Enter your element's number of isotopes (up to 4), then fill in each isotope mass and its natural abundance — the Average Atomic Mass Calculator computes the average atomic mass along with a total abundance check and weighted contribution sum so you can verify every part of your calculation.
Average Atomic Mass
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Total Abundance Check
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Weighted Contribution Sum
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Average atomic mass is calculated by multiplying each isotope's mass by its fractional abundance, then summing all contributions. Use the formula: AM = (m₁ × f₁) + (m₂ × f₂) + ... where m is mass and f is fractional abundance (percentage ÷ 100).
Chlorine has two stable isotopes: Cl-35 (mass 34.969 amu, 75.78% abundance) and Cl-37 (mass 36.966 amu, 24.22% abundance). The average atomic mass is approximately 35.45 amu.
Atomic mass is not a whole number because it represents a weighted average of all isotopes of an element. Each isotope has a different mass, and their natural abundances result in a decimal value when averaged.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This gives them different atomic masses while maintaining the same chemical properties.
Fractional abundance is the proportion of each isotope found in nature, expressed as a decimal (percentage divided by 100). For example, if an isotope has 25% abundance, its fractional abundance is 0.25.
Average atomic mass reflects the actual composition of an element as it appears in nature. Since elements exist as mixtures of isotopes with different abundances, the average provides a more practical value for calculations and the periodic table.
Yes, the abundances of all naturally occurring isotopes of an element must total 100%. If your abundances don't add up to 100%, check your data or consider if you're missing any isotopes.