All calculatorsAverage Atomic Mass Calculator
Compute the average atomic mass from isotopic masses and percent abundances. Add up to 6 isotopes, use quick picks, and get step-by-step contributions. Optionally solve for one unknown abundance.
Background
The periodic table’s atomic mass is a weighted average of an element’s naturally occurring isotopes. Each isotope contributes its mass multiplied by its fractional abundance. For example, chlorine’s two stable isotopes (~75.78% ³⁵Cl and ~24.22% ³⁷Cl) average to ~35.45 amu.
How to use this calculator
- Enter isotopic data: Provide mass (amu) and percent abundance for each isotope. Add/remove rows as needed.
- Compute average: We calculate Σ(mass × fractional abundance) across all isotopes.
- Optional solve mode: Leave one % blank and enter a target average to compute the missing abundance.
- Quick picks: Load common elements (C, Cl, Cu, Br, Ga, B) with standard natural abundances.
- Export: Download a CSV of your inputs and step-by-step contributions.
Formula & Equation Used
For isotopes with masses mi and abundances pi (percent):
Solving for a single missing abundance pk given a target average
Abundances should be between 0–100% and sum to ~100%.
Example Problems & Step-by-Step Solutions
Example 1 (Chlorine):
masses: 34.96885 amu (75.78%), 36.96590 amu (24.22%) ⇒ average ≈ 35.45 amu.
Example 2 (Boron):
masses: 10.0129 (19.9%), 11.0093 (80.1%) ⇒ average ≈ 10.81 amu.
Frequently Asked Questions
Q: Do my % abundances have to add to exactly 100?
Small rounding is fine; we allow a ±0.5% tolerance and offer a one-click normalize.
Q: What’s the unit for mass here?
Atomic mass unit (amu), also called unified atomic mass unit (u).
Q: Can I solve for an unknown abundance?
Yes—pick “solve missing %”, leave exactly one % blank, and provide the target average.
