Atomic Mass Calculator
Calculate the weighted average atomic mass of an element based on its isotopes.
Results:
Average Atomic Mass: amu
Total Abundance: %
Understanding Atomic Mass and Isotopes
The atomic mass of an element listed on the periodic table is not the mass of a single atom. Instead, it is a "weighted average" of all the naturally occurring isotopes of that element. Because different atoms of the same element can have different numbers of neutrons (isotopes), their masses vary.
What are Isotopes?
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.
The Weighted Average Formula
To find the average atomic mass, you must account for how common each isotope is in nature. This is known as relative abundance. The formula is:
Average Atomic Mass = (Mass1 × Abundance1) + (Mass2 × Abundance2) + … + (Massn × Abundancen)
Note: Abundance should be used as a decimal in the calculation (e.g., 75% becomes 0.75).
Step-by-Step Calculation Example
Chlorine has two main isotopes:
- Chlorine-35: Mass = 34.969 amu, Abundance = 75.78%
- Chlorine-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 mass by decimal abundance.
(34.969 × 0.7578) = 26.4995
(36.966 × 0.2422) = 8.9532
Step 3: Add the products together.
26.4995 + 8.9532 = 35.4527 amu
Why Isn't It a Simple Average?
If you took a simple average of Chlorine-35 and Chlorine-37, you would get 36 amu. However, because Chlorine-35 is much more common (about 3/4 of all chlorine atoms), the final "weighted" average is pulled closer to 35, resulting in the standard value of 35.45 amu seen on periodic tables.