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Average Atomic Mass Calculator

Calculate the average atomic mass of an element from its isotope masses and natural abundances. Supports up to 4 isotopes.

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What is a Average Atomic Mass Calculator?

The Average Atomic Mass Calculator computes the weighted average atomic mass of an element based on its naturally occurring isotopes. Most elements exist as a mixture of two or more isotopes, each with a different mass and a specific natural abundance. The average atomic mass, which is the value you see on the periodic table, is calculated by multiplying each isotope's mass by its fractional abundance and summing the results. For example, chlorine has two stable isotopes: Cl-35 (mass 34.969 amu, 75.77% abundance) and Cl-37 (mass 36.966 amu, 24.23% abundance). The average atomic mass is (34.969 ร— 0.7577) + (36.966 ร— 0.2423) = 35.45 amu. This calculator supports up to 4 isotopes and validates that the abundances sum to approximately 100%. Understanding how to calculate average atomic mass is essential for chemistry courses and helps explain why periodic table values are not whole numbers.

Formula

Mห‰=โˆ‘i=1nmiร—fi\bar{M} = \sum_{i=1}^{n} m_i \times f_i

Where Mฬ„ is the average atomic mass, mแตข is the mass of isotope i, and fแตข is the fractional abundance (percent/100) of isotope i. The abundances must sum to 1 (or 100%).

How to Calculate

  1. 1

    Identify all naturally occurring isotopes of the element and their masses in amu.

  2. 2

    Find the natural abundance (percentage) of each isotope.

  3. 3

    Enter each isotope's mass and abundance percentage in the calculator (at least 2 isotopes required).

  4. 4

    Verify that your abundances sum to approximately 100%.

  5. 5

    Click Calculate to get the weighted average atomic mass.

Worked Examples

Average Atomic Mass of Chlorine

Input: Cl-35: 34.969 amu (75.77%), Cl-37: 36.966 amu (24.23%)

  1. Cl-35: 34.969 ร— 0.7577 = 26.496 amu
  2. Cl-37: 36.966 ร— 0.2423 = 8.957 amu
  3. Sum: 26.496 + 8.957 = 35.453 amu

Result: 35.453 amu

Average Atomic Mass of Boron

Input: B-10: 10.013 amu (19.9%), B-11: 11.009 amu (80.1%)

  1. B-10: 10.013 ร— 0.199 = 1.993 amu
  2. B-11: 11.009 ร— 0.801 = 8.818 amu
  3. Sum: 1.993 + 8.818 = 10.811 amu

Result: 10.811 amu

Average Atomic Mass of Magnesium (3 isotopes)

Input: Mg-24: 23.985 amu (78.99%), Mg-25: 24.986 amu (10.00%), Mg-26: 25.983 amu (11.01%)

  1. Mg-24: 23.985 ร— 0.7899 = 18.947 amu
  2. Mg-25: 24.986 ร— 0.1000 = 2.499 amu
  3. Mg-26: 25.983 ร— 0.1101 = 2.861 amu
  4. Sum: 18.947 + 2.499 + 2.861 = 24.307 amu

Result: 24.307 amu

Frequently Asked Questions

Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, taking into account their relative natural abundances. It is the value reported on the periodic table.
The abundances represent what fraction of all atoms of that element are each isotope in nature. Since every atom must be one of the isotopes, all the percentages must account for the total, adding up to 100%.
Atomic mass usually refers to the mass of a specific isotope. Average atomic mass is the weighted average across all natural isotopes, which is what appears on the periodic table.
Isotope masses and natural abundances are published by IUPAC and can be found in chemistry textbooks, the NUBASE database, or resources like the NIST Atomic Weights and Isotopic Compositions database.
Because they are weighted averages of multiple isotopes with different masses. Even individual isotope masses are not exact integers due to nuclear binding energy differences (mass defect).

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