How do you calculate average atomic mass?

Multiply each isotope’s mass by its fractional abundance and add the results. For chlorine: 34.969 × 0.7576 + 36.966 × 0.2424 = 35.45.

Why is the atomic mass on the periodic table not a whole number?

It is an average over the element’s natural isotopes, weighted by how common each one is. Individual isotope masses are also not exact whole numbers, because of the nuclear binding energy.

How do I find the abundance of two isotopes from the average mass?

Let x be the fraction of the lighter isotope. Then average = m₁x + m₂(1 − x), so x = (m₂ − average) ÷ (m₂ − m₁). Use the “Abundances from the average” mode.

What if my abundances don’t add up to 100 %?

The calculator divides by the total you entered, so abundances given as ratios or rounded percentages still work. It also warns you that they did not sum to 100.

Average Atomic Mass Calculator | PeriodicElementTable

Load an element's natural isotopes

Isotope 1 mass (u)

Abundance (%)

Isotope 2 mass (u)

Abundance (%)

Average atomic mass = 35.4529 u

34.96885 u × 75.76 % = 26.4924 u
36.96590 u × 24.24 % = 8.96053 u
Sum of contributions = 35.4529 u

How it works

Average atomic mass = Σ (isotope mass × abundance ÷ 100). The element presets use isotope masses and natural abundances from the IAEA Live Chart of Nuclides, the same data behind the isotope tables on the element pages.

IUPAC gives some elements, such as hydrogen, carbon and lead, an interval rather than a single atomic weight, because their isotope ratios vary between samples. Your result for a preset may therefore differ slightly from the value in the periodic table.

Frequently asked questions

How do you calculate average atomic mass?: Multiply each isotope’s mass by its fractional abundance and add the results. For chlorine: 34.969 × 0.7576 + 36.966 × 0.2424 = 35.45.
Why is the atomic mass on the periodic table not a whole number?: It is an average over the element’s natural isotopes, weighted by how common each one is. Individual isotope masses are also not exact whole numbers, because of the nuclear binding energy.
How do I find the abundance of two isotopes from the average mass?: Let x be the fraction of the lighter isotope. Then average = m₁x + m₂(1 − x), so x = (m₂ − average) ÷ (m₂ − m₁). Use the “Abundances from the average” mode.
What if my abundances don’t add up to 100 %?: The calculator divides by the total you entered, so abundances given as ratios or rounded percentages still work. It also warns you that they did not sum to 100.

How it works

Frequently asked questions

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