Atomic Mass of Elements: Calculation Activity

Activity Overview

Students will use the atomic mass of an element often throughout any chemistry course. One common misconception is that some atoms have a mass equal to the atomic mass, but that is false in many cases. Because the atomic mass is a weighted average that reflects the abundance of isotopes in nature, a model helps students better understand this concept.

In this activity, students will create examples to illustrate the percent abundances for an element and calculate the atomic mass for an element. This will help students understand this idea visually. The example above can also be edited and used as an example for students rather than an activity. Copying the assignment will also copy the completed example into your account!

In order to calculate the atomic mass of an element, scientists must know what isotopes exist and how abundant they are. Once this is known, the average atomic mass is calculated by taking into account the masses of the isotopes and how prevalent they are. This can be compared to finding the average age of students in a class. When most of the students are 16, but some of the students are 17, the average age can be predicted to be close to 16. Students typically understand this intuitively. The average age is not necessarily 16.5 (the average of 16 and 17) unless their are equal numbers of students who are 16 or 17. This model can be extended to isotopes: the more abundant an isotope, the closer the average will be to the mass of that isotope and the average will not necessarily be the midpoint between the masses of the isotopes.