Isotope Abundance Calculator

Calculate average atomic mass from isotope masses and natural abundances. Find percent abundance of one isotope when given average atomic mass and other isotope data.

☢️ Nuclear Chemistry📐 M̄ = Σ(fractionᵢ × massᵢ)🧪 Chemistry

Mass of isotope 1 (amu)

Abundance of isotope 1 (%)

Mass of isotope 2 (amu)

Abundance of isotope 2 (%)

Abundances must sum to 100%. For 2-isotope: enter both. Result = average atomic mass.

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Isotope Abundance Calculator	—	M̄ = Σ(fractionᵢ × massᵢ)	g/mol

Step-by-Step Examples

Example 1

Boron

B-10: mass=10.013 amu, abundance=19.9%. B-11: mass=11.009 amu, abundance=80.1%.

M = 10.013x(19.9/100) + 11.009x(80.1/100)
= 1.993 + 8.818 = 10.811 amu

✓ Average atomic mass of boron = 10.811 amu

Example 2

Chlorine

Cl-35: 34.969 amu, 75.76%. Cl-37: 36.966 amu, 24.24%.

M = 34.969x0.7576 + 36.966x0.2424
= 26.495 + 8.960 = 35.455 amu

✓ Average atomic mass of Cl = 35.455 amu

Example 3

Find Unknown Abundance

Two isotopes: m1=6.015 amu (Li-6), m2=7.016 amu (Li-7). Average M=6.941 amu. Find abundances.

Let x = fraction of Li-6: 6.015x + 7.016(1-x) = 6.941
7.016 - 1.001x = 6.941; x = 0.075/1.001 = 0.0749
Li-6: 7.49%, Li-7: 92.51%

✓ Li-6: 7.49%, Li-7: 92.51%

Real-World Applications

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Periodic Table Values

Atomic masses on periodic table are weighted averages based on isotope abundances.

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Isotope Geochemistry

Isotope ratios in rocks reveal geological age and history of formation.

🧪

Mass Spectrometry

MS measures isotope masses and abundances directly, used to verify or discover isotopic composition.

💡

Enrichment Technology

Nuclear fuel uses enriched U-235 (natural 0.72%) increased to 3-5% for reactors, 90%+ for weapons.

Common Mistakes to Avoid

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Abundances sum to 100%

Percent abundances of all isotopes of an element must sum to 100% exactly. Check this before calculating.

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Natural abundance varies by location

Isotope ratios can differ slightly by geographic location. Periodic table values are IUPAC averages.

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Mass is in amu not g/mol

Isotope mass in atomic mass units (amu). Average = g/mol for molar mass. Same number, different context.

Frequently Asked Questions

Why do atomic masses on the periodic table have decimals? ▾

Elements have multiple stable isotopes. The table gives the weighted average based on natural abundance. Carbon: 98.9% C-12 (12.000) + 1.1% C-13 (13.003) = 12.011.

What is the mass defect? ▾

Actual nuclear mass is slightly less than sum of proton and neutron masses. Mass defect x c^2 = binding energy (nuclear stability).

Why is carbon-12 defined as 12.000 exactly? ▾

The atomic mass unit (amu or u) is defined as 1/12 mass of C-12. All other masses measured relative to this standard.

What is mass spectrometry? ▾

Technique that measures mass-to-charge ratio of ions. Gives exact isotopic masses and relative abundances. Used to identify compounds and measure isotope ratios.

Do isotope ratios change in living things? ▾

Yes, slightly. Metabolic processes slightly favor lighter isotopes (isotope fractionation). C-13/C-12 ratios differ between C3 and C4 plants. Used in food fraud detection.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship connects isotopic composition, decay, radiation, mass defect or nuclear energy to a measurable quantity. Assumption: Use the correct nuclide, decay constant, branching behavior and time units. Radiation estimates also depend on geometry, shielding and detector response.

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