Calculate the weighted average atomic mass from isotope masses and abundances. Supports 2-6 isotopes with automatic normalization and visual abundance charts.
Every element on the periodic table lists an atomic mass that is the weighted average of its naturally occurring isotopes. For example, carbon's atomic mass of 12.011 amu comes from 98.93% ¹²C (12.000 amu) and 1.07% ¹³C (13.003 amu). Calculating this average is a fundamental chemistry skill.
The weighted average is computed by multiplying each isotope's mass by its fractional abundance, then summing the products. This process relates directly to mass spectrometry data, where the mass spectrum shows peaks at each isotope's mass with heights proportional to relative abundance.
This calculator supports up to six isotopes and automatically normalizes abundances if they don't sum to 100%. It shows each isotope's contribution to the total, a visual abundance breakdown, and comparison with the periodic table value. It's ideal for general chemistry homework, mass spectrometry analysis, and understanding why atomic masses aren't whole numbers.
For best results, combine calculator output with direct observation and periodic check-ins with a veterinarian or qualified advisor. Small adjustments made early usually improve comfort, safety, and long-term outcomes more than large corrective changes made later.
Essential for general chemistry courses, mass spectrometry data analysis, and understanding isotopic composition. The visual breakdown makes it easy to see how each isotope contributes to the overall atomic mass. This average atomic mass calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
Average Atomic Mass = Σ (fractional abundance_i × isotopic mass_i)\n\nwhere fractional abundance = percent abundance / 100\n\nFor two isotopes:\nM_avg = f₁ × m₁ + f₂ × m₂ (f₁ + f₂ = 1) This keeps planning practical and lowers the chance of preventable errors.
Result: 35.453 amu
Chlorine has two stable isotopes. Average = 0.7577 × 34.969 + 0.2423 × 36.966 = 26.496 + 8.957 = 35.453 amu, matching the periodic table value.
A mass spectrometer ionizes atoms, accelerates them through magnetic and electric fields, and separates them by mass-to-charge ratio. The resulting spectrum directly shows which isotopes exist and their relative abundances. Modern instruments can measure isotopic masses to six decimal places.
Hydrogen is unique: its isotopes differ by 100% and 200% in mass (¹H, ²H, ³H). Chlorine's two-isotope pattern is visible in mass spectra of chlorinated compounds. Lead's atomic mass varies geographically because radiogenic lead (from U and Th decay) changes the isotopic mix.
Heavier isotopes form slightly stronger bonds (lower zero-point energy), leading to kinetic and equilibrium isotope effects. This is measurable for hydrogen/deuterium (up to 7× rate difference) and detectable for ¹²C/¹³C and ¹⁶O/¹⁸O ratios in biochemistry and geochemistry.
Fractional abundance is the decimal form of percent abundance (e.g., 75.77% = 0.7577). Both are equivalent; the calculator accepts percentages for convenience.
The calculator automatically normalizes them. This is useful when working with mass spectra where relative intensities may not be calibrated.
This calculator is designed for single elements with isotopes. For molecular mass (molar mass), use a molar mass calculator that sums atomic masses of all atoms in the formula.
By definition. The amu scale is defined so that ¹²C has a mass of exactly 12 amu. All other masses are measured relative to this standard.
Most elements have 2-3 stable isotopes. Tin holds the record with 10 stable isotopes. About 20 elements (like gold, fluorine, sodium) have only one stable isotope.
A mass spectrum plots ion abundance vs. mass-to-charge ratio (m/z). For an element, it shows peaks at each isotope's mass, and the peak heights correspond to natural abundances.