What does amu mean?
An amu, or atomic mass unit, is the standard unit scientists use to express the masses of atoms, molecules, and subatomic particles. Defined as exactly one twelfth of the mass of a single unbound neutral carbon-12 atom in its ground state, it equals approximately 1.66 × 10⁻²⁷ kilograms — far too small to be useful in everyday units like grams or kilograms. On this scale, a proton weighs about 1.0073 amu and a water molecule about 18 amu, which makes the unit ideal for chemistry and physics calculations involving relative atomic and molecular weights. The abbreviation is almost always written in lowercase as 'amu', and the same quantity is also officially named the dalton (Da) in honour of John Dalton, the pioneer of atomic theory. Although modern scientific bodies prefer the dalton, amu remains deeply entrenched in textbooks, laboratory practice, and everyday scientific communication.
nounAn atomic mass unit (amu) is a unit of mass equal to one twelfth of the mass of an unbound neutral atom of carbon-12 in its ground state, approximately 1.66 × 10⁻²⁷ kilograms. It is used to express the masses of atoms, molecules, and subatomic particles.
- A unit of mass equal to one twelfth the mass of a carbon-12 atom (~1.66 × 10⁻²⁷ kg), used for expressing atomic and molecular masses.
"A proton has a mass of roughly 1.0073 amu."
"The molecular weight of water is about 18 amu."
"The two isotopes of chlorine differ by nearly 2 amu in mass."
As an abbreviation, 'amu' is used unchanged when referring to multiple units; quantities are stated numerically, e.g. 'a difference of 2 amu'.
"The two isotopes differ in mass by about 2 amu."
Chemists gave the humble amu a proper name — the dalton, after John Dalton, father of atomic theory — yet scientists still stubbornly write 'amu' anyway.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of amu
The term derives from 'atomic mass unit', built on the word 'atom', which traces back through Latin 'atomus' to Greek 'átomos' meaning 'uncuttable'. The unit was originally defined relative to hydrogen and later oxygen before being fixed in 1961 to one twelfth of the mass of carbon-12, chosen because carbon's stable isotopes made it a convenient reference. In recognition of John Dalton's foundational work on atomic theory, the unit was given the alternative name 'dalton' (Da), which is now the name preferred by international standards bodies.
How amu is actually used
Almost always encountered as the lowercase abbreviation 'amu' rather than spelled out; in formal scientific literature the SI-preferred name is the dalton (symbol Da), though amu remains widespread in chemistry education and older texts.
Easily confused with amu
An emu is a large flightless Australian bird, whereas an amu is a unit of atomic mass; they are unrelated apart from similar spelling.
The dalton (Da) and the atomic mass unit are the same quantity — 'dalton' is simply the SI-accepted name for what was traditionally abbreviated as amu.