Atomic Mass from Isotopes Calculator
Σ (isotope mass × abundance fraction). Periodic-table atomic mass from isotope mix.
Result
Atomic mass
35.4844 u
Sum of % checks to 100.00%.
- Isotope 135 u × 75.78% = 26.5230
- Isotope 237 u × 24.22% = 8.9614
- Sum of abundances100.000%
- Atomic mass35.484400 u
Step-by-step
- M = Σ (mass × abundance).
- = 35 × 0.7578 + 37 × 0.2422
- = 35.4844 u.
How to use this calculator
- Enter mass + abundance for each isotope.
- Sum should be ~100%.
About this calculator
Periodic-table atomic mass is a weighted average of natural-abundance isotopes. Chlorine: 75.78% ³⁵Cl (~35 u) + 24.22% ³⁷Cl (~37 u) = 35.45 u. Atomic mass differs from mass number — mass number is integer count of nucleons; atomic mass includes binding energy mass defect and isotope ratios. Modern measurements via mass spectrometry, accurate to ~10 ppm.
Frequently asked
Mass number = integer protons+neutrons (e.g. ³⁵Cl mass # = 35). Atomic mass = real measured mass (35.45 for Cl element).
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