Atomic Mass from Isotopes Calculator

Σ (isotope mass × abundance fraction). Periodic-table atomic mass from isotope mix.

Result

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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 vs. atomic mass?+

Mass number = integer protons+neutrons (e.g. ³⁵Cl mass # = 35). Atomic mass = real measured mass (35.45 for Cl element).

Why average?+

Natural samples contain a mix of isotopes. Periodic-table mass = abundance-weighted mean.

Common element examples?+

C: 98.93% ¹²C (12.0) + 1.07% ¹³C (13.003). Average: 12.011. Cl: 35 + 37 mix → 35.45.

Mass defect?+

Bound nucleus mass < sum of free nucleons (binding energy via E=mc²). Atomic mass already includes this.

Stable vs. radioactive isotopes?+

Stable isotopes contribute. Radioactive (e.g. ¹⁴C, U) have negligible natural abundance for most elements.

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