Worked example

Cl: 0.7577×34.97+0.2423×36.97≈35.45 u.

Average Atomic Mass

Weighted average isotopic masses by fractional abundance.

Equation

$\bar{M}=\sum f_i M_i$

Inline: $\bar{M}=\sum f_i M_i$$

Variables

Symbol	Quantity	Unit	Notes
$M̄$	average mass	u or g·mol⁻¹	—
$f_i$	fractional abundance	—	—
$M_i$	isotopic mass	u	—

Conditions and limitations

f_i sum to 1; masses are isotopic exact masses.

Rearrangements

—

Dimensional check: u ✓

Worked example

Cl: 0.7577×34.97+0.2423×36.97≈35.45 u.

Steps: write equation → substitute with units → check dimensional consistency → report with correct significant figures.

Common errors

• Using mass numbers instead of isotopic masses

Related resources

• atomic-structure topic
• Calculators — linked only when a real calculator implements this formula.
• Constants — sourced constants with version when values are required.

Sources

• NIST Physical Constants 2022 — CODATA.

Reviewed 2026-08-21 by Editorial Team; due 2027-08-21.