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Isotope Abundance Calculator

Two solvers: compute the average atomic mass from isotope masses and abundances (leave one abundance blank and it's found from the 100% rule), or work backwards from the periodic-table average to find both abundances of a two-isotope element.

The weighted average formula

Average atomic mass

average = Σ (isotope mass × fractional abundance)

“Fractional abundance” is the percentage divided by 100. The result is the decimal number printed on the periodic table — an average that usually matches no single real atom.

Worked example: chlorine

Chlorine is 75.76% Cl-35 (34.9689 amu) and 24.24% Cl-37 (36.9659 amu). Find its average atomic mass.

  1. Cl-35: 34.9689 × 0.7576 = 26.49 amu
  2. Cl-37: 36.9659 × 0.2424 = 8.96 amu
  3. Sum: 26.49 + 8.96

Answer: ≈ 35.45 amu — exactly the periodic table value.

Solving backwards for abundances

For a two-isotope element you can reverse the problem: let x be one isotope's fraction, so the other is (1 − x), and solve average = m₁x + m₂(1 − x). Exam favourites include chlorine, boron, copper and silver. The calculator's second mode does this algebra and shows every step.

⚠ Common mistake: The average must sit between the two isotope masses — if your calculated abundance comes out negative or above 100%, one of the input values is wrong.

Frequently asked questions

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

It averages isotopes of different masses weighted by natural abundance. Chlorine's 35.45 reflects roughly three Cl-35 atoms for every Cl-37.

What if abundances don't add up to 100%?

They must — the calculator flags totals off by more than rounding. With one abundance left blank, it's filled using the 100% rule automatically.

Are isotope masses whole numbers?

Close to, but not exactly — Cl-35 weighs 34.9689 amu, not 35. Mass numbers count particles; measured masses include binding-energy effects.

Related chemistry tools

Interactive Periodic TableExplore all 118 elements with hover details, filters, trends, temperature states, a formula builder and more.Molar Mass CalculatorCalculate the molar mass of any chemical formula with a full element-by-element breakdown and percent composition.Electron Configuration CalculatorFind the full and noble-gas electron configuration, shell distribution and valence electrons of any element.

Learn the concepts

What Are Isotopes?Isotopes are atoms of the same element with different neutron counts. Learn notation, why atomic masses are decimals, and real uses from dating to medicine.Atomic Number vs Atomic MassAtomic number counts protons; atomic mass averages isotope masses. Learn the difference, what mass number means, and how to use all three correctly.