# Debye

The debye (symbol: D) (/dɛˈb/;[1] Dutch: [dəˈbɛiə]) is a CGS unit[2] (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.

 1 D = 10−18 statC·cm = 10−10 esu·Å[note 2] = .mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}1⁄299,792,458×10−21 C·m[note 5] ≈ 3.33564×10−30 C·m ≈ 1.1004×1023 qPlP ≈ 0.3934303 e·Bohr[3] ≈ 0.2081943 eÅ ≈ 0.02081943 e·nm

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.[4]

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.

## Notes

1. ^ Electric dipole moment is defined as charge times displacement:
p = qr.
2. ^ a b The statcoulomb is also known as the franklin or electrostatic unit of charge.
1 statC = 1 Fr = 1 esu.
3. ^ 10−10 statcoulomb is approximately 0.2083 units of elementary charge.
4. ^ The ångström is of an order of magnitude close to that of a typical covalent bond.
1 Å = 100 pm = 10−8 cm = 10−10 m.
5. ^ One debye equals 1×10−21 C·m2/s divided by the speed of light in vacuum. Conversely, 1 C·m = 2.9979×1029 D.
6. ^ Yocto-, with a value of 10−24, is the smallest SI prefix. Note that SI disallows the application of prefixes to both members of a compound unit or the compounding of prefixes, thus ruling out such units as the femtocoulomb-femtometre or the microyoctocoulomb-metre (both approximately 0.3 D) respectively.

## References

1. ^ "Debye". Random House Dictionary. 2013.
2. ^ CGS units R. Rowlett (University of North Carolina at Chapel Hill).
3. ^
4. ^ Physical chemistry 2d Edition (1966) G. M. Barrow. McGraw-Hill.