Dalton (unit)
Unit of mass for atomic-scale objects, defined via carbon-12.
War Department. The Adjutant General's Office. 3/4/1907-9/18/1947 · Public domain
The dalton (Da), also called the unified atomic mass unit (u), is a mass unit set to exactly one-twelfth the mass of a single, stationary carbon-12 atom that is unbound, neutral, and in its lowest energy state. The word "unified" marks the joint agreement on this definition by the physics body IUPAP and the chemistry body IUPAC. The atomic mass constant (symbol mᵤ) is the same reference mass, but it is not a unit; it is defined as mᵤ = mₐ(¹²C)/12 = 1 Da. A proton’s mass is roughly 1 dalton.
The constant’s value in kilograms provides a conversion factor from daltons to metric mass units. The 2019 SI revision redefined the kilogram by fixing Planck’s constant (h), which improved the precision of the atomic mass constant in SI units by tying it to fixed physical constants. Even though the dalton is still defined via carbon-12, this change made atomic mass measurements more traceable and accurate. The dalton’s value in terms of the fixed-h kilogram is an experimentally determined number, updated periodically with its uncertainty. The 2022 CODATA recommended value for the atomic mass constant in kilograms is mᵤ = 1.66053906892(52)×10⁻²⁷ kg. The earlier 2018 CODATA value was used in the traditional definition of the Avogadro number to get g/Da = 6.0221407620(18)×10²³, which was then rounded to nine significant figures and set as the exact value for the mole’s 2019 redefinition.
The mole, a unit for amount of substance, works so that the mass of one mole of a substance in grams (its molar mass in g/mol or kg/kmol) equals the average mass of one of its elementary entities (atom, molecule, or formula unit) in daltons. For instance, a water molecule averages about 18.0153 Da, and a mole of water is about 18.0153 g. A protein with an average molecular mass of 64 kDa has a molar mass of 64 kg/mol. However, this equality is only approximate after the 2019 mole redefinition, though it holds for practical purposes.
In physics and chemistry, the dalton expresses the mass of atomic-scale objects—atoms, molecules, and elementary particles—both for single instances and ensemble averages. A helium-4 atom has a mass of 4.0026 Da, an intrinsic property shared by all such atoms. Acetylsalicylic acid (aspirin, C₉H₈O₄) has an average mass near 180.157 Da, but no single molecule has that mass; the two most common masses are 180.0423 Da (with the most common isotopes) and 181.0456 Da (with one carbon-13). Large polymers like proteins and nucleic acids often use kilodaltons (kDa) and megadaltons (MDa). Titin, one of the largest known proteins, ranges from 3 to 3.7 MDa. Human chromosome 1 DNA has about 249 million base pairs, each averaging 650 Da, totaling 156 gigadaltons (GDa).
An atom’s mass in daltons is numerically close to, but not exactly, its number of nucleons (protons and neutrons). So a compound’s molar mass in grams per mole is near the average nucleon count per molecule. By definition, carbon-12 is exactly 12 Da, matching its 12 nucleons. But binding energy of nucleons and electrons affects actual masses, so this equality holds only for carbon-12 under the stated conditions. For example, a free protium atom is 1.007825032241(94) Da, a proton is 1.0072764665789(83) Da, a free neutron is 1.00866491606(40) Da, and a deuterium atom is 2.014101778114(122) Da. The absolute mass excess is generally under 0.1%, except for hydrogen-1 (about 0.8%), helium-3 (0.5%), lithium-6 (0.25%), and beryllium (0.14%). The dalton is not the same as the atomic unit of mass, which is the electron rest mass (mₑ).
The atomic mass constant also has an energy equivalent, mᵤc², with CODATA recommended values. This mass-equivalent is often used as a mass unit in particle physics and is important for determining relative atomic masses in practice.
The concept of a natural atomic mass unit began in 1803, when John Dalton proposed using the mass of the lightest atom, hydrogen, as the unit. This formed the basis of the atomic weight scale. In 1898, chemist Wilhelm Ostwald and others suggested redefining the unit as 1/16 the mass of an oxygen atom, which was formally adopted by the International Committee on Atomic Weights in 1903. Oxygen was easier to work with experimentally, even though it was roughly the mass of a hydrogen atom.
- status_from_2026
- Remains accepted for use with the SI by the BIPM as of the latest SI Brochure; no change has been announced.
Lore & Background
The dalton (Da), also called the unified atomic mass unit (u), is defined as exactly one-twelfth of the mass of a single, unbound, neutral atom of carbon-12 in its nuclear and electronic ground state at rest. This definition was jointly adopted by the international physics and chemistry unions, hence the word “unified.” The atomic mass constant, denoted mᵤ, is numerically identical to the dalton but is considered a reference mass rather than a unit. A proton’s mass is approximately one dalton. The dalton is used across physics and chemistry to express the masses of atoms, molecules, elementary particles, and large polymers. For large biomolecules, kilodaltons (kDa) and megadaltons (MDa) are common; the protein titin, for instance, ranges from 3 to 3.7 MDa. The mass in daltons of an atom is numerically close—but not exactly equal—to its number of nucleons, due to nuclear binding energy and electron mass. Carbon-12 is exactly 12 Da, matching its six protons and six neutrons, but hydrogen-1 is about 1.0078 Da, a deviation of roughly 0.8%. The dalton’s value in kilograms is experimentally determined and periodically updated by CODATA. The 2019 SI revision redefined the kilogram via the Planck constant, improving the precision of the atomic mass constant. One mole of a substance has a mass in grams numerically equal to the average mass of its elementary entities in daltons; for example, one water molecule averages about 18.0153 Da, so one mole of water is about 18.0153 g. This equality is approximate after the 2019 mole redefinition. Historically, John Dalton first proposed using hydrogen’s mass as the natural unit. In 1898, Wilhelm Ostwald and others suggested using 1/16 of an oxygen atom, adopted in 1903, before isotopes were discovered. Jean Perrin used that same oxygen-based definition in 1909 experiments. The carbon-12 standard replaced it later.
Reader's Guide
The dalton is essential for expressing masses of atoms, molecules, and elementary particles, with common usage in kilodaltons (kDa) and megadaltons (MDa) for large polymers like proteins and DNA. Its definition ensures that the mass of one mole of a substance in grams is numerically equal to the average mass of its elementary entities in daltons, though this equality is approximate after the 2019 redefinition of the mole. The unit's history reflects the need for a consistent standard across physics and chemistry, resolving earlier discrepancies between oxygen-based scales. The 2019 revision of the SI redefined the kilogram by fixing the Planck constant, enhancing traceability and accuracy in atomic mass measurements while the dalton's definition via carbon-12 remains unchanged. The atomic mass constant (mu) is an atomic-scale reference mass, not a unit, and its energy equivalent (muc²) is used in particle physics.
Did You Know?
- The dalton is defined as 1/12 of the mass of an unbound neutral carbon-12 atom in its ground state.
- A protein with an average mass of 64 kDa has a molar mass of 64 kg/mol.
Definition and the 'Unified' Identity
The dalton, symbolized as Da and also called the unified atomic mass unit (u), represents one-twelfth of the mass of a single, unbound, neutral carbon-12 atom resting in its lowest nuclear and electronic energy state. The word 'unified' is not decorative; it signals that the definition was jointly endorsed by the physics community through IUPAP and the chemistry community through IUPAC, bridging two disciplines that had historically relied on slightly different reference standards. A closely related but distinct concept is the atomic mass constant, written as mu, which equals the mass of carbon-12 divided by twelve. Although numerically identical to one dalton, mu is formally classified as a reference mass rather than a unit of mass. A proton's mass sits very close to one dalton, giving the unit an intuitive anchor at the scale of individual nucleons.
Practical Usage Across Scales
In routine laboratory and theoretical work, the dalton is the default measure for atomic-scale objects—individual atoms, molecules, and subatomic particles. A helium-4 atom, for instance, carries an intrinsic mass of 4.0026 Da, a fixed value shared by every helium-4 atom. By contrast, a molecule such as aspirin (acetylsalicylic acid, C9H8O4) is assigned an average mass of roughly 180.157 Da, yet no single aspirin molecule actually weighs that amount; the two most common individual masses are 180.0423 Da for the most abundant isotope combination and 181.0456 Da when one carbon is the heavier carbon-13 isotope. For macromolecules, scientists scale up to kilodaltons, megadaltons, and even gigadaltons. Titin, one of the largest known proteins, spans 3 to 3.7 MDa, while the DNA of human chromosome 1, with roughly 249 million base pairs averaging about 650 Da each, totals approximately 156 GDa.
Ties to the SI System and the Mole
The dalton's connection to the broader metric system runs through the atomic mass constant expressed in kilograms. The 2019 revision of the SI redefined the kilogram by fixing the Planck constant, which in turn sharpened the precision with which the atomic mass constant can be stated in SI terms. The 2022 CODATA recommended value places mu at 1.66053906892(52) × 10⁻²⁷ kg, a figure that is experimentally determined and updated as measurement techniques improve. This value also underpins the mole: the mass of one mole of a substance in grams is numerically equal to the average mass of one elementary entity in daltons. Water, for example, has an average molecular mass of about 18.0153 Da, and one mole of water weighs about 18.0153 g. A protein at 64 kDa corresponds to a molar mass of 64 kg/mol. Because the mole was redefined in 2019 using a fixed Avogadro constant, this numerical equality is now only approximate rather than exact.
Historical Roots and the Road to Carbon-12
The idea of a standardized atomic mass unit traces back to 1803, when John Dalton suggested that the mass of a hydrogen atom—the lightest known—should serve as the natural reference. This hydrogen-based scale persisted for nearly a century until, in 1898, chemist Wilhelm Ostwald and colleagues argued for a shift to one-sixteenth of an oxygen atom's mass, a change formally adopted by the International Committee on Atomic Weights in 1903. Oxygen was chosen partly because it was easier to measure experimentally, and the proposal predated the 1912 discovery of isotopes. Physicist Jean Perrin independently adopted the same oxygen-based definition in 1909 while measuring atomic masses and the Avogadro constant, and he also coined the term 'mole' for the number of molecules in 32 grams of O2. That oxygen standard held until 1961, when the field moved to the carbon-12 definition that endures today.
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Frequently Asked Questions
What is the dalton (Da)?
The dalton is a mass unit designed for weighing atoms and subatomic particles. It is defined as exactly one-twelfth of the mass of a single, free, neutral carbon-12 atom in its lowest-energy nuclear and electronic state, at rest.
Is the dalton the same thing as the unified atomic mass unit (u)?
Yes — 'dalton' and 'unified atomic mass unit' are simply two names for the identical quantity, with Da and u serving as interchangeable symbols. The word 'unified' reflects the joint endorsement of the definition by both IUPAP and IUPAC, bridging the physics and chemistry communities.
Why is the dalton important in chemistry?
It gives chemists an intuitive, atom-sized scale so they can say 'a water molecule is about 18 Da' rather than juggling a tiny kilogram figure. Molar-mass lookups, stoichiometry, and molecular-weight calculations all rest on this unit.
Who is the dalton named after?
The unit honors John Dalton, the early-19th-century English chemist who formulated the modern atomic theory. His idea that elements consist of discrete, weighable particles is precisely the concept the dalton quantifies.
Is the dalton still officially accepted alongside the SI?
As of the most recent BIPM SI Brochure, the dalton remains accepted for use with the SI, and no deprecation has been announced. It coexists with the kilogram rather than replacing it, simply because atomic-scale masses are far more practical to express in Da.
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