Isotope Abundance & Relative Atomic Mass Calculator
Computes the standard weighted atomic weight of chemical elements from isotopic masses and percent fractional abundances, with presets for Carbon, Chlorine, Uranium, and Oxygen.
Isotope Abundance & Relative Atomic Mass Calculator
Compute weighted average atomic weight from isotopic masses and fractional abundances
What is Relative Atomic Mass?
An element's standard relative atomic mass (atomic weight) is the weighted average of the masses of its naturally occurring isotopes, weighted by their natural percentage abundances on Earth.
Because elements exist as mixtures of isotopes with varying numbers of neutrons, the atomic mass on the periodic table is rarely a whole integer.
Formula & Step-by-Step Calculation
Sum of each isotope's atomic mass multiplied by its fractional natural abundance.
Worked Step-by-Step Examples
Calculate Chlorine atomic mass: ³⁵Cl (75.76%, 34.969 u) and ³⁷Cl (24.24%, 36.966 u)
Common Real-World & Academic Use Cases
- ✓ Mass spectrometry isotope ratio interpretation
- ✓ High school and general chemistry stoichiometry homework
- ✓ Nuclear physics isotopic composition analysis
- ✓ Understanding atomic weight variations in nature
How to Use the Isotope Abundance & Relative Atomic Mass Calculator
Select Preset or Custom
Choose a standard element preset (Carbon, Chlorine, Uranium, Oxygen) or enter custom isotopes.
Enter Masses & Abundances
Input each isotope's exact isotopic mass (in unified atomic mass units, u) and percent natural abundance.
Compute Weighted Mass
Get the exact weighted average atomic weight and proportional contribution breakdown.
Frequently Asked Questions
Q: Must total percentage abundances equal 100%?
Yes, natural fractional abundances must sum to 100%. The calculator automatically normalizes abundances or alerts you if your entered percentages don't add to 100%.