Debye

The debye (symbol: D) (/dɛˈbaɪ/;[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-centimetre.[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]
=  1⁄299,792,458×10−21 C·m[note 5]
≈ 3.33564×10−30 C·m
≈ 1.10048498×1023 qPlP
≈ 0.393430307 ea0[3]
≈ 0.20819434 eÅ
≈ 0.020819434 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-metre, 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.

See also

Buckingham (unit) (CGS unit of electric quadrupole)

Notes

  1. ↑ Electric dipole moment is defined as charge times displacement:
    p = qr.
  2. 1 2 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. ↑ Atomic unit of electric dipole moment, NIST.
  4. ↑ Physical chemistry 2d Edition (1966) G. M. Barrow. McGraw-Hill.
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