Atomic Mass Constant

One twelfth the mass of a carbon-12 atom — the dalton, the unit in which atomic masses are quoted.

Core Principles & Analysis

• Defined as one twelfth the mass of an unbound carbon-12 atom at rest.
• Equals 931.494 MeV/c², the conversion behind nuclear binding-energy calculations.
• Since 2019 it is measured rather than exact: the redefined mole no longer references carbon-12.

Specifications & Metrics

• SYMBOL: u
• STATUS: Measured (CODATA 2022)
• VALUE: 1.66053906892 × 10⁻²⁷ ± 5.2 × 10⁻³⁷ kg

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