Average Atomic Mass Calculator

What is an Average Atomic Mass?

Average atomic mass is the weighted average of an element's isotope masses. More abundant isotopes affect the result more. Use isotope mass and fractional abundance in this formula: $A = \Sigma(mf)$.

Equation:

$A = (m_1 \times f_1) + (m_2 \times f_2) + ...$

Here, $A$ is average atomic mass, $m$ is isotope mass, and $f$ is fractional abundance.

How to calculate Average Atomic Mass manually

Use $A = (m_1f_1) + (m_2f_2) + ...$, where $m$ is isotope mass and $f$ is fractional abundance.

Formula

$A = \Sigma(m \times f)$

Step by step guide

1. List each isotope mass and abundance.
2. Divide each percentage by 100.
3. Multiply each isotope mass by its fractional abundance.
4. Add the isotope contributions.
5. Round the final answer correctly.

Practical Example

Chlorine has $^{35}Cl$ at 34.969 u and 75.78%, plus $^{37}Cl$ at 36.966 u and 24.22%. Calculate $A = (34.969 \times 0.7578) + (36.966 \times 0.2422)$. The average is about 35.45 u.

Average Atomic Masses Table of some elements

Isotope Abundance and Average Atomic Mass

Isotope abundance tells you how common each isotope is in a natural sample. The calculator uses this value to weight each isotope mass.

What Is Isotopic Abundance?

Isotopic abundance shows the percentage of an element found as a specific isotope. Chlorine has common chlorine-35 and chlorine-37 isotopes.

What Is Fractional Abundance?

Fractional abundance changes a percentage into a decimal. Divide the percentage by 100, then use it in the weighted average formula.

How Does Isotope Abundance Affect the Average Atomic Mass?

A more abundant isotope has a greater effect on average atomic mass. Its mass gets a larger weight in the weighted average calculation.

Why Does the Average Usually Fall Between Isotope Masses?

The weighted average usually sits between the isotope masses. It moves closer to the mass of the isotope with higher abundance.

Average Atomic Mass vs. Related Mass Terms