Average Atomic Mass Calculator

Average atomic mass is the weighted mean of the masses of an element's naturally occurring isotopes. Most elements occur as mixtures rather than as a single isotope, so the number printed on a periodic table is usually not a whole number. It reflects both the exact mass of each isotope and how frequently that isotope appears in a representative natural sample. The unit atomic mass unit, abbreviated amu, is also commonly written as the unified atomic mass unit, u. The calculation multiplies each isotope mass by its fractional abundance, then adds the products. A percentage abundance becomes a fraction by division by 100. For two isotopes, the familiar expression is average mass equals mass one times abundance one plus mass two times abundance two, with percentages converted to fractions. This calculator uses an equivalent normalized form: it divides the weighted sum by the total entered abundance. Normalization means values such as 75.77 and 24.23 work directly, while rounded values whose sum is 99.99 still produce a properly scaled result. Exact isotope mass is different from mass number. Mass number counts protons and neutrons and is always an integer, while measured isotopic mass includes nuclear binding effects and is generally decimal. For example, chlorine-35 has mass number 35 but an isotopic mass near 34.969 amu. Use experimentally reported isotope masses whenever precision matters. Entering mass numbers may be useful for an estimate, but it will not reproduce the standard atomic weight shown in a high-quality reference table. Natural abundance can vary slightly among samples because geological, biological, and industrial processes can fractionate isotopes. Published standard atomic weights may therefore be intervals or values with uncertainty. This calculator returns the arithmetic result for the data supplied; it does not add measurement uncertainty or choose abundance data for you. Keep enough significant figures during the calculation, then round the final result to the precision supported by the least precise input. Weighted atomic mass calculations are common in introductory chemistry, isotope geochemistry, mass spectrometry, and stoichiometry. They also help explain why a periodic-table atomic weight lies closer to the mass of the more abundant isotope. Check that each abundance is paired with the correct mass, use the same abundance units for both isotopes, and review the displayed total before interpreting the result.