the average atomic mass based on the atomic masses of the different isotopes and the natural abundance. The unit that is used for the average atomic mass is amu (atomic mass unit).

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