Average Atomic Mass Calculation
Atoms of the same element can have different numbers of neutrons. These variations are called isotopes. Each isotope has a different atomic mass, but the element itself has a single average atomic mass, which is a weighted average based on how common each isotope is.
Formula for Average Atomic Mass
To find the average atomic mass of an element, use this formula:
Average Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + (Mass of Isotope 3 × Abundance of Isotope 3) + …
Since abundance is usually given in percentages, convert it to a decimal by dividing by 100.
Example Calculation
Let’s say an element has two isotopes:
- Isotope 1 has a mass of 10 amu and an abundance of 20%.
- Isotope 2 has a mass of 12 amu and an abundance of 80%.
Applying the formula:
Average Atomic Mass = (10 × 20/100) + (12 × 80/100)
Average Atomic Mass = (10 × 0.20) + (12 × 0.80)
Average Atomic Mass = 2 + 9.6
Average Atomic Mass = 11.6 amu
Why Is This Important?
The periodic table shows the average atomic masses of elements. These values are not whole numbers because they reflect the weighted average of all naturally occurring isotopes. For example, the atomic mass of carbon is 12.01 amu, which accounts for its isotopes carbon-12 and carbon-13.
Key Takeaways
- Isotopes are atoms of the same element with different neutron counts.
- The average atomic mass is a weighted average based on isotope mass and abundance.
- To calculate, multiply each isotope’s mass by its abundance and sum the results.
- This concept is important for chemistry, physics, and material science.
Use this Average Atomic Mass Calculator to quickly and accurately compute the atomic mass of any element with known isotopes!
