Average Atomic Weight Calculator
Calculate the weighted average mass of an element's isotopes.
Results:
How to Calculate Average Atomic Weight
Average atomic weight (also known as atomic mass) is the weighted average of the masses of all naturally occurring isotopes of an element. This value is what you see listed on the Periodic Table for each element.
The Atomic Weight Formula
The calculation relies on two variables for each isotope: its mass (measured in atomic mass units or amu) and its natural abundance (the percentage of that isotope found in nature).
*Note: Abundance must be expressed in decimal form (e.g., 75% = 0.75) for the formula to work correctly.
Step-by-Step Calculation Example
Let's calculate the average atomic weight of Chlorine, which has two primary isotopes:
- Isotope 1 (Cl-35): Mass = 34.969 amu | Abundance = 75.78%
- Isotope 2 (Cl-37): Mass = 36.966 amu | Abundance = 24.22%
Step 1: Convert percentages to decimals.
75.78% → 0.7578
24.22% → 0.2422
Step 2: Multiply each mass by its decimal abundance.
(34.969 × 0.7578) = 26.499 amu
(36.966 × 0.2422) = 8.953 amu
Step 3: Sum the results.
26.499 + 8.953 = 35.452 amu
Common Use Cases
This calculation is vital in chemistry for stoichiometry, calculating molar mass, and determining the amount of reactants needed in a chemical reaction. Because elements in nature are mixtures of isotopes, using the average atomic weight ensures high precision in laboratory and industrial environments.