In chemistry and molecular physics, measuring the mass of individual atoms or molecules using standard units like kilograms or grams results in impractically small numbers. To solve this, scientists use the dalton (Da)—also known as the unified atomic mass unit (u).
This fundamental unit serves as the bridge between microscopic atomic masses and macroscopic quantities measured in laboratory environments.
What Is the Definition of a Dalton (Da)?
The dalton (Da) or unified atomic mass unit (u) is defined strictly as 1/12 of the mass of a single unbound, neutral atom of carbon-12 (12C) in its nuclear and electronic ground state and at rest.
Key Terminology
Unified Definition: The term "unified" signifies that both the International Union of Pure and Applied Physics (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC) jointly agreed on this single definition.
The Atomic Mass Constant ($m_u$): The atomic mass constant is defined identically as
While numerically equal to 1mu serves as the physical reference mass constant rather than the unit symbol itself.
Quick Benchmark: For simple mental calculations, a single proton or neutron has a mass roughly equal to 1 Da.
Converting Daltons to Kilograms (2022 CODATA Value)
Because the dalton is an experimentally determined value relative to the SI system, its exact value in kilograms (kg) is periodically updated based on high-precision physical measurements.
According to the 2022 CODATA recommended values, the atomic mass constant is:
mu = 1.66053906892(52) X 10^27 kg
(Note: The numbers in parentheses indicate the standard uncertainty in the final digits).
Impact of the 2019 SI Redefinition
In 2019, the International System of Units (SI) redefined the kilogram by fixing the value of the Planck constant (h). While the definition of the dalton remains anchored to carbon-12, this modern SI revision significantly improved the precision, traceability, and measurement accuracy of the atomic mass constant in SI base units.
How the Dalton Relates to Moles and Molar Mass
The relationship between the dalton (Da) and the mole (mol) forms the core foundation of quantitative chemical calculations and stoichiometry.
The mole is defined such that the mass of one mole of a substance expressed in grams (g/mol) is numerically equal to the average mass of a single entity (atom, molecule, or formula unit) expressed in daltons (Da).
Real-World Examples
Substance | Average Molecular/Atomic Mass | Molar Mass |
Water H2O | ≈18.0153 Da | 18.0153 g/mol |
Hemoglobin / Large Protein | ≈ (64 kDa= 64,000 Da ) | ≈ 64 kg/mol |
Summary Key Takeaways
Standard Reference: 1 Da = 1/12 the mass of a carbon-12 atom.
Primary Use: Expressing atomic, molecular, and macromolecular weights (commonly written in kilodaltons, or kDa, for proteins).
Conversion Factor: 1 Da ≈ 1.66054 X10^-27 kg.
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