Calculadora de Massa Atômica Média
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
M̄ = Σ(mᵢ · aᵢ) / 100
About this calculator
The average atomic mass calculator determines the weighted average atomic mass of a chemical element from the masses and abundances of its two most common isotopes. The average atomic mass is the weighted mean of the isotopic masses, considering the percentage of each isotope in nature. This value is what appears on the periodic table for most elements.
The calculation uses the formula M̄ = (m₁ · a₁ + m₂ · a₂) / 100, where m₁ and m₂ are the isotopic masses (in atomic mass units, u) and a₁ and a₂ are their percent abundances. For example, for chlorine, with isotopes Cl-35 (34.97 u, 75.78%) and Cl-37 (36.97 u, 24.22%), the average atomic mass is (34.97·75.78 + 36.97·24.22)/100 ≈ 35.45 u.
Use this calculator in general chemistry problems, such as determining the atomic mass of an element from mass spectrometry data or in educational exercises. It is also useful for verifying tabulated values or understanding how isotopic abundances affect atomic mass.
Caution: Abundances must be provided as percentages (sum equal to 100). If data are in decimal fractions, multiply by 100 before use. The calculator considers only two isotopes; for elements with more isotopes, the result will be approximate. Ensure isotopic masses are in the same unit (u).
Frequently asked questions
What is average atomic mass?
It is the weighted average of the masses of an element's isotopes, considering their natural abundances. This value is what appears on the periodic table.
How to calculate average atomic mass with more than two isotopes?
The formula extends to M̄ = Σ(mᵢ · aᵢ) / 100, where i represents each isotope. You can use this calculator for two isotopes and then combine results, but a calculator that accepts multiple isotopes is ideal.
Why is the average atomic mass not a whole number?
Because it is a weighted average of isotopic masses that are usually not integers, and abundances vary. Also, the mass of an isotope is not exactly the mass number due to nuclear binding energy.
Does the calculator work for any element?
Yes, as long as you have the masses and abundances of the two most common isotopes. For elements with many isotopes, the result may differ slightly from the tabulated value.
What if the abundances do not sum to 100%?
Check your data: abundances must sum to 100%. If they are in decimal fractions, multiply by 100. Otherwise, the calculation will be incorrect.