Average Atomic Mass Calculator

Enter the isotope mass (in amu) and the relative abundance (in %) for each isotope. Leave rows blank if the element has fewer isotopes.

What is Average Atomic Mass?

The average atomic mass, also called the atomic weight, is the weighted average mass of all the naturally occurring isotopes of an element. It is the value listed on the periodic table for each element and is expressed in atomic mass units (amu or u). Because a sample of an element is a mixture of isotopes, the average atomic mass reflects both the mass of each isotope and how common it is.

Why It Is a Weighted Average

A simple average would treat every isotope as equally common, but that is rarely true. Instead, each isotope's mass is multiplied (weighted) by its fractional abundance before adding. Isotopes that occur more frequently contribute more to the final value, which is why the average atomic mass usually sits closer to the mass of the most abundant isotope.

For chlorine, the lighter isotope (chlorine-35, 34.969 amu) makes up about 75.77% of natural chlorine, while the heavier isotope (chlorine-37, 36.966 amu) accounts for 24.23%. The weighted average is therefore pulled toward the lighter isotope, giving about 35.45 amu rather than the midpoint of the two masses.

Isotopes and Relative Abundance

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, which gives them different masses. The relative abundance of an isotope is the percentage of atoms in a natural sample that are that isotope. These abundances are measured with mass spectrometry and, for most elements, are remarkably consistent across the planet.

• Abundances are normally given as percentages that add up to about 100%.
• If your abundances do not sum to 100%, the calculator still computes a result but flags the discrepancy.
• Isotope masses are very close to the mass number but not exactly equal, because of nuclear binding energy.

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