The Dalton (abbreviated Da) is a unit used in expressing atomic or molecular mass, defined as 1/12 the mass of a single atom of carbon-12 (i.e., a carbon atom containing 6 protons and 6 neutrons). As a consequence of this definition and the fact that electrons have very small mass relative to protons and neutrons, the dalton is approximately equal to the mass of one nuclear particle: a proton weighs 1.009 Da, whereas a neutron weighs 1.007 Da.

1 Da is a unit of mass that is almost unfathomably small; 1 gram is equivalent to $6.02 \times 10^{23}$ daltons. Accordingly, the Dalton is only suitable for measuring the mass of objects on the molecular scale.