Atomic Mass Calculator
Calculate the average atomic mass of an element from the masses and natural abundances of its isotopes. Useful for chemistry coursework, spectroscopy, and nuclear science.
About this calculator
Elements in nature exist as a mixture of isotopes — atoms with the same number of protons but different numbers of neutrons. The average atomic mass is the abundance-weighted mean of each isotope's mass: average mass = (m₁ × a₁/100) + (m₂ × a₂/100), where m is mass in atomic mass units (amu) and a is percent abundance. This value matches what appears on the periodic table and is what chemists use for molar mass calculations. The formula extends to any number of isotopes by adding additional (mᵢ × aᵢ/100) terms. Abundances must sum to 100% for the result to be physically meaningful. One amu is defined as 1/12 the mass of a carbon-12 atom, approximately 1.66054 × 10⁻²⁷ kg.
How to use
Chlorine has two stable isotopes: Cl-35 (mass = 34.969 amu, abundance = 75.77%) and Cl-37 (mass = 36.966 amu, abundance = 24.23%). Step 1: Contribution of Cl-35 = 34.969 × (75.77/100) = 34.969 × 0.7577 = 26.496 amu. Step 2: Contribution of Cl-37 = 36.966 × (24.23/100) = 36.966 × 0.2423 = 8.957 amu. Step 3: Average atomic mass = 26.496 + 8.957 = 35.453 amu. This matches the periodic table value for chlorine almost exactly.
Frequently asked questions
Why does the periodic table show non-integer atomic masses for most elements?
Periodic table masses are weighted averages over all naturally occurring isotopes of an element, not the mass of a single atom. Because each isotope occurs in a specific natural abundance, the average lands between whole numbers. For example, chlorine is approximately 75.77% Cl-35 and 24.23% Cl-37, giving an average of ~35.45 amu. Only monoisotopic elements like fluorine (100% F-19) display a near-integer atomic mass on the table.
How do isotope abundances affect atomic mass calculations?
Each isotope's contribution to the average atomic mass is proportional to its natural abundance. An isotope present at 90% abundance dominates the average, pulling it close to its own mass, while a rare isotope at 1% barely shifts the result. This is why the atomic mass of boron (10B: 19.9%, 11B: 80.1%) is 10.81 amu — much closer to 11 than to 10. Accurate abundance values, typically measured by mass spectrometry, are essential for a precise result.
What is the difference between atomic mass and atomic number in chemistry?
Atomic number (Z) is the count of protons in the nucleus and defines which element an atom is — it is always a whole number. Atomic mass is the total mass of an atom's protons, neutrons, and electrons expressed in amu, and varies between isotopes of the same element. The average atomic mass reported on the periodic table is a weighted mean across all isotopes. Atomic number determines chemical behavior; atomic mass determines mass-based stoichiometry in calculations like molar mass and formula weight.