Atomic Mass Calculator
Enter each isotope's mass and percent natural abundance to get an element's weighted-average atomic mass.
Enter every isotope's mass (amu) and its percent natural abundance. Abundances must sum to 100%.
34.9689 × 0.7576 + 36.9659 × 0.2424 = 35.4530 amu.
Isotopes and Weighted-Average Atomic Mass
Most elements occur naturally as a mix of isotopes, each with a slightly different mass and a different share of a real sample. The periodic table's atomic mass is the weighted average of every naturally occurring isotope: each isotope's mass multiplied by its fractional abundance, all added together.
This is why the periodic table's numbers aren't whole -- chlorine's 35.45 isn't any single chlorine atom's real mass, it's what you'd get averaging a huge natural sample of chlorine, most of which is chlorine-35 with a smaller share of the heavier chlorine-37.
Worked example
Chlorine has two stable isotopes: chlorine-35 (34.9689 amu, 75.76% abundant) and chlorine-37 (36.9659 amu, 24.24% abundant). Find the weighted-average atomic mass.
(34.9689 × 0.7576) + (36.9659 × 0.2424) = 35.4530 amu.
That matches chlorine's published periodic-table value of 35.45 -- a real check that this calculation method is correct, not just a formula that looks right.
Common mistake
Taking a simple average of the isotope masses instead of a weighted one -- (34.9689+36.9659)/2 = 35.97, noticeably off from the real 35.45. Abundance has to be factored in, not just the number of isotopes.
Keep going
- An element's atomic mass is exactly what the molar mass calculator's periodic-table data draws on for every element in a formula. Molar Mass Calculator
Frequently Asked Questions
Why isn't an element's atomic mass a whole number?
Because most elements exist naturally as a mix of isotopes -- atoms with the same number of protons but different numbers of neutrons, so slightly different masses. The periodic table lists a weighted average across all naturally occurring isotopes, not any single isotope's exact mass.
What does 'weighted average' mean here?
Each isotope's mass is multiplied by how common it actually is (its fractional abundance) before adding them together -- so a rare, heavy isotope shifts the average far less than a common one does. It's the same idea as a weighted grade average, where a bigger assignment counts for more.
Why must abundances add up to 100%?
Because the abundances describe how a real 100%-complete sample of the element's natural isotopes is divided up. If they don't sum to 100%, either an isotope is missing from the list or a percentage was entered wrong, and the average would be meaningless.
Is isotope mass the same as mass number?
No, though they're close. Mass number (like the 35 in chlorine-35) is just the whole-number count of protons plus neutrons; isotope mass (34.9689 amu for that same isotope) is the actual measured mass, which is always slightly different from the whole-number mass number.
Related Calculators
- Molar Mass Calculator — Atomic & Molecular
- Moles Calculator — Atomic & Molecular
- Empirical Formula Calculator — Atomic & Molecular
- Half-Life Calculator — Atomic & Molecular