dalton

Non-SI unit accepted for use with SI

Name	Symbol	Quantity		SI units
dalton	Da	mass	1.660 539 068 92(52) × 10⁻²⁷ kg
The dalton, symbol Da, or unified atomic mass unit, is a non-SI unit of mass accepted for use with the SI. One dalton is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest.
Definition				h c⁻² Δν_Cs
$1 \mspace{4mu} \text{Da} \mspace{6mu} \approx \dfrac{(1.660 \mspace{4mu} 539 \mspace{4mu} 068 \mspace{4mu} 92 \times10^{-27}) (299 \mspace{4mu} 792 \mspace{4mu} 458)^2}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)} \mspace{6mu} \dfrac{h \mspace{4mu} \Delta \nu _{Cs}}{c^2}\\ \\ \\ 1 \mspace{4mu} \text{Da} \mspace{6mu} \approx 2.450 \mspace{4mu} 160 \mspace{4mu} 931 \mspace{4mu} 287 \mspace{4mu} 942 \times10^{13} \mspace{6mu} h \mspace{4mu} c^{-2} \mspace{4mu} \Delta \nu _{Cs}$

In SI units, one dalton is 1.660 539 068 92(52) × 10⁻²⁷ kg, or approximately 1.66 yoctograms.

$1 \ \text{Da} = \dfrac{1}{12} \ m \mspace{4mu}(^{12} \text{}C)$

The dalton is used as a unit of mass for quantities on an atomic or molecular scale. One dalton is approximately the mass of one nucleon (either a single proton or neutron), and is approximately equivalent to 1 g mol^-1.

Examples of atomic and molecular masses

Name	Symbol	Mass in Da	Mass in SI units
hydrogen	H	1.00	1.66 yg
carbon-12	¹²C	12.00	19.93 yg
silicon-28	²⁸Si	27.98	46.46 yg
iron-56	⁵⁶Fe	55.93	92.88 yg
uranium-235	²³⁵U	235.04	390.29 yg
water	H₂O	18.01	29.91 yg
carbon dioxide	CO₂	44.01	73.08 yg
ethanol	C₂H₅OH	46.07	76.50 yg
benzene	C₆H₆	78.11	129.70 yg
human haemoglobin		≈ 64 000	≈ 106 000 yg

History

Prior to 2019, the mole was defined in terms of the kilogram and the dalton, and was equal to

“the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12”.

The numerical value of the mole is now defined exactly, and the value of the dalton in SI units must be obtained experimentally.