Average Atomic Mass Calculator

Calculate the average atomic mass for any element from its isotopic masses and natural abundances. Select from common elements with known isotopes, or enter custom isotope data to calculate weighted averages.

Element*

Isotope 1 Mass (u)

Isotope 1 Abundance (%)

Isotope 2 Mass (u)

Isotope 2 Abundance (%)

Isotope 3 Mass (u)

Isotope 3 Abundance (%)

AVERAGE ATOMIC MASS FORMULA: Average Mass = (Mass₁ × Abundance₁ + Mass₂ × Abundance₂ + ...) / (Total Abundance) Where: - Mass = isotopic mass in atomic mass units (u) - Abundance = fractional abundance (or divide by 100 if using %) - Must account for all naturally occurring isotopes FOR PERCENTAGE ABUNDANCE: Average = (Mass₁ × %₁ + Mass₂ × %₂) / 100 EXAMPLE: Chlorine Isotope 1: 34.969 u at 75.77% Isotope 2: 36.966 u at 24.23% Avg = (34.969 × 75.77 + 36.966 × 24.23) / 100 = 2648.74 + 895.50 / 100 = 3544.24 / 100 = 35.44 u

Example 1: Using preset for Chlorine (Cl) - Cl-35: 34.969 u at 75.77% - Cl-37: 36.966 u at 24.23% - Average = 35.453 u (matches periodic table 35.45) Example 2: Custom calculation - Isotope 1: mass 20.0 u, abundance 90% - Isotope 2: mass 22.0 u, abundance 10% - Average = (20×90 + 22×10) / 100 = 1800 + 220 / 100 - = 2020 / 100 = 20.2 u Note: If abundances don't add to 100%, the calculator normalizes them automatically before computing the average.

What is average atomic mass?

Average atomic mass is the weighted average of all naturally occurring isotopes of an element, weighted by their natural abundance. Since isotopes exist in different proportions, the atomic mass in the periodic table reflects this distribution. For example, Chlorine has Cl-35 (75.77%) and Cl-37 (24.23%), giving average 35.45 u, not 35 or 37.

How do I calculate average atomic mass from isotopes?

Multiply each isotope mass by its fractional abundance, sum all products, divide by total. Formula: average mass = Σ(massᵢ × abundanceᵢ). Example for Cl: (34.969 × 0.7577) + (36.966 × 0.2423) = 26.50 + 8.96 = 35.45 u. Abundance should be fractions (0.7577), not percentages, OR divide final sum by 100.

Why do atomic masses have decimals if isotopes are integers?

Isotope masses are not exactly whole numbers because: (1) neutron mass isn't exactly 1 u (1.008), (2) nuclear binding energy slightly changes mass (E=mc2), (3) electron masses subtract slightly, (4) isotope masses are measured values, not theoretically calculated. The values in the periodic table are precisely measured.

What determines natural isotope abundances?

Abundances come from stellar nucleosynthesis patterns. Even-numbered isotopes more stable. Different processes favor different isotopes: Big Bang made H and He. Stars make up to Fe. Supernovae make heavier. The current mix reflects formation history plus stability. Unstable isotopesdecayed over 4.5 billion years.

Can I use this calculator for custom isotopes?

Yes! Select "Custom Isotopes" to enter up to three unknown isotopes with their masses and abundances. The calculator computes the weighted average from your data. Great for labs working with enriched isotopes or studying specific isotope mixtures.

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