The debye (symbol: D) (; Dutch: [d?'b?i?]) is a CGS unit (a non-SI metric unit) of electric dipole moment[note 1] named in honour of the physicist Peter J. W. Debye. It is defined as 1×10−18 statcoulomb-centimeters.[note 2] Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb[note 3] (generally called e.s.u. (electrostatic unit) in older literature), which were separated by 1 Ångström.[note 4] This gave a convenient unit for molecular dipole moments.
Typical dipole moments for simple diatomic molecules are in the range of 0 to 11 D. Symmetric homoatomic species, e.g. chlorine, Cl2, have zero dipole moment, and highly ionic molecular species have a very large dipole moment, e.g. gas-phase potassium bromide, KBr, with a dipole moment of 10.5 D.
The debye is still used in atomic physics and chemistry because SI units are inconveniently large. The smallest SI unit of electric dipole moment is the yoctocoulomb-meter, which is roughly 300,000 D.[note 6] There is currently no satisfactory solution to this problem of notation without resorting to the use of scientific notation.