Dalton: The atomic mass Unit and Its Origins
The dalton is a unit of atomic mass defined through carbon-12, rather than hydrogen. One dalton is one twelfth of the mass of an unbound, neutral carbon-12 atom in its nuclear and electronic ground state and at rest. The alternative name, unified atomic mass unit, reflects agreement between international physics and chemistry organizations. Its symbols are Da and u. The associated constant is defined using the same reference mass, but the constant and the unit have different roles. In physics and chemistry, daltons express the masses of atoms, molecules, and elementary particles, including averages across groups of such objects.
The conditions in the definition matter because mass includes more than a count of particles in the nucleus. Protons and neutrons contribute most of an atom's mass, so the value in daltons generally lies close to the number of these nucleons. Binding energy and electrons affect the total, however. Carbon-12 has exactly 12 daltons under the specified conditions, corresponding to its six protons and six neutrons. Other atoms depart from that numerical equality. Hydrogen-1 differs by about 0.8%, while helium-3 differs by about 0.5%. The dalton also differs from the mass unit in the atomic units system, which uses electron rest mass.
An average molecular mass can describe a mixture without matching any individual molecule in it. Aspirin provides an example: its average mass is about 180.157 daltons, yet individual molecules do not have that mass. Two common individual masses are 180.0423 and 181.0456 daltons, the latter involving one carbon-13 atom. Larger molecules use larger unit prefixes. Protein and nucleic-acid masses often appear in kilodaltons or megadaltons. Titin, among the largest known proteins, measures several megadaltons. The DNA in human chromosome 1 has a total mass of about 156 gigadaltons, extending the same unit across a much larger molecular structure.
The atomic mass scale has a history of changing references. In 1803, John Dalton proposed using hydrogen, the lightest atom, as the natural reference. Wilhelm Ostwald and others suggested oxygen in 1898 because it was easier to determine experimentally, and an international committee adopted that proposal in 1903. The reference became one sixteenth of an oxygen atom's mass. Jean Perrin adopted the same definition for experiments in 1909. At that stage, isotopes had not yet been discovered, leaving a distinction that later work would make unavoidable: naturally occurring oxygen was not a collection of atoms all having one identical mass.
After oxygen isotopes were discovered in 1929, chemists and physicists used different definitions. Chemists based the unit on an abundance-weighted average of natural oxygen atoms, while physicists used oxygen-16 specifically. The resulting discrepancy mattered in high-precision work. Oxygen's natural isotope proportions also differed between water and air. In April 1957, Alfred O. C. Nier suggested carbon-12 to Josef Mattauch, drawing on carbon's role as a secondary standard in mass spectrometry. International physics and chemistry assemblies approved the change in 1960 and 1961. The new unified unit received the symbol u, replacing amu for the older oxygen-based reference.
The shorter name dalton arrived later. IUPAC proposed it with the symbol Da in 1993, and the international physics union endorsed it in 2005. The Consultative Committee for Units had recommended the name to the International Bureau of Weights and Measures in 2003 because it was shorter and worked better with prefixes. The bureau included it in the 2006 edition of its SI brochure. An international standard listed dalton as an alternative name in 2009. In English, the unit name remains lowercase even though the first letter of its symbol is capitalized, following the pattern used for names such as watt and newton.
The 2019 SI revision left the carbon-12 definition intact while changing the definitions of the kilogram and mole. The kilogram became tied to a fixed Planck constant, and the mole received an exact entity count. As a consequence, molecular mass in daltons and molar mass in grams per mole remain numerically very close but no longer exactly equal. A water molecule averages about 18.0153 daltons, for example, while a mole of water has about 18.0153 grams of mass. The conversion between daltons and kilograms still depends on experiment, with periodically updated values and associated measurement uncertainty.
Experimental atomic mass work also distinguishes neutral atoms from measured ions. Mass spectrometry measures ions, so results require corrections for removed electrons and their binding energy. Earlier estimates followed other routes: Josef Loschmidt obtained an indirect value in 1865 by estimating particles in a gas volume. Perrin used several methods to estimate the Avogadro number and received the 1926 Nobel Prize in Physics largely for that work. Another approach used electrolysis and measured silver lost from an electrode while current flowed for a known time, with corrections for mechanical silver losses and analysis of the sample's isotopic composition.