Dalton (unit) — unified atomic mass unit (Da, u, amu)

Definition and approximate value

By definition, one dalton equals one twelfth of the mass of a neutral carbon‑12 atom. That reference makes the dalton a relative atomic mass unit linked to a specific isotope, carbon‑12. In SI terms the dalton is a very small mass: about 1.66054 × 10⁻²⁷ kilograms (written approximately as 1.66054×10⁻²⁷ kg). Typical particle masses expressed in daltons are: a proton ≈ 1.00728 Da, a neutron ≈ 1.00866 Da and an electron ≈ 0.000549 Da. For larger molecules, chemists report kilodaltons (kDa, 1 kDa = 1,000 Da), megadaltons and so on.

History and terminology

The concept of comparing atomic masses dates to early atomic theory; the unit name "dalton" honors John Dalton, a pioneer of modern atomic theory and atomic weight determination. The term "unified atomic mass unit" arose to distinguish the carbon‑12 based scale from earlier oxygen‑based scales. Usage has varied historically: "amu" was common in older literature, while "u" and "Da" are widely used in contemporary scientific publishing.

Uses and importance

The dalton is the standard unit in chemistry, biochemistry and molecular biology for reporting relative atomic masses, formula weights and protein sizes. Mass spectrometry outputs are often reported in daltons because the unit corresponds directly to the mass‑to‑charge ratio when z = 1. In biochemistry, protein sizes are typically stated in kilodaltons to indicate molecular mass; for example, a protein of 50 kDa has a mass of about 50,000 daltons. The dalton also links microscopic mass to macroscopic molar quantities: by using Avogadro's number, atomic mass units relate to grams per mole and thereby to laboratory mass measurements.

Key facts and distinctions

• The dalton equals 1/12 of a carbon‑12 atom's mass and is written as Da or u.
• It is convenient for atoms, molecules and supramolecular complexes; larger assemblies are expressed in kDa or MDa.
• Historical names include amu and atomic mass unit; modern practice favors Da or u.
• Named after John Dalton, whose work laid foundations for atomic mass concepts.

The dalton remains a practical, widely accepted unit for molecular-scale mass and provides a direct bridge between atomic physics, chemistry and the macroscopic laboratory scale.