This atomic mass calculator computes an element’s average atomic mass from its isotopes — the weighted average that explains why the periodic table’s numbers are almost never whole.
Why Atomic Masses Are Not Whole Numbers
Two reasons stack up. First, individual isotopes are not exactly whole either (a chlorine-35 atom is 34.96885 amu, because binding energy and the proton-neutron mass difference shave off fractions). Second — the reason this calculator exists — elements in nature are isotope mixtures, and the table reports the abundance-weighted average: average = Σ (isotope mass × its fraction). It is the same weighted-average math as a course grade, which is why our Molar Mass Calculator can treat elements as single numbers afterward. Part of our Chemistry Calculators collection.
Worked Example (Verified): Chlorine
- Chlorine-35: 34.96885 amu × 75.78% = 26.499
- Chlorine-37: 36.96590 amu × 24.22% = 8.953
- Average: 35.45 amu — the periodic table's 35.45, reproduced from isotope data. Notice the average is nowhere near either isotope's mass: no chlorine atom anywhere actually weighs 35.45 amu. The number is a population statistic, not a property of any single atom.
- Try carbon: 12.00000 × 98.93% + 13.00335 × 1.07% = 12.011 — another familiar table value.
Reading Isotope Data Like a Chemist
- The dominant isotope rules: boron is the famous swing case — 10.81 average from ~20% B-10 and ~80% B-11, and its average genuinely varies slightly by source, one of the few elements whose table value has been revised into an interval.
- Abundances must total 100% (or the missing isotopes are distorting the average); the calculator flags this for you.
- Mass spectrometers are how these numbers are measured: they sort atoms by mass and count them, outputting exactly the mass/abundance pairs this calculator consumes.
Related Tools
- Molar Mass Calculator — where average atomic masses get used
- Weighted Average Calculator — the same math, general-purpose
- Chemistry Calculators — the full collection
Frequently Asked Questions
How is average atomic mass calculated?
Multiply each isotope's mass by its fractional abundance and add the products: for chlorine, 34.96885×0.7578 + 36.96590×0.2422 = 35.45 amu.
Why is chlorine's atomic mass 35.45 and not a whole number?
Because natural chlorine is a mix — about 76% Cl-35 and 24% Cl-37 — and the table prints the weighted average. No individual chlorine atom has mass 35.45.
What is the difference between mass number and atomic mass?
Mass number is the count of protons + neutrons in one specific isotope (always whole: 35, 37). Atomic mass is the measured mass of an atom or the weighted average for an element (34.96885 for one Cl-35 atom; 35.45 for chlorine overall).
What does "amu" mean?
Atomic mass unit (unified, symbol u; also called the dalton): defined as 1/12 the mass of a carbon-12 atom, ≈ 1.66 × 10⁻²⁷ kg. One mole of amu-sized things conveniently equals that many grams — the bridge between atomic masses and lab scales.
Which element's atomic mass varies most by source?
Lithium, boron and a few others fractionate in nature enough that their averages differ measurably between deposits — IUPAC now publishes their atomic weights as intervals rather than single values.
Is this atomic mass calculator free?
Yes — free, instant, no sign-up.