
Quantity 
Name of unit 
Symbol for unit 
Value in SI units ^{(a)} 

Units accepted for use with the SI 
energy 
electronvolt ^{(b)} 
eV 
1 eV = 1.602 176 565(35) x 10^{–19} J 
mass 
dalton ^{(c)} 
Da 
1 Da = 1.660 538 921(73) x 10^{–27} kg 

unified atomic mass unit 
u 
1 u = 1 Da 
Natural units (n.u.) 
speed 
n.u. of speed (speed of light in vacuum) 
c_{0} 
299 792 458 m/s (exact) 
action 
n.u. of action (reduced Planck constant) 

1.054 571 726(47) x 10^{–34} J s 
mass 
n.u. of mass (electron mass) 
m_{e} 
9.109 382 91(40) x 10^{–31} kg 
time 
n.u. of time 
/(m_{e}c_{0}^{2}) 
1.288 088 668 33(83) x 10^{–21} s 
Atomic units (a.u.) 
charge 
a.u. of charge (elementary charge) 
e 
1.602 176 565(35) x 10^{–19} C 
mass 
a.u. of mass (electron mass) 
m_{e} 
9.109 382 91(40) x 10^{–31} kg 
action 
a.u. of action (reduced Planck constant) 

1.054 571 726(47) x 10^{–34} J s 
length 
a.u. of length, bohr (Bohr radius) 
a_{0} 
0.529 177 210 92(17) x 10^{–10} m 
energy 
a.u. of energy, hartree (Hartree energy) 
E_{h} 
4.359 744 34(19) x 10^{–18} J 
time 
a.u. of time 
/E_{h} 
2.418 884 326 502(12) x 10^{–17} s 
(a) 
The values in SI units of all units in this table are taken from the 2010 CODATA set of recommended values of the fundamental physical constants, P.J. Mohr, B.N. Taylor and D.B. Newell, Rev. Mod. Phys., 2012, 84, 15271605. The standard uncertainty in the last two digits is given in parentheses (see section 5.3.5). 
(b) 
The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of one volt in vacuum. The electronvolt is often combined with the SI prefixes. 
(c) 
The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 times the mass of a free carbon 12 atom, at rest and in its ground state. The dalton is often combined with SI prefixes, for example to express the masses of large molecules in kilodaltons, kDa, or megadaltons, MDa, or to express the values of small mass differences of atoms or molecules in nanodaltons, nDa, or even picodaltons, pDa. 

