Relative atomic mass and isotope composition
Relative atomic mass is dimensionless and depends on the isotope composition being averaged.
Key knowledge
The atomic mass constant mu is one twelfth of the mass of a free carbon-12 atom at rest in its ground state. A sample's relative atomic mass is its mean atomic mass divided by mu. Isotope fractions determine the mean; this is distinct from an isotope's integer mass number. Standard atomic weights describe specified normal terrestrial materials and may have intervals because isotope abundances vary.
Values and equations
Ar = sum_i(x_i m_i) / mu
- Ar
- relative atomic mass of the specified sample 1
- x_i
- number fraction of isotope i 1
- m_i
- mass of one atom of isotope i kg
- mu
- atomic mass constant kg
Conditions and scope
- Use number fractions for the stated sample, with nonnegative fractions summing to one.
- A standard atomic weight is not a universal exact mass for every sample or isotope; enriched, extraterrestrial or anomalous samples may differ.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected SI Brochure printed pp.130 and 141, mole/mass-constant relations and note (i), and CIAAW's natural-variation explanation. The weighted sum is explicit averaging, not a revalidated atomic-weight table or historical molar-mass definition.
2026-10-10 · Independent Mathematics and Information editor (/root/core_math_information). Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.
Entry details
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- foundation-chem-relative-atomic-mass
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- foundations-refresh.1
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